The Objective-C 2.0 Programming Language

The Objective-C 2.0 Programming Language Cocoa > Objective-C Language 2008-02-05 Apple Inc. © 2008 Apple Inc. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, mechanical, electronic, photocopying, recording, or otherwise, without prior written permission of Apple Inc., with the following exceptions: Any person is hereby authorized to store documentation on a single computer for personal use only and to print copies of documentation for personal use provided that the documentation contains Apple’s copyright notice. The Apple logo is a trademark of Apple Inc. Use of the “keyboard” Apple logo (Option-Shift-K) for commercial purposes without the prior written consent of Apple may constitute trademark infringement and unfair competition in violation of federal and state laws. No licenses, express or implied, are granted with respect to any of the technology described in this document. 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This warranty gives you specific legal rights, and you may also have other rights which vary from state to state. Contents Introduction Introduction to The Objective-C 2.0 Programming Language 11 Who Should Read This Document 12 Organization of This Document 12 Conventions 13 See Also 13 Chapter 1 Objects and Classes 15 Objects 15 id 15 Dynamic Typing 16 Object Messaging 16 Message Syntax 17 Sending Messages to nil 18 The Receiver’s Instance Variables 18 Polymorphism 19 Dynamic Binding 19 Classes 20 Inheritance 20 Class Types 23 Class Objects 25 Class Names in Source Code 29 Chapter 2 Defining a Class 31 Source Files 31 Class Interface 32 Importing the Interface 33 Referring to Other Classes 33 The Role of the Interface 34 Class Implementation 34 Referring to Instance Variables 35 The Scope of Instance Variables 36 Chapter 3 Categories and Extensions 41 Adding Methods to Classes 41 3 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. How you Use Categories 42 Categories of the Root Class 43 Extensions 43 Chapter 4 Properties 45 Overview 45 Motivation 45 Declaring Properties 46 Accessing Properties 46 Property Declaration and Implementation 46 Property Declaration 47 Property Implementation Directives 47 Property Declaration Attributes 48 Copy 50 Property Re-declaration 51 Performance and Threading 51 Markup and Deprecation 52 Core Foundation 52 Example 53 The Dot Syntax 54 Overview 54 nil Values 56 self 56 Performance and Threading 56 Usage Summary 57 Abusage Summary 57 Properties and Key-Value Coding 58 Subclassing with Properties 59 Property Introspection 59 Runtime Differences 60 Chapter 5 Protocols 61 Declaring Interfaces for Others to Implement 61 Methods for Others to Implement 62 Declaring Interfaces for Anonymous Objects 63 Non-Hierarchical Similarities 63 Formal Protocols 64 Declaring a Protocol 64 Optional Protocol Methods 65 Informal Protocols 65 Protocol Objects 66 Adopting a Protocol 66 Conforming to a Protocol 67 Type Checking 68 4 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CONTENTS Protocols Within Protocols 68 Referring to Other Protocols 69 Chapter 6 Fast Enumeration 71 The for…in Feature 71 Using Fast Enumeration 72 Implementing the NSFastEnumeration Protocol 73 Chapter 7 How Messaging Works 75 The objc_msgSend Function 75 Selectors 78 Methods and Selectors 78 Method Return and Argument Types 79 Varying the Message at Runtime 79 The Target-Action Design Pattern 79 Avoiding Messaging Errors 80 Dynamic Method Resolution 81 Using Hidden Arguments 81 Messages to self and super 82 An Example 83 Using super 84 Redefining self 85 Chapter 8 Enabling Static Behavior 87 Static Typing 87 Type Checking 88 Return and Argument Types 89 Static Typing to an Inherited Class 89 Getting a Method Address 90 Chapter 9 Exception Handling 91 Handling Exceptions 91 Throwing Exceptions 92 Processing Exceptions 92 Chapter 10 Threading 95 Synchronizing Thread Execution 95 Chapter 11 Using C++ With Objective-C 97 Overview 97 5 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CONTENTS Mixing Objective-C and C++ Language Features 97 C++ Lexical Ambiguities and Conflicts 100 Chapter 12 The Runtime System 103 Interacting with the Runtime System 103 Allocating and Initializing Objects 104 The Returned Object 105 Arguments 106 Coordinating Classes 106 The Designated Initializer 108 Combining Allocation and Initialization 110 Memory Management 111 Forwarding 112 Forwarding and Multiple Inheritance 113 Surrogate Objects 114 Forwarding and Inheritance 115 Dynamic Method Resolution 116 Dynamic Loading 117 Remote Messaging 117 Distributed Objects 118 Language Support 119 Type Encodings 123 Appendix A Language Summary 127 Messages 127 Defined Types 127 Preprocessor Directives 128 Compiler Directives 128 Classes 130 Categories 130 Deprecation Syntax 131 Formal Protocols 131 Method Declarations 132 Method Implementations 132 Naming Conventions 133 Appendix B Grammar 135 External Declarations 136 Type Specifiers 139 Type Qualifiers 139 Primary Expressions 140 Exceptions 141 Synchronization 141 6 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CONTENTS Glossary 143 Document Revision History 147 Index 151 7 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CONTENTS 8 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CONTENTS Figures, Tables, and Listings Chapter 1 Objects and Classes 15 Figure 1-1 Some Drawing Program Classes 21 Figure 1-2 Rectangle Instance Variables 22 Figure 1-3 Inheritance hierarchy for NSCell 27 Listing 1-1 Implementation of the initialize method 29 Chapter 2 Defining a Class 31 Figure 2-1 The scope of instance variables 37 Chapter 4 Properties 45 Listing 4-1 Declaring properties in a class 46 Listing 4-2 Using @synthesize 47 Listing 4-3 Using @dynamic with direct method implementations 48 Listing 4-4 Declaring properties for a class 53 Listing 4-5 Accessing properties using the dot syntax 55 Listing 4-6 Accessing properties using accessor methods 55 Chapter 7 How Messaging Works 75 Figure 7-1 Messaging Framework 77 Figure 7-2 High, Mid, Low 83 Chapter 9 Exception Handling 91 Listing 9-1 An exception handler 92 Chapter 10 Threading 95 Listing 10-1 Locking a method using self 95 Listing 10-2 Locking a method using _cmd 96 Listing 10-3 Locking a method using a custom semaphore 96 Chapter 11 Using C++ With Objective-C 97 Listing 11-1 Using C++ and Objective-C instances as instance variables 97 9 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. Chapter 12 The Runtime System 103 Figure 12-1 Incorporating an Inherited Initialization Method 107 Figure 12-2 Covering an Inherited Initialization Model 108 Figure 12-3 Covering the Designated Initializer 109 Figure 12-4 Initialization Chain 110 Figure 12-5 Forwarding 114 Figure 12-6 Remote Messages 118 Figure 12-7 Round-Trip Message 120 Table 12-1 Objective-C type encodings 123 Table 12-2 Objective-C method encodings 125 10 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. FIGURES, TABLES, AND LISTINGS An object-oriented approach to application development makes programs more intuitive to design, faster to develop, more amenable to modification, and easier to understand. Most object-oriented development environments consist of at least three parts: ■ An object-oriented programming language and support library ■ A library of objects ■ A set of development tools This document is about the first component of the development environment—the programming language and its runtime environment. It fully describes the Objective-C language, and provides a foundation for learning about the second component, the Mac OS X Objective-C application frameworks—collectively known as Cocoa. You can start to learn more about Cocoa by reading Getting Started with Cocoa. The two main development tools you use are Xcode and Interface Builder, described in Xcode User Guide and Interface Builder respectively. Important: This document describes version 2.0 of the Objective-C language which is released with Mac OS X v10.5. Several new features are introduced in this version, including properties (see “Properties” (page 45)), fast enumeration (see “Fast Enumeration” (page 71)), optional protocols, and (on modern platforms) non-fragile instance variables. These features are not available on versions of Mac OS X prior to 10.5. If you use these features, therefore, your application cannot run on versions of Mac OS X prior to 10.5. To learn about version 1.0 of the Objective-C language, read Object Oriented Programming and the Objective-C Programming Language 1.0. The Objective-C language is a simple computer language designed to enable sophisticated object-oriented programming. Objective-C is defined as a small but powerful set of extensions to the standard ANSI C language. Its additions to C are mostly based on Smalltalk, one of the first object-oriented programming languages. Objective-C is designed to give C full object-oriented programming capabilities, and to do so in a simple and straightforward way. For those who have never used object-oriented programming to create applications before, this document is also designed to help you become familiar with object-oriented development. It spells out some of the implications of object-oriented design and gives you a flavor of what writing an object-oriented program is really like. 11 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. INTRODUCTION Introduction to The Objective-C 2.0 Programming Language Who Should Read This Document The document is intended for readers who might be interested in: ■ Learning about object-oriented programming ■ Finding out about the basis for the Cocoa application framework ■ Programming in Objective-C This document both introduces the object-oriented model that Objective-C is based upon and fully documents the language. It concentrates on the Objective-C extensions to C, not on the C language itself. Because this isn’t a document about C, it assumes some prior acquaintance with that language. However, it doesn’t have to be an extensive acquaintance. Object-oriented programming in Objective-C is sufficiently different from procedural programming in ANSI C that you won’t be hampered if you’re not an experienced C programmer. Organization of This Document This document is divided into several chapters and two appendixes. The following chapters describe the Objective-C language They cover all the features that the language adds to standard C and C++. These chapters present the language, but also touch on important elements of the runtime system. ■ “Objects and Classes” (page 15) ■ “Defining a Class” (page 31) ■ “Properties” (page 45) ■ “Protocols” (page 61) ■ “Fast Enumeration” (page 71) ■ “How Messaging Works” (page 75) ■ “Enabling Static Behavior” (page 87) ■ “Exception Handling” (page 91) ■ “Threading” (page 95) The Apple compilers are based on the compilers of the GNU Compiler Collection. Objective-C syntax is a superset of GNU C/C++ syntax, and the Objective-C compiler works for C, C++ and Objective-C source code. The compiler recognizes Objective-C source files by the filename extension .m, just as it recognizes files containing only standard C syntax by filename extension .c. Similarly, the compiler recognizes C++ files that use Objective-C by the extension .mm. Other issues when using Objective-C with C++ are covered in “Using C++ With Objective-C” (page 97). 12 Who Should Read This Document 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. INTRODUCTION Introduction to The Objective-C 2.0 Programming Language “The Runtime System” (page 103) looks at the NSObject class and how Objective-C programs interact with the runtime system. In particular, it examines the paradigms for managing object allocations, dynamically loading new classes at runtime, and forwarding messages to other objects. The appendixes contain reference material that might be useful for understanding the language. They are: ■ “Language Summary” (page 127) lists and briefly comments on all of the Objective-C extensions to the C language. ■ “Grammar” (page 135) presents, without comment, a formal grammar of the Objective-C extensions to the C language. This reference manual is meant to be read as a companion to the reference manual for C presented in The C Programming Language by Brian W. Kernighan and Dennis M. Ritchie, published by Prentice Hall. Conventions Where this document discusses functions, methods, and other programming elements, it makes special use of computer voice and italic fonts. Computer voice denotes words or characters that are to be taken literally (typed as they appear). Italic denotes words that represent something else or can be varied. For example, the syntax: @interfaceClassName(CategoryName) means that @interface and the two parentheses are required, but that you can choose the class name and category name. Where example code is shown, ellipsis points indicates the parts, often substantial parts, that have been omitted: - (void)encodeWithCoder:(NSCoder *)coder { [super encodeWithCoder:coder]; ... } The conventions used in the reference appendix are described in that appendix. See Also Object-Oriented Programming with Objective-C describes object-oriented programming and development from the perspective of an Objective-C developer. Objective-C 2.0 Runtime Reference describes the data structures and functions of the Objective-C runtime support library. Your programs can use these interfaces to interact with the Objective-C runtime system. For example, you can add classes or methods, or obtain a list of all class definitions for loaded classes. Garbage Collection Programming Guide describes the garbage collection system used by Objective-C. Conventions 13 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. INTRODUCTION Introduction to The Objective-C 2.0 Programming Language Objective-C Release Notes describes some of the changes in the Objective-C runtime in the latest release of Mac OS X. 14 See Also 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. INTRODUCTION Introduction to The Objective-C 2.0 Programming Language Objects As the name implies, object-oriented programs are built around objects. An object associates data with the particular operations that can use or affect that data. In Objective-C, these operations are known as the object’s methods; the data they affect are its instance variables. In essence, an object bundles a data structure (instance variables) and a group of procedures (methods) into a self-contained programming unit. For example, if you are writing a drawing program that allows a user to create images composed of lines, circles, rectangles, text, bit-mapped images, and so forth, you might create classes for many of the basic shapes that a user can manipulate. A Rectangle object, for instance, might have instance variables that identify the position of the rectangle within the drawing along with its width and its height. Other instance variables could define the rectangle’s color, whether or not it is to be filled, and a line pattern that should be used to display the rectangle. A Rectangle class would have methods to set an instance’s position, size, color, fill status, and line pattern, along with a method that causes the instance to display itself. In Objective-C, an object’s instance variables are internal to the object; generally, you get access to an object’s state only through the object’s methods (you can specify whether subclasses or other objects can access instance variables directly by using scope directives, see “The Scope of Instance Variables” (page 36)). For others to find out something about an object, there has to be a method to supply the information. For example, a Rectangle would have methods that reveal its size and its position. Moreover, an object sees only the methods that were designed for it; it can’t mistakenly perform methods intended for other types of objects. Just as a C function protects its local variables, hiding them from the rest of the program, an object hides both its instance variables and its method implementations. id In Objective-C, object identifiers are a distinct data type: id. This type is defined as a pointer to an object—in reality, a pointer to the instance variables of the object, the object’s unique data. Like a C function or an array, an object is identified by its address. All objects, regardless of their instance variables or methods, are of type id. id anObject; Objects 15 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 1 Objects and Classes For the object-oriented constructs of Objective-C, such as method return values, id replaces int as the default data type. (For strictly C constructs, such as function return values, int remains the default type.) The keyword nil is defined as a null object, an id with a value of 0. id, nil, and the other basic types of Objective-C are defined in the header file objc/objc.h. Dynamic Typing The id type is completely nonrestrictive. By itself, it yields no information about an object, except that it is an object. But objects aren’t all the same. A Rectangle won’t have the same methods or instance variables as an object that represents a bit-mapped image. At some point, a program needs to find more specific information about the objects it contains—what the object’s instance variables are, what methods it can perform, and so on. Since the id type designator can’t supply this information to the compiler, each object has to be able to supply it at runtime. This is possible because every object carries with it an isa instance variable that identifies the object’s class—what kind of object it is. Every Rectangle object would be able to tell the runtime system that it is a Rectangle. Every Circle can say that it is a Circle. Objects with the same behavior (methods) and the same kinds of data (instance variables) are members of the same class. Objects are thus dynamically typed at runtime. Whenever it needs to, the runtime system can find the exact class that an object belongs to, just by asking the object. Dynamic typing in Objective-C serves as the foundation for dynamic binding, discussed later. The isa pointer also enables objects to perform introspection—to find out about themselves (or other objects). The compiler records information about class definitions in data structures for the runtime system to use. The functions of the runtime system use isa, to find this information at runtime. Using the runtime system, you can, for example, determine whether an object implements a particular method, or discover the name of its superclass. Object classes are discussed in more detail under “Classes” (page 20). It’s also possible to give the compiler information about the class of an object by statically typing it in source code using the class name. Classes are particular kinds of objects, and the class name can serve as a type name. See “Class Types” (page 23) and “Enabling Static Behavior” (page 87). Object Messaging This section explains the syntax of sending messages, including how you can nest message expressions. It also discusses the “visibility” of an object’s instance variables, and the concepts of polymorphism and dynamic binding. 16 Object Messaging 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 1 Objects and Classes Message Syntax To get an object to do something, you send it a message telling it to apply a method. In Objective-C, message expressions are enclosed in brackets: [receiver message] The receiver is an object, and the message tells it what to do. In source code, the message is simply the name of a method and any arguments that are passed to it. When a message is sent, the runtime system selects the appropriate method from the receiver’s repertoire and invokes it. For example, this message tells the myRect object to perform its display method, which causes the rectangle to display itself: [myRect display]; Methods can also take arguments. The imaginary message below tells myRect to set its location within the window to coordinates (30.0, 50.0): [myRect setOrigin:30.0 :50.0]; Here the method name, setOrigin::, has two colons, one for each of its arguments. The arguments are inserted after the colons. This method name uses unlabeled arguments. Unlabeled arguments make it difficult to determine the kind and purpose of a method’s arguments. Instead, method names should include labels describing each of their arguments. Argument labels precede each colon in the method name. The setWidth:height: method, for example, makes the purpose of its two arguments clear: [myRect setWidth:10.0 height:15.0]; Methods that take a variable number of arguments are also possible, though they’re somewhat rare. Extra arguments are separated by commas after the end of the method name. (Unlike colons, the commas aren’t considered part of the name.) In the following example, the imaginary makeGroup: method is passed one required argument (group) and three that are optional: [receiver makeGroup:group, memberOne, memberTwo, memberThree]; Like standard C functions, methods can return values. The following example sets the variable isFilled to YES if myRect is drawn as a solid rectangle, or NO if it’s drawn in outline form only. BOOL isFilled; isFilled = [myRect isFilled]; Note that a variable and a method can have the same name. One message expression can be nested inside another. Here, the color of one rectangle is set to the color of another: [myRect setPrimaryColor:[otherRect primaryColor]]; Objective-C 2.0 also provides a dot (.) operator that offers a compact and convenient syntax for invoking an object’s accessor methods. This is typically used in conjunction with the declared properties feature (see “Properties” (page 45)), and is described in “The Dot Syntax” (page 54). Object Messaging 17 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 1 Objects and Classes Sending Messages to nil In Objective-C, it is valid to send a message to nil—it simply has no effect at runtime. There are several patterns in Cocoa that take advantage of this fact. The value returned from a message to nil may also be valid: ■ If the method returns an object, any pointer type, any integer scalar of size less than or equal to sizeof(void*), a float, a double, a long double, or a long long, then a message sent to nil returns 0. ■ If the method returns a struct, as defined by the Mac OS X ABI Function Call Guide to be returned in registers, then a message sent to nil returns 0.0 for every field in the data structure. Other struct data types will not be filled with zeros. ■ If the method returns anything other than the aforementioned value types the return value of a message sent to nil is undefined. The following code fragment illustrates valid use of sending a message to nil. id anObjectMaybeNil = nil; // this is valid if ([anObjectMaybeNil methodThatReturnsADouble] == 0.0) { // implementation continues... } Note: The behavior of sending messages to nil changed with Mac OS X v10.5; for the behavior of previous versions, see prior documentation. The Receiver’s Instance Variables A method has automatic access to the receiving object’s instance variables. You don’t need to pass them to the method as arguments. For example, the primaryColor method illustrated above takes no arguments, yet it can find the primary color for otherRect and return it. Every method assumes the receiver and its instance variables, without having to declare them as arguments. This convention simplifies Objective-C source code. It also supports the way object-oriented programmers think about objects and messages. Messages are sent to receivers much as letters are delivered to your home. Message arguments bring information from the outside to the receiver; they don’t need to bring the receiver to itself. A method has automatic access only to the receiver’s instance variables. If it requires information about a variable stored in another object, it must send a message to the object asking it to reveal the contents of the variable. The primaryColor and isFilled methods shown above are used for just this purpose. See “Defining a Class” (page 31) for more information on referring to instance variables. 18 Object Messaging 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 1 Objects and Classes Polymorphism As the examples above illustrate, messages in Objective-C appear in the same syntactic positions as function calls in standard C. But, because methods “belong to” an object, messages behave differently than function calls. In particular, an object can be operated on by only those methods that were defined for it. It can’t confuse them with methods defined for other kinds of objects, even if another object has a method with the same name. This means that two objects can respond differently to the same message. For example, each kind of object sent a display message could display itself in a unique way. A Circle and a Rectangle would respond differently to identical instructions to track the cursor. This feature, referred to as polymorphism, plays a significant role in the design of object-oriented programs. Together with dynamic binding, it permits you to write code that might apply to any number of different kinds of objects, without you having to choose at the time you write the code what kinds of objects they might be. They might even be objects that will be developed later, by other programmers working on other projects. If you write code that sends a display message to an id variable, any object that has a display method is a potential receiver. Dynamic Binding A crucial difference between function calls and messages is that a function and its arguments are joined together in the compiled code, but a message and a receiving object aren’t united until the program is running and the message is sent. Therefore, the exact method that’s invoked to respond to a message can only be determined at runtime, not when the code is compiled. The precise method that a message invokes depends on the receiver. Different receivers may have different method implementations for the same method name (polymorphism). For the compiler to find the right method implementation for a message, it would have to know what kind of object the receiver is—what class it belongs to. This is information the receiver is able to reveal at runtime when it receives a message (dynamic typing), but it’s not available from the type declarations found in source code. The selection of a method implementation happens at runtime. When a message is sent, a runtime messaging routine looks at the receiver and at the method named in the message. It locates the receiver’s implementation of a method matching the name, “calls” the method, and passes it a pointer to the receiver’s instance variables. (For more on this routine, see “How Messaging Works” (page 75).) The method name in a message thus serves to “select” a method implementation. For this reason, method names in messages are often referred to as selectors. This dynamic binding of methods to messages works hand-in-hand with polymorphism to give object-oriented programming much of its flexibility and power. Since each object can have its own version of a method, a program can achieve a variety of results, not by varying the message itself, but by varying just the object that receives the message. This can be done as the program runs; receivers can be decided “on the fly” and can be made dependent on external factors such as user actions. When executing code based upon the Application Kit, for example, users determine which objects receive messages from menu commands like Cut, Copy, and Paste. The message goes to whatever object controls the current selection. An object that displays text would react to a copy message differently from an object that displays scanned images. An object that represents a set of shapes Object Messaging 19 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 1 Objects and Classes would respond differently from a Rectangle. Since messages don’t select methods (methods aren’t bound to messages) until runtime, these differences are isolated in the methods that respond to the message. The code that sends the message doesn’t have to be concerned with them; it doesn’t even have to enumerate the possibilities. Each application can invent its own objects that respond in their own way to copy messages. Objective-C takes dynamic binding one step further and allows even the message that’s sent (the method selector) to be a variable that’s determined at runtime. This is discussed in the section “How Messaging Works” (page 75). Classes An object-oriented program is typically built from a variety of objects. A program based on the Cocoa frameworks might use NSMatrix objects, NSWindow objects, NSDictionary objects, NSFont objects, NSText objects, and many others. Programs often use more than one object of the same kind or class—several NSArray objects or NSWindow objects, for example. In Objective-C, you define objects by defining their class. The class definition is a prototype for a kind of object; it declares the instance variables that become part of every member of the class, and it defines a set of methods that all objects in the class can use. The compiler creates just one accessible object for each class, a class object that knows how to build new objects belonging to the class. (For this reason it’s traditionally called a “factory object.”) The class object is the compiled version of the class; the objects it builds are instances of the class. The objects that do the main work of your program are instances created by the class object at runtime. All instances of a class have the same set of methods, and they all have a set of instance variables cut from the same mold. Each object gets its own instance variables, but the methods are shared. By convention, class names begin with an uppercase letter (such as “Rectangle”); the names of instances typically begin with a lowercase letter (such as “myRect”). Inheritance Class definitions are additive; each new class that you define is based on another class from which it inherits methods and instance variables. The new class simply adds to or modifies what it inherits. It doesn’t need to duplicate inherited code. Inheritance links all classes together in a hierarchical tree with a single class at its root. When writing code that is based upon the Foundation framework, that root class is typically NSObject. Every class (except a root class) has a superclass one step nearer the root, and any class (including a root class) can be the superclass for any number of subclasses one step farther from the root. Figure 1-1 illustrates the hierarchy for a few of the classes used in the drawing program. 20 Classes 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 1 Objects and Classes Figure 1-1 Some Drawing Program Classes Image Text NSObject Graphic Shape Line CircleRectangle Square This figure shows that the Square class is a subclass of the Rectangle class, the Rectangle class is a subclass of Shape, Shape is a subclass of Graphic, and Graphic is a subclass of NSObject. Inheritance is cumulative. So a Square object has the methods and instance variables defined for Rectangle, Shape, Graphic, and NSObject, as well as those defined specifically for Square. This is simply to say that a Square object isn’t only a Square, it’s also a Rectangle, a Shape, a Graphic, and an NSObject. Every class but NSObject can thus be seen as a specialization or an adaptation of another class. Each successive subclass further modifies the cumulative total of what’s inherited. The Square class defines only the minimum needed to turn a Rectangle into a Square. When you define a class, you link it to the hierarchy by declaring its superclass; every class you create must be the subclass of another class (unless you define a new root class). Plenty of potential superclasses are available. Cocoa includes the NSObject class and several frameworks containing definitions for more than 250 additional classes. Some are classes that you can use “off the shelf”—incorporate into your program as is. Others you might want to adapt to your own needs by defining a subclass. Some framework classes define almost everything you need, but leave some specifics to be implemented in a subclass. You can thus create very sophisticated objects by writing only a small amount of code, and reusing work done by the programmers of the framework. The NSObject Class NSObject is a root class, and so doesn’t have a superclass. It defines the basic framework for Objective-C objects and object interactions. It imparts to the classes and instances of classes that inherit from it the ability to behave as objects and cooperate with the runtime system. A class that doesn’t need to inherit any special behavior from another class should nevertheless be made a subclass of the NSObject class. Instances of the class must at least have the ability to behave like Objective-C objects at runtime. Inheriting this ability from the NSObject class is much simpler and much more reliable than reinventing it in a new class definition. Classes 21 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 1 Objects and Classes Note: Implementing a new root class is a delicate task and one with many hidden hazards. The class must duplicate much of what the NSObject class does, such as allocate instances, connect them to their class, and identify them to the runtime system. For this reason, you should generally use the NSObject class provided with Cocoa as the root class. For more information, see the Foundation framework documentation for the NSObject class and the NSObject protocol. Inheriting Instance Variables When a class object creates a new instance, the new object contains not only the instance variables that were defined for its class but also the instance variables defined for its superclass and for its superclass’s superclass, all the way back to the root class. Thus, the isa instance variable defined in the NSObject class becomes part of every object. isa connects each object to its class. Figure 1-2 shows some of the instance variables that could be defined for a particular implementation of Rectangle, and where they may come from. Note that the variables that make the object a Rectangle are added to the ones that make it a Shape, and the ones that make it a Shape are added to the ones that make it a Graphic, and so on. Figure 1-2 Rectangle Instance Variables Class NSPoint NSColor Pattern . . . float float BOOL NSColor . . . declared in Shape declared in Rectangle declared in NSObject declared in Graphic isa; origin; *primaryColor; linePattern; width; height; filled; *fillColor; A class doesn’t have to declare instance variables. It can simply define new methods and rely on the instance variables it inherits, if it needs any instance variables at all. For example, Square might not declare any new instance variables of its own. Inheriting Methods An object has access not only to the methods defined for its class, but also to methods defined for its superclass, and for its superclass’s superclass, all the way back to the root of the hierarchy. For instance, a Square object can use methods defined in the Rectangle, Shape, Graphic, and NSObject classes as well as methods defined in its own class. Any new class you define in your program can therefore make use of the code written for all the classes above it in the hierarchy. This type of inheritance is a major benefit of object-oriented programming. When you use one of the object-oriented frameworks provided by Cocoa, your programs can take advantage of the basic functionality coded into the framework classes. You have to add only the code that customizes the standard functionality to your application. Class objects also inherit from the classes above them in the hierarchy. But because they don’t have instance variables (only instances do), they inherit only methods. 22 Classes 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 1 Objects and Classes Overriding One Method With Another There’s one useful exception to inheritance: When you define a new class, you can implement a new method with the same name as one defined in a class farther up the hierarchy. The new method overrides the original; instances of the new class perform it rather than the original, and subclasses of the new class inherit it rather than the original. For example, Graphic defines a display method that Rectangle overrides by defining its own version of display. The Graphic method is available to all kinds of objects that inherit from the Graphic class—but not to Rectangle objects, which instead perform the Rectangle version of display. Although overriding a method blocks the original version from being inherited, other methods defined in the new class can skip over the redefined method and find the original (see “Messages to self and super” (page 82) to learn how). A redefined method can also incorporate the very method it overrides. When it does, the new method serves only to refine or modify the method it overrides, rather than replace it outright. When several classes in the hierarchy define the same method, but each new version incorporates the version it overrides, the implementation of the method is effectively spread over all the classes. Although a subclass can override inherited methods, it can’t override inherited instance variables. Since an object has memory allocated for every instance variable it inherits, you can’t override an inherited variable by declaring a new one with the same name. If you try, the compiler will complain. Abstract Classes Some classes are designed only or primarily so that other classes can inherit from them. These abstract classes group methods and instance variables that can be used by a number of different subclasses into a common definition. The abstract class is typically incomplete by itself, but contains useful code that reduces the implementation burden of its subclasses. The NSObject class is the prime example of an abstract class. Although programs often define NSObject subclasses and use instances belonging to the subclasses, they never use instances belonging directly to the NSObject class. An NSObject instance wouldn’t be good for anything; it would be a generic object with the ability to do nothing in particular. The NSView class provides an example of an abstract class that may be used directly. Abstract classes often contain code that helps define the structure of an application. When you create subclasses of these classes, instances of your new classes fit effortlessly into the application structure and work automatically with other objects. (Because abstract classes must have subclasses to be useful, they’re sometimes also called abstract superclasses.) Class Types A class definition is a specification for a kind of object. The class, in effect, defines a data type. The type is based not just on the data structure the class defines (instance variables), but also on the behavior included in the definition (methods). Classes 23 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 1 Objects and Classes A class name can appear in source code wherever a type specifier is permitted in C—for example, as an argument to the sizeof operator: int i = sizeof(Rectangle); Static Typing You can use a class name in place of id to designate an object’s type: Rectangle *myRect; Because this way of declaring an object type gives the compiler information about the kind of object it is, it’s known as static typing. Just as id is defined as a pointer to an object, objects are statically typed as pointers to a class. Objects are always typed by a pointer. Static typing makes the pointer explicit; id hides it. Static typing permits the compiler to do some type checking—for example, to warn if an object could receive a message that it appears not to be able to respond to—and to loosen some restrictions that apply to objects generically typed id. In addition, it can make your intentions clearer to others who read your source code. However, it doesn’t defeat dynamic binding or alter the dynamic determination of a receiver’s class at runtime. An object can be statically typed to its own class or to any class that it inherits from. For example, since inheritance makes a Rectangle a kind of Graphic, a Rectangle instance could be statically typed to the Graphic class: Graphic *myRect; This is possible because a Rectangle is a Graphic. It’s more than a Graphic since it also has the instance variables and method capabilities of a Shape and a Rectangle, but it’s a Graphic nonetheless. For purposes of type checking, the compiler considers myRect to be a Graphic, but at runtime it’s treated as a Rectangle. See “Enabling Static Behavior” (page 87) for more on static typing and its benefits. Type Introspection Instances can reveal their types at runtime. The isMemberOfClass: method, defined in the NSObject class, checks whether the receiver is an instance of a particular class: if ( [anObject isMemberOfClass:someClass] ) ... The isKindOfClass: method, also defined in the NSObject class, checks more generally whether the receiver inherits from or is a member of a particular class (whether it has the class in its inheritance path): if ( [anObject isKindOfClass:someClass] ) ... The set of classes for which isKindOfClass: returns YES is the same set to which the receiver can be statically typed. Introspection isn’t limited to type information. Later sections of this chapter discuss methods that return the class object, report whether an object can respond to a message, and reveal other information. 24 Classes 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 1 Objects and Classes See the NSObject class specification in the Foundation framework reference for more on isKindOfClass:, isMemberOfClass:, and related methods. Class Objects A class definition contains various kinds of information, much of it about instances of the class: ■ The name of the class and its superclass ■ A template describing a set of instance variables ■ The declarations of method names and their return and argument types ■ The method implementations This information is compiled and recorded in data structures made available to the runtime system. The compiler creates just one object, a class object, to represent the class. The class object has access to all the information about the class, which means mainly information about what instances of the class are like. It’s able to produce new instances according to the plan put forward in the class definition. Although a class object keeps the prototype of a class instance, it’s not an instance itself. It has no instance variables of its own and it can’t perform methods intended for instances of the class. However, a class definition can include methods intended specifically for the class object—class methods as opposed to instance methods. A class object inherits class methods from the classes above it in the hierarchy, just as instances inherit instance methods. In source code, the class object is represented by the class name. In the following example, the Rectangle class returns the class version number using a method inherited from the NSObject class: int versionNumber = [Rectangle version]; However, the class name stands for the class object only as the receiver in a message expression. Elsewhere, you need to ask an instance or the class to return the class id. Both respond to a class message: id aClass = [anObject class]; id rectClass = [Rectangle class]; As these examples show, class objects can, like all other objects, be typed id. But class objects can also be more specifically typed to the Class data type: Class aClass = [anObject class]; Class rectClass = [Rectangle class]; All class objects are of type Class. Using this type name for a class is equivalent to using the class name to statically type an instance. Class objects are thus full-fledged objects that can be dynamically typed, receive messages, and inherit methods from other classes. They’re special only in that they’re created by the compiler, lack data structures (instance variables) of their own other than those built from the class definition, and are the agents for producing instances at runtime. Classes 25 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 1 Objects and Classes Note: The compiler also builds a “metaclass object” for each class. It describes the class object just as the class object describes instances of the class. But while you can send messages to instances and to the class object, the metaclass object is used only internally by the runtime system. Creating Instances A principal function of a class object is to create new instances. This code tells the Rectangle class to create a new Rectangle instance and assign it to the myRect variable: id myRect; myRect = [Rectangle alloc]; The alloc method dynamically allocates memory for the new object’s instance variables and initializes them all to 0—all, that is, except the isa variable that connects the new instance to its class. For an object to be useful, it generally needs to be more completely initialized. That’s the function of an init method. Initialization typically follows immediately after allocation: myRect = [[Rectangle alloc] init]; This line of code, or one like it, would be necessary before myRect could receive any of the messages that were illustrated in previous examples in this chapter. The alloc method returns a new instance and that instance performs an init method to set its initial state. Every class object has at least one method (like alloc) that enables it to produce new objects, and every instance has at least one method (like init) that prepares it for use. Initialization methods often take arguments to allow particular values to be passed and have keywords to label the arguments (initWithPosition:size:, for example, is a method that might initialize a new Rectangle instance), but they all begin with “init”. Customization With Class Objects It’s not just a whim of the Objective-C language that classes are treated as objects. It’s a choice that has intended, and sometimes surprising, benefits for design. It’s possible, for example, to customize an object with a class, where the class belongs to an open-ended set. In the Application Kit, for example, an NSMatrix object can be customized with a particular kind of NSCell object. An NSMatrix object can take responsibility for creating the individual objects that represent its cells. It can do this when the matrix is first initialized and later when new cells are needed. The visible matrix that an NSMatrix object draws on the screen can grow and shrink at runtime, perhaps in response to user actions. When it grows, the matrix needs to be able to produce new objects to fill the new slots that are added. But what kind of objects should they be? Each matrix displays just one kind of NSCell, but there are many different kinds. The inheritance hierarchy in Figure 1-3 shows some of those provided by the Application Kit. All inherit from the generic NSCell class: 26 Classes 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 1 Objects and Classes Figure 1-3 Inheritance hierarchy for NSCell NSObject NSCell NSActionCell NSTextFieldCell NSSliderCellNSButtonCell NSFormCell NSMenuCell NSBrowserCell When a matrix creates NSCell objects, should they be NSButtonCell objects to display a bank of buttons or switches, NSTextFieldCell objects to display fields where the user can enter and edit text, or some other kind of NSCell? The NSMatrix object must allow for any kind of cell, even types that haven’t been invented yet. One solution to this problem is to define the NSMatrix class as an abstract class and require everyone who uses it to declare a subclass and implement the methods that produce new cells. Because they would be implementing the methods, users of the class could be sure that the objects they created were of the right type. But this requires others to do work that ought to be done in the NSMatrix class, and it unnecessarily proliferates the number of classes. Since an application might need more than one kind of NSMatrix, each with a different kind of NSCell, it could become cluttered with NSMatrix subclasses. Every time you invented a new kind of NSCell, you’d also have to define a new kind of NSMatrix. Moreover, programmers on different projects would be writing virtually identical code to do the same job, all to make up for NSMatrix's failure to do it. A better solution, the solution the NSMatrix class actually adopts, is to allow NSMatrix instances to be initialized with a kind of NSCell—with a class object. It defines a setCellClass: method that passes the class object for the kind of NSCell object an NSMatrix should use to fill empty slots: [myMatrix setCellClass:[NSButtonCell class]]; The NSMatrix object uses the class object to produce new cells when it’s first initialized and whenever it’s resized to contain more cells. This kind of customization would be difficult if classes weren’t objects that could be passed in messages and assigned to variables. Variables and Class Objects When you define a new class, you can specify instance variables. Every instance of the class can maintain its own copy of the variables you declare—each object controls its own data. There is, however, no “class variable” counterpart to an instance variable. Only internal data structures, initialized from the class definition, are provided for the class. Moreover, a class object has no access to the instance variables of any instances; it can’t initialize, read, or alter them. For all the instances of a class to share data, you must define an external variable of some sort. The simplest way to do this is to declare a variable in the class implementation file as illustrated in the following code fragment. int MCLSGlobalVariable; Classes 27 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 1 Objects and Classes @implementation MyClass // implementation continues In a more sophisticated implementation, you can declare a variable to be static, and provide class methods to manage it. Declaring a variable static limits its scope to just the class—and to just the part of the class that’s implemented in the file. (Thus unlike instance variables, static variables cannot be inherited by, or directly manipulated by, subclasses.) This pattern is commonly used to define shared instances of a class (such as singletons, see Creating a Singleton Instance). static MyClass *MCLSSharedInstance; @implementation MyClass + (MyClass *)sharedInstance { // check for existence of shared instance // create if necessary return MCLSSharedInstance; } // implementation continues Static variables help give the class object more functionality than just that of a “factory” producing instances; it can approach being a complete and versatile object in its own right. A class object can be used to coordinate the instances it creates, dispense instances from lists of objects already created, or manage other processes essential to the application. In the case when you need only one object of a particular class, you can put all the object’s state into static variables and use only class methods. This saves the step of allocating and initializing an instance. Note: It is also possible to use external variables that are not declared static, but the limited scope of static variables better serves the purpose of encapsulating data into separate objects. Initializing a Class Object If a class object is to be used for anything besides allocating instances, it may need to be initialized just as an instance is. Although programs don’t allocate class objects, Objective-C does provide a way for programs to initialize them. If a class makes use of static or global variables, the initialize method is a good place to set their initial values. For example, if a class maintains an array of instances, the initialize method could set up the array and even allocate one or two default instances to have them ready. The runtime system sends an initialize message to every class object before the class receives any other messages and after its superclass has received the initialize message. This gives the class a chance to set up its runtime environment before it’s used. If no initialization is required, you don’t need to write an initialize method to respond to the message. Because of inheritance, an initialize message sent to a class that doesn’t implement the initialize method is forwarded to the superclass, even though the superclass has already received the initialize message. For example, assume class A implements the initialize method, and class B inherits from class A but does not implement the initialize method. Just before class B is to receive its first message, the runtime system sends initialize to it. But, because class B doesn’t implement initialize, class A’s initialize is executed instead. Therefore, class A should ensure that its initialization logic is performed only once. 28 Classes 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 1 Objects and Classes To avoid performing initialization logic more than once, use the template in Listing 1-1 when implementing the initialize method. Listing 1-1 Implementation of the initialize method + (void)initialize { static BOOL initialized = NO; if (!initialized) { // Perform initialization here. ... initialized = YES; } } Note: Remember that the runtime system sends initialize to each class individually. Therefore, in a class’s implementation of the initialize method, you must not send the initialize message to its superclass. Methods of the Root Class All objects, classes and instances alike, need an interface to the runtime system. Both class objects and instances should be able to introspect about their abilities and to report their place in the inheritance hierarchy. It’s the province of the NSObject class to provide this interface. So that NSObject's methods don’t have to be implemented twice—once to provide a runtime interface for instances and again to duplicate that interface for class objects—class objects are given special dispensation to perform instance methods defined in the root class. When a class object receives a message that it can’t respond to with a class method, the runtime system determines whether there’s a root instance method that can respond. The only instance methods that a class object can perform are those defined in the root class, and only if there’s no class method that can do the job. For more on this peculiar ability of class objects to perform root instance methods, see the NSObject class specification in the Foundation framework reference. Class Names in Source Code In source code, class names can be used in only two very different contexts. These contexts reflect the dual role of a class as a data type and as an object: ■ The class name can be used as a type name for a kind of object. For example: Rectangle * anObject; Here anObject is statically typed to be a pointer to a Rectangle. The compiler expects it to have the data structure of a Rectangle instance and the instance methods defined and inherited by the Rectangle class. Static typing enables the compiler to do better type checking and makes source code more self-documenting. See “Enabling Static Behavior” (page 87) for details. Only instances can be statically typed; class objects can’t be, since they aren’t members of a class, but rather belong to the Class data type. Classes 29 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 1 Objects and Classes ■ As the receiver in a message expression, the class name refers to the class object. This usage was illustrated in several of the earlier examples. The class name can stand for the class object only as a message receiver. In any other context, you must ask the class object to reveal its id (by sending it a class message). The example below passes the Rectangle class as an argument in an isKindOfClass: message. if ( [anObject isKindOfClass:[Rectangle class]] ) ... It would have been illegal to simply use the name “Rectangle” as the argument. The class name can only be a receiver. If you don’t know the class name at compile time but have it as a string at runtime, NSClassFromString will return the class object: NSString *className; ... if ( [anObject isKindOfClass:NSClassFromString(className)] ) ... This function returns nil if the string it’s passed is not a valid class name. Classnames exist in the same namespace as global variables and function names. A class and a global variable can’t have the same name. Classnames are about the only names with global visibility in Objective-C. 30 Classes 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 1 Objects and Classes Much of object-oriented programming consists of writing the code for new objects—defining new classes. In Objective-C, classes are defined in two parts: ■ An interface that declares the methods and instance variables of the class and names its superclass ■ An implementation that actually defines the class (contains the code that implements its methods) These are typically split between two files, sometimes however a class definition may span several files through the use of a feature called a “category.” Categories can compartmentalize a class definition or extend an existing one. Categories are described in “Categories and Extensions” (page 41). Source Files Although the compiler doesn’t require it, the interface and implementation are usually separated into two different files. The interface file must be made available to anyone who uses the class. A single file can declare or implement more than one class. Nevertheless, it’s customary to have a separate interface file for each class, if not also a separate implementation file. Keeping class interfaces separate better reflects their status as independent entities. Interface and implementation files typically are named after the class. The name of the implementation file has the .m extension, indicating that it contains Objective-C source code. The interface file can be assigned any other extension. Because it’s included in other source files, the name of the interface file usually has the .h extension typical of header files. For example, the Rectangle class would be declared in Rectangle.h and defined in Rectangle.m. Separating an object’s interface from its implementation fits well with the design of object-oriented programs. An object is a self-contained entity that can be viewed from the outside almost as a “black box.” Once you’ve determined how an object interacts with other elements in your program—that is, once you’ve declared its interface—you can freely alter its implementation without affecting any other part of the application. Source Files 31 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 2 Defining a Class Class Interface The declaration of a class interface begins with the compiler directive @interface and ends with the directive @end. (All Objective-C directives to the compiler begin with “@”.) @interface ClassName : ItsSuperclass { instance variable declarations } method declarations @end The first line of the declaration presents the new class name and links it to its superclass. The superclass defines the position of the new class in the inheritance hierarchy, as discussed under “Inheritance” (page 20). If the colon and superclass name are omitted, the new class is declared as a root class, a rival to the NSObject class. Following the first part of the class declaration, braces enclose declarations of instance variables, the data structures that are part of each instance of the class. Here’s a partial list of instance variables that might be declared in the Rectangle class: float width; float height; BOOL filled; NSColor *fillColor; Methods for the class are declared next, after the braces enclosing instance variables and before the end of the class declaration. The names of methods that can be used by class objects, class methods, are preceded by a plus sign: + alloc; The methods that instances of a class can use, instance methods, are marked with a minus sign: - (void)display; Although it’s not a common practice, you can define a class method and an instance method with the same name. A method can also have the same name as an instance variable. This is more common, especially if the method returns the value in the variable. For example, Circle has a radius method that could match a radius instance variable. Method return types are declared using the standard C syntax for casting one type to another: - (float)radius; Argument types are declared in the same way: - (void)setRadius:(float)aRadius; If a return or argument type isn’t explicitly declared, it’s assumed to be the default type for methods and messages—an id. The alloc method illustrated earlier returns id. When there’s more than one argument, the arguments are declared within the method name after the colons. Arguments break the name apart in the declaration, just as in a message. For example: 32 Class Interface 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 2 Defining a Class - (void)setWidth:(float)width height:(float)height; Methods that take a variable number of arguments declare them using a comma and ellipsis points, just as a function would: - makeGroup:group, ...; Importing the Interface The interface file must be included in any source module that depends on the class interface—that includes any module that creates an instance of the class, sends a message to invoke a method declared for the class, or mentions an instance variable declared in the class. The interface is usually included with the #import directive: #import "Rectangle.h" This directive is identical to #include, except that it makes sure that the same file is never included more than once. It’s therefore preferred and is used in place of #include in code examples throughout Objective-C–based documentation. To reflect the fact that a class definition builds on the definitions of inherited classes, an interface file begins by importing the interface for its superclass: #import "ItsSuperclass.h" @interface ClassName : ItsSuperclass { instance variable declarations } method declarations @end This convention means that every interface file includes, indirectly, the interface files for all inherited classes. When a source module imports a class interface, it gets interfaces for the entire inheritance hierarchy that the class is built upon. Note that if there is a precomp—a precompiled header—that supports the superclass, you may prefer to import the precomp instead. Referring to Other Classes An interface file declares a class and, by importing its superclass, implicitly contains declarations for all inherited classes, from NSObject on down through its superclass. If the interface mentions classes not in this hierarchy, it must import them explicitly or declare them with the @class directive: @class Rectangle, Circle; This directive simply informs the compiler that “Rectangle” and “Circle” are class names. It doesn’t import their interface files. An interface file mentions class names when it statically types instance variables, return values, and arguments. For example, this declaration Class Interface 33 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 2 Defining a Class - (void)setPrimaryColor:(NSColor *)aColor; mentions the NSColor class. Since declarations like this simply use the class name as a type and don’t depend on any details of the class interface (its methods and instance variables), the @class directive gives the compiler sufficient forewarning of what to expect. However, where the interface to a class is actually used (instances created, messages sent), the class interface must be imported. Typically, an interface file uses @class to declare classes, and the corresponding implementation file imports their interfaces (since it will need to create instances of those classes or send them messages). The @class directive minimizes the amount of code seen by the compiler and linker, and is therefore the simplest way to give a forward declaration of a class name. Being simple, it avoids potential problems that may come with importing files that import still other files. For example, if one class declares a statically typed instance variable of another class, and their two interface files import each other, neither class may compile correctly. The Role of the Interface The purpose of the interface file is to declare the new class to other source modules (and to other programmers). It contains all the information they need to work with the class (programmers might also appreciate a little documentation). ■ The interface file tells users how the class is connected into the inheritance hierarchy and what other classes—inherited or simply referred to somewhere in the class—are needed. ■ The interface file also lets the compiler know what instance variables an object contains, and tells programmers what variables subclasses inherit. Although instance variables are most naturally viewed as a matter of the implementation of a class rather than its interface, they must nevertheless be declared in the interface file. This is because the compiler must be aware of the structure of an object where it’s used, not just where it’s defined. As a programmer, however, you can generally ignore the instance variables of the classes you use, except when defining a subclass. ■ Finally, through its list of method declarations, the interface file lets other modules know what messages can be sent to the class object and instances of the class. Every method that can be used outside the class definition is declared in the interface file; methods that are internal to the class implementation can be omitted. Class Implementation The definition of a class is structured very much like its declaration. It begins with the @implementation directive and ends with the @end directive: @implementation ClassName : ItsSuperclass { instance variable declarations } method definitions @end 34 Class Implementation 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 2 Defining a Class However, every implementation file must import its own interface. For example, Rectangle.m imports Rectangle.h. Because the implementation doesn’t need to repeat any of the declarations it imports, it can safely omit: ■ The name of the superclass ■ The declarations of instance variables This simplifies the implementation and makes it mainly devoted to method definitions: #import "ClassName.h" @implementation ClassName method definitions @end Methods for a class are defined, like C functions, within a pair of braces. Before the braces, they’re declared in the same manner as in the interface file, but without the semicolon. For example: + alloc { ... } - (BOOL)isfilled { ... } - (void)setFilled:(BOOL)flag { ... } Methods that take a variable number of arguments handle them just as a function would: #import ... - getGroup:group, ... { va_list ap; va_start(ap, group); ... } Referring to Instance Variables By default, the definition of an instance method has all the instance variables of the object within its scope. It can refer to them simply by name. Although the compiler creates the equivalent of C structures to store instance variables, the exact nature of the structure is hidden. You don’t need either of the structure operators (. or ->) to refer to an object’s data. For example, the following method definition refers to the receiver’s filled instance variable: Class Implementation 35 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 2 Defining a Class - (void)setFilled:(BOOL)flag { filled = flag; ... } Neither the receiving object nor its filled instance variable is declared as an argument to this method, yet the instance variable falls within its scope. This simplification of method syntax is a significant shorthand in the writing of Objective-C code. When the instance variable belongs to an object that’s not the receiver, the object’s type must be made explicit to the compiler through static typing. In referring to the instance variable of a statically typed object, the structure pointer operator (->) is used. Suppose, for example, that the Sibling class declares a statically typed object, twin, as an instance variable: @interface Sibling : NSObject { Sibling *twin; int gender; struct features *appearance; } As long as the instance variables of the statically typed object are within the scope of the class (as they are here because twin is typed to the same class), a Sibling method can set them directly: - makeIdenticalTwin { if ( !twin ) { twin = [[Sibling alloc] init]; twin->gender = gender; twin->appearance = appearance; } return twin; } The Scope of Instance Variables Although they’re declared in the class interface, instance variables are more a matter of the way a class is implemented than of the way it’s used. An object’s interface lies in its methods, not in its internal data structures. Often there’s a one-to-one correspondence between a method and an instance variable, as in the following example: - (BOOL)isFilled { return filled; } But this need not be the case. Some methods might return information not stored in instance variables, and some instance variables might store information that an object is unwilling to reveal. 36 Class Implementation 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 2 Defining a Class As a class is revised from time to time, the choice of instance variables may change, even though the methods it declares remain the same. As long as messages are the vehicle for interacting with instances of the class, these changes won’t really affect its interface. To enforce the ability of an object to hide its data, the compiler limits the scope of instance variables—that is, limits their visibility within the program. But to provide flexibility, it also lets you explicitly set the scope at three different levels. Each level is marked by a compiler directive: MeaningDirective The instance variable is accessible only within the class that declares it.@private The instance variable is accessible within the class that declares it and within classes that inherit it. @protected The instance variable is accessible everywhere.@public On 64-bit, an @package instance variable acts like @public inside the image that implements the class, but @private outside. This is analogous to private_extern for variables and functions. Any code outside the class implementation’s image that tries to use the instance variable will get a link error. This is most useful for instance variables in framework classes, where @private may be too restrictive but @protected or @public too permissive. @package This is illustrated in Figure 2-1. Figure 2-1 The scope of instance variables Unrelated code The class that declares the instance variable A class that inherits the instance variable @private @protected @public A directive applies to all the instance variables listed after it, up to the next directive or the end of the list. In the following example, the age and evaluation instance variables are private, name, job, and wage are protected, and boss is public. Class Implementation 37 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 2 Defining a Class @interface Worker : NSObject { char *name; @private int age; char *evaluation; @protected id job; float wage; @public id boss; } By default, all unmarked instance variables (like name above) are @protected. All instance variables that a class declares, no matter how they’re marked, are within the scope of the class definition. For example, a class that declares a job instance variable, such as the Worker class shown above, can refer to it in a method definition: - promoteTo:newPosition { id old = job; job = newPosition; return old; } Obviously, if a class couldn’t access its own instance variables, the instance variables would be of no use whatsoever. Normally, a class also has access to the instance variables it inherits. The ability to refer to an instance variable is usually inherited along with the variable. It makes sense for classes to have their entire data structures within their scope, especially if you think of a class definition as merely an elaboration of the classes it inherits from. The promoteTo: method illustrated earlier could just as well have been defined in any class that inherits the job instance variable from the Worker class. However, there are reasons why you might want to restrict inheriting classes from directly accessing an instance variable: ■ Once a subclass accesses an inherited instance variable, the class that declares the variable is tied to that part of its implementation. In later versions, it can’t eliminate the variable or alter the role it plays without inadvertently breaking the subclass. ■ Moreover, if a subclass accesses an inherited instance variable and alters its value, it may inadvertently introduce bugs in the class that declares the variable, especially if the variable is involved in class-internal dependencies. To limit an instance variable’s scope to just the class that declares it, you must mark it @private. Instance variables marked @private are only available to subclasses by calling public accessor methods, if they exist. At the other extreme, marking a variable @public makes it generally available, even outside of class definitions that inherit or declare the variable. Normally, to get information stored in an instance variable, other objects must send a message requesting it. However, a public instance variable can be accessed anywhere as if it were a field in a C structure. For example: Worker *ceo = [[Worker alloc] init]; 38 Class Implementation 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 2 Defining a Class ceo->boss = nil; Note that the object must be statically typed. Marking instance variables @public defeats the ability of an object to hide its data. It runs counter to a fundamental principle of object-oriented programming—the encapsulation of data within objects where it’s protected from view and inadvertent error. Public instance variables should therefore be avoided except in extraordinary cases. Class Implementation 39 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 2 Defining a Class 40 Class Implementation 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 2 Defining a Class A category allows you to add methods to an existing class—even to one to which you do not have the source. This is a powerful feature that allows you to extend the functionality of existing classes without subclassing. Using categories, you can also split the implementation of your own classes between several files. Class extensions are similar, but allow additional required API to be declared for a class in locations other than within the primary class @interface block Adding Methods to Classes You can add methods to a class by declaring them in an interface file under a category name and defining them in an implementation file under the same name. The category name indicates that the methods are additions to a class declared elsewhere, not a new class. You cannot, however, use a category to add additional instance variables to a class. The methods the category adds become part of the class type. For example, methods added to the NSArray class in a category are among the methods the compiler expects an NSArray instance to have in its repertoire. Methods added to the NSArray class in a subclass are not included in the NSArray type. (This matters only for statically typed objects, since static typing is the only way the compiler can know an object’s class.) Category methods can do anything that methods defined in the class proper can do. At runtime, there’s no difference. The methods the category adds to the class are inherited by all the class’s subclasses, just like other methods. The declaration of a category interface looks very much like a class interface declaration—except the category name is listed within parentheses after the class name and the superclass isn’t mentioned. Unless its methods don’t access any instance variables of the class, the category must import the interface file for the class it extends: #import "ClassName.h" @interface ClassName ( CategoryName ) // method declarations @end The implementation, as usual, imports its own interface. A common naming convention is that the base file name of the category is the name of the class the category extends followed by “+” followed by the name of the category. A category implementation (in a file named ClassName+CategoryName.m) might therefore look like this: Adding Methods to Classes 41 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 3 Categories and Extensions #import "ClassName+CategoryName.h" @implementation ClassName ( CategoryName ) // method definitions @end Note that a category can’t declare additional instance variables for the class; it includes only methods. However, all instance variables within the scope of the class are also within the scope of the category. That includes all instance variables declared by the class, even ones declared @private. There’s no limit to the number of categories that you can add to a class, but each category name must be different, and each should declare and define a different set of methods. How you Use Categories You can use categories to extend classes defined by other implementors—for example, you can add methods to the classes defined in the Cocoa frameworks. The added methods are inherited by subclasses and are indistinguishable at runtime from the original methods of the class. A category can be an alternative to a subclass. Rather than define a subclass to extend an existing class, through a category you can add methods to the class directly. For example, you could add categories to NSArray and other Cocoa classes. As in the case of a subclass, you don’t need source code for the class you’re extending. The methods added in a category can be used to extend the functionality of the class or override methods the class inherits. A category can also override methods declared in the class interface. However, it cannot reliably override methods declared in another category of the same class. A category is not a substitute for a subclass. It’s best if categories don’t attempt to redefine methods that are explicitly declared in the class’s @interface section. Also note that a class can’t define the same method more than once. When a category overrides an inherited method, the new version can, as usual, incorporate the inherited version through a message to super. But there’s no way for a category method to incorporate a method with the same name defined for the same class. You can also use categories to distribute the implementation of a new class into separate source files—for example, you could group the methods of a large class into several categories and put each category in a different file. When used like this, categories can benefit the development process in a number of ways: ■ They provide a simple way of grouping related methods. Similar methods defined in different classes can be kept together in the same source file. ■ They simplify the management of a large class when several developers contribute to the class definition. ■ They let you achieve some of the benefits of incremental compilation for a very large class. ■ They can help improve locality of reference for commonly used methods. ■ They enable you to configure a class differently for separate applications, without having to maintain different versions of the same source code. 42 How you Use Categories 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 3 Categories and Extensions Categories are also used to declare informal protocols (see “Informal Protocols ” (page 65)), as discussed under “Declaring Interfaces for Others to Implement” (page 61). Categories of the Root Class A category can add methods to any class, including the root class. Methods added to NSObject become available to all classes that are linked to your code. While this can be useful at times, it can also be quite dangerous. Although it may seem that the modifications the category makes are well understood and of limited impact, inheritance gives them a wide scope. You may be making unintended changes to unseen classes; you may not know all the consequences of what you’re doing. Moreover, others who are unaware of your changes won’t understand what they’re doing. In addition, there are two other considerations to keep in mind when implementing methods for the root class: ■ Messages to super are invalid (there is no superclass). ■ Class objects can perform instance methods defined in the root class. Normally, class objects can perform only class methods. But instance methods defined in the root class are a special case. They define an interface to the runtime system that all objects inherit. Class objects are full-fledged objects and need to share the same interface. This feature means that you need to take into account the possibility that an instance method you define in a category of the NSObject class might be performed not only by instances but by class objects as well. For example, within the body of the method, self might mean a class object as well as an instance. See the NSObject class specification in the Foundation framework reference for more information on class access to root instance methods. Extensions Class extensions are like “anonymous” categories, except that the methods they declare must be implemented in the the main @implementation block for the corresponding class. It is common for a class to have a publicly declared API and to then have additional API declared privately for use solely by the class or the framework within which the class resides. You can declare such API in a category (or in more than one category) in a private header file or implementation file as described above. This works, but the compiler cannot verify that all declared methods are implemented. For example, the compiler will compile without error the following declarations and implementation: @interface MyObject : NSObject { NSNumber *number; } - (NSNumber *)number; @end @interface MyObject (Setter) Categories of the Root Class 43 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 3 Categories and Extensions - (void)setNumber:(NSNumber *)newNumber; @end @implementation MyObject - (NSNumber *)number { return number; } @end Note that there is no implementation of the setNumber method. If it is invoked at runtime, this will generate an error. Class extensions allow you to declare additional required API for a class in locations other than within the primary class @interface block, as illustrated in the following example: @interface MyObject : NSObject { NSNumber *number; } - (NSNumber *)number; @end @interface MyObject () - (void)setNumber:(NSNumber *)newNumber; @end @implementation MyObject - (NSNumber *)number { return number; } - (void)setNumber(NSNumber *)newNumber { number = newNumber; } @end Notice that in this case: ■ No name is given in the parentheses in the second @interface block; ■ The implementation of the setNumber method appears within the main @implementation block for the class. The implementation of the setNumber method must appear within the main @implementation block for the class (you cannot implement it in a category). If this is not the case, the compiler will emit a warning that it cannot find a method definition for setNumber. 44 Extensions 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 3 Categories and Extensions The Objective-C “declared properties” feature provides a simple way to implement an object’s properties declaratively. It also provides a concise syntax for accessing properties of objects without the need to write accessor methods. Overview There are two aspects to properties: the syntactic elements used to declare properties, and the syntactic elements used to access properties. This section gives a brief overview of both, and discusses the motivation behind the feature. Motivation You typically access instance variables through a pair of accessor (getter/setter) methods. You can think of state managed through this accessor style API as being the properties of an instance of the class. By using accessor methods, you adhere to the principle of encapsulation (see “Mechanisms Of Abstraction” in Object-Oriented Programming with Objective-C > The Object Model). You can exercise tight control of the behavior of the getter/setter pair and the underlying state management while clients of the API remain insulated from the implementation changes. Although using accessor methods therefore has significant advantages, writing accessor methods is nevertheless a tedious process—particularly if you have to write code to support both garbage collected and managed memory environments. Moreover, aspects of the property that may be important to consumers of the API are left obscured—such as whether the accessor methods are thread-safe or whether new values are copied when set. The Objective-C declared properties feature offers the following advantages: ■ The property declaration provides a clear, explicit specification of how the accessor methods behave. ■ The compiler can synthesize accessor methods for you, according to the specification you provide in the declaration. This means you have less code to write and maintain. ■ Properties are represented syntactically as identifiers and are scoped, so the compiler can detect use of undeclared properties. ■ Runtime introspection of the properties declared by a class. Overview 45 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 4 Properties Declaring Properties The declaration of a property is, effectively, a shorthand for declaring the setter and getter for an attribute of an instance of a class, although it also provides a specification for the accessors’ behavior. In addition to the declaration itself, there are implementation directives to instruct the compiler to synthesize the accessors and to inform the compiler that you will provide the methods yourself at runtime. ■ You use the compiler directive @property to declare a property. It can appear anywhere in the method declaration section of a class, category, or protocol declaration. ■ You use the @synthesize and @dynamic directives in @implementation blocks to trigger specific compiler actions: @synthesize instructs the compiler to synthesize the relevant accessors; @dynamic informs the compiler that you will provide the methods yourself at runtime. Listing 4-1 illustrates the declaration of a property, value, and use of the @synthesize directive to instruct the compiler to create accessor methods to match the specification given in the declaration. Listing 4-1 Declaring properties in a class @interface MyClass : NSObject { NSString *value; } @property(copy, readwrite) NSString *value; @end @implementation MyClass @synthesize value; @end Accessing Properties Using the dot syntax, you can access a property using the same pattern as accessing structure elements. MyClass *myInstance = [[MyClass alloc] init]; myInstance.value = @"New value"; NSLog(@"myInstance value: %@", myInstance.value); The dot syntax is purely “syntactic sugar”—it is transformed by the compiler into invocation of accessor methods (so you are not actually accessing an instance variable directly). The code example above is exactly equivalent to the following: MyClass *myInstance = [[MyClass alloc] init]; [myInstance setValue:@"New value"]; NSLog(@"myInstance value: %@", [myInstance value]); Property Declaration and Implementation There are two parts to a property, its declaration and its implementation. 46 Property Declaration and Implementation 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 4 Properties Property Declaration A property declaration begins with the keyword @property. @property can appear anywhere in the method declaration list found in the @interface of a class. @property can also appear in the declaration of a protocol or category. @property(attributes) type name; @property declares a property. An optional parenthesized set of attributes provides additional details about the storage semantics and other behaviors of the property—see “Property Declaration Attributes” (page 48) for possible values. Like any other Objective-C type, each property has a type specification and a name. Property Implementation Directives You can use the @synthesize and @dynamic directives in @implementation blocks to trigger specific compiler actions. Note that neither is required for any given @property declaration. Important: The default value is @dynamic. If, therefore, you do not specify either @synthesize or @dynamic for a particular property, you must provide a getter and setter (or just a getter in the case of a readonly property) method implementation for that property. @synthesize You use the @synthesize keyword to tell the compiler that it should synthesize the setter and/or getter methods for the property if you do not supply them within the @implementation block. Listing 4-2 Using @synthesize @interface MyClass : NSObject { NSString *value; } @property(copy, readwrite) NSString *value; @end // assume using garbage collection @implementation MyClass @synthesize value; @end You can use the form property=ivar to indicate that a particular instance variable should be used for the property, for example: @synthesize firstName, lastName, age = yearsOld; This specifies that the accessor methods for firstName, lastName, and age should be synthesized and that the property age is represented by the instance variable yearsOld. Other aspects of the synthesized methods are determined by the optional attributes (see “Property Declaration Attributes” (page 48)). There are differences in the behavior that depend on the runtime (see also “Runtime Differences” (page 60)): Property Declaration and Implementation 47 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 4 Properties ■ For the legacy runtimes, instance variables must already be declared in the @interface block. If an instance variable of the same name and compatible type as the property exists, it is used—otherwise, you get a compiler error. ■ For the modern runtimes, instance variables are synthesized as needed. If an instance variable of the same name already exists, it is used. @dynamic You use the @dynamic keyword to tell the compiler that you will fulfill the API contract implied by a property either by providing method implementations directly or at runtime using other mechanisms such as dynamic loading of code or dynamic method resolution. The example shown in Listing 4-3 illustrates using direct method implementations—it is equivalent to the example given in Listing 4-2 (page 47). Listing 4-3 Using @dynamic with direct method implementations @interface MyClass : NSObject { NSString *value; } @property(copy, readwrite) NSString *value; @end // assume using garbage collection @implementation MyClass @dynamic value; - (NSString *)value { return value; } - (void)setValue:(NSString *)newValue { if (newValue != value) { value = [newValue copy]; } } @end Property Declaration Attributes You can decorate a property with attributes by using the form @property(attribute [, attribute2, ...]). Like methods, properties are scoped to their enclosing interface declaration. For property declarations that use a comma delimited list of variable names, the property attributes apply to all of the named properties. If you use garbage collection, you can use the storage modifiers __weak and __strong in a property’s declaration, but they are not a formal part of the list of attributes. getter=getterName, setter=setterName getter= and setter= specify respectively the names of get and set accessors for the property. The getter must return a type matching the property’s type and take no arguments. The setter method must take a single argument of a type matching the property’s type and must return void. The default names are propertyName and setPropertyName: respectively—for example, given a property “foo”, the accessors would be foo and setFoo:. Typically you should specify accessor method names that are key-value coding compliant (see Key-Value Coding Programming 48 Property Declaration and Implementation 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 4 Properties Guide)—a common reason for using this decorator is to adhere to the isPropertyName convention for Boolean values. readonly Indicates that the property is read-only. The default is read/write. If you specify readonly, only a getter method is required in the @implementation, or if you use @synthesize only the getter method is synthesized. Moreover, if you attempt to assign a value using the dot syntax, you get a compiler error. readwrite Indicates that the property should be treated as read/write. This is the default. Both a getter and setter method will be required in the @implementation, or if you use @synthesize the getter and setter methods are synthesized. assign Specifies that the setter uses simple assignment. This is the default. If your application uses garbage collection, if you want to use assign for a property whose class adopts the NSCopying protocol you should specify the attribute explicitly rather than simply relying on the default—otherwise you will get a compiler warning. (This is to reassure the compiler that you really do want to assign the value, even though it’s copyable.) retain Specifies that retain should be invoked on the object upon assignment. (The default is assign.) This attribute is valid only for Objective-C object types. (You cannot specify retain for Core Foundation objects—see “Core Foundation” (page 52).) copy Specifies that a copy of the object should be used for assignment. (The default is assign.) The copy is made by invoking the copy method. This attribute is valid only for object types, which must implement the NSCopying protocol. For further discussion, see “Copy” (page 50). nonatomic Specifies that accessors are non-atomic. By default, accessors are atomic. By default, accessors are atomic. This is so that default synthesized accessors provide robust access to properties in a multi-threaded environment—that is, the value returned from the getter or set via the setter is always fully retrieved or set regardless of what other threads are executing simultaneously. For more details, see “Performance and Threading” (page 51). If you do not specify nonatomic, then in a managed memory environment a synthesized get accessor for an object property retains and autoreleases the returned value; if you specify nonatomic, then a synthesized accessor for an object property simply returns the value directly. If you use the @synthesize directive to instruct the compiler to create the accessor method(s), the code it generates matches the specification given by the keywords. If you implement your the accessor method(s) yourself, you should ensure that it/they match the specification (for example, if you specify copy you must make sure that you do copy the input value in the setter method). You can use the interface attributes getter=, setter=, readonly, readwrite in a class, category or protocol declaration. You can only use one of readonly and readwrite in the attribute list. setter=/getter= are both optional and may appear with any other attribute save for readonly. If you specify that a property is readonly then also specify a setter with setter=, you will get a compiler warning. assign, retain, and copy are mutually exclusive. Different constraints apply depending on whether or not you use garbage collection: Property Declaration and Implementation 49 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 4 Properties ■ If you do not use garbage collection, for object properties you must explicitly specify one of assign, retain or copy—otherwise you will get a compiler warning. (This encourages you think about what memory management behavior you want and type it explicitly.) ■ If you use garbage collection, you don't get a warning if you use the default (that is, if you don’t specify any of assign, retain or copy) unless the property's type is a class that conforms to NSCopying. The default is usually what you want; if the property type can be copied, however, to preserve encapsulation you often want to make a private copy of the object. Copy If you use the copy declaration attribute, you specify that a value is copied during assignment. If you synthesize the corresponding accessor, the synthesized method uses the copy method. This is useful for attributes such as string objects where there is a possibility that the new value passed in a setter may be mutable (for example, an instance of NSMutableString) and you want to ensure that your object has its own private immutable copy. For example, if you declare a property as follows: @property (nonatomic, copy) NSString *string; then the synthesized setter method is similar to the following: -(void)setString:(NSString *)newString { if (string != newString) { [string release]; string = [newString copy]; } } Although this works well for strings, it may present a problem if the attribute is a collection such as an array or a set. Typically you want such collections to be mutable, but the copy method returns an immutable version of the collection. In this situation, you have to provide your own implementation of the setter method, as illustrated in the following example. @interface MyClass : NSObject { NSMutableArray *myArray; } @property (nonatomic, copy) NSMutableArray *myArray; @end @implementation MyClass @synthesize myArray; - (void)setMyArray:(NSArray *)newArray { if (myArray != newArray) { [myArray release]; myArray = [newArray mutableCopy]; } } 50 Property Declaration and Implementation 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 4 Properties @end Property Re-declaration You can re-declare a property in a subclass, but (with the exception of readonly vs. readwrite) you must repeat its attributes in whole in the subclasses. The same holds true for a property declared in a category or protocol—while the property may be redeclared in a category or protocol, the property’s attributes must be repeated in whole. If you declare a property in one class as readonly, you can redeclare it as readwrite in a class extension (see “Extensions” (page 43)), a protocol, or a subclass—see “Subclassing with Properties” (page 59). In the case of a class extension redeclaration, the fact that the property was redeclared prior to any @synthesize statement will cause the setter to be synthesized. The ability to redeclare a read-only property as read/write enables two common implementation patterns: a mutable subclass of an immutable class (NSString, NSArray, and NSDictionary are all examples) and a property that has public API that is readonly but a private readwrite implementation internal to the class. The following example shows using a class extension to provide a property that is declared as read-only in the public header but is redeclared privately as read/write. // public header file @interface MyObject : NSObject { NSString *language; } @property (readonly, copy) NSString *language; @end // private implementation file @interface MyObject () @property (readwrite, copy) NSString *language; @end @implementation MyObject @synthesize language; @end Performance and Threading If you supply your own method implementation, the fact that you declared a property has no effect on its efficiency or thread safety. If you use synthesized properties, the method implementations generated by the compiler depend on the specification you supply. The declaration attributes that affect performance and threading are retain, assign, copy, and nonatomic. The first three of these affect only the implementation of the assignment part of the set method, as illustrated below (note that the implementation may not be exactly as shown): // assign property = newValue; // retain if (property != newValue) { Property Declaration and Implementation 51 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 4 Properties [property release]; property = [newValue retain]; } // copy if (property != newValue) { [property release]; property = [newValue copy]; } The effect of the nonatomic attribute depends on the environment. By default, the synthesized accessors are atomic. In a managed memory environment, guaranteeing atomic behavior requires the use of a lock; moreover a returned object is retained and autoreleased. If such accessors are invoked frequently, this may have a significant impact on performance. In a garbage collected environment, most synthesized methods are atomic without incurring this overhead. It is important to understand that the goal of the atomic implementation is to provide robust accessors—it does not guarantee correctness of your code. Although “atomic” means that access to the property is thread-safe, simply making all the properties in your class atomic does not mean that your class or more generally your object graph is “thread safe”—thread safety cannot be expressed at the level of individual accessor methods. For more about multi-threading, see Threading Programming Guide. Markup and Deprecation Properties support the full range of C style decorators. Properties can be deprecated and support __attribute__ style markup, as illustrated in the following example. @property CGFloat x AVAILABLE_MAC_OS_X_VERSION_10_1_AND_LATER_BUT_DEPRECATED_IN_MAC_OS_X_VERSION_10_4; @property CGFloat y __attribute__((...)); Core Foundation As noted in “Property Implementation Directives” (page 47), you cannot specify the retain attribute for non-object types. If, therefore, you declare a property whose type is a CFType and synthesize the accessors as illustrated in the following example: @interface MyClass : NSObject { CGImageRef myImage; } @property(readwrite) CGImageRef myImage; @end @implementation MyClass @synthesize myImage; @end then in a managed memory environment the generated set accessor will simply assign the new value to the instance variable (the new value is not retained and the old value is not released). This is typically incorrect, so you should not synthesize the methods, you should implement them yourself. 52 Property Declaration and Implementation 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 4 Properties In a garbage collected environment, if the variable is declared __strong: ... __strong CGImageRef myImage; ... @property CGImageRef myImage; then the accessors are synthesized appropriately—the image will not be CFRetain'd, but the setter will trigger a write barrier. Example The following example illustrates the use of properties in several different ways: ■ The Link protocol declares a property, next. ■ MyClass adopts the Link protocol so implicitly also declares the property next. MyClass also declares several other properties. ■ creationTimestamp and next are synthesized but use existing instance variables with different names; ■ name is synthesized, and uses instance variable synthesis (recall that instance variable synthesis is not supported using the 32-bit runtime—see “Property Implementation Directives” (page 47) and “Runtime Differences” (page 60)); ■ gratuitousFloat has a dynamic directive—it is supported using direct method implementations; ■ nameAndAge does not have a dynamic directive, but this is the default value; it is supported using a direct method implementation (since it is read-only, it only requires a getter) with a specified name (nameAndAgeAsString). Listing 4-4 Declaring properties for a class @protocol Link @property id next; @end @interface MyClass : NSObject { NSTimeInterval intervalSinceReferenceDate; CGFloat gratuitousFloat; id nextLink; } @property(readonly) NSTimeInterval creationTimestamp; @property(copy) __strong NSString *name; @property CGFloat gratuitousFloat; @property(readonly, getter=nameAndAgeAsString) NSString *nameAndAge; @end @implementation MyClass @synthesize creationTimestamp = intervalSinceReferenceDate, name; Property Declaration and Implementation 53 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 4 Properties // synthesizing 'name' is an error in legacy runtimes // in modern runtimes, the instance variable is synthesized @synthesize next = nextLink; // uses instance variable "nextLink" for storage @dynamic gratuitousFloat; // will warn unless -gratuitousFloat and -setGratuitousFloat: occur in @implementation - (CGFloat)gratuitousFloat { return gratuitousFloat; } - (void)setGratuitousFloat:(CGFloat)aValue { gratuitousFloat = aValue; } - (NSString *)nameAndAgeAsString { return [NSString stringWithFormat:@"%@ (%fs)", [self name], [NSDate timeIntervalSinceReferenceDate] - intervalSinceReferenceDate]; } - init { if (self = [super init]) { intervalSinceReferenceDate = [NSDate timeIntervalSinceReferenceDate]; } return self; } @end The Dot Syntax Objective-C provides a dot (.) operator that offers a compact and convenient syntax for invoking accessor methods. It is particularly useful when you want to access or modify a property that is a property of another object (that is a property of another object (...)). Overview You can read and write properties using the dot (.) operator, as illustrated in the following example. 54 The Dot Syntax 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 4 Properties Listing 4-5 Accessing properties using the dot syntax Graphic *graphic = [[Graphic alloc] init]; NSColor *color = graphic.color; CGFloat xLoc = graphic.xLoc; BOOL hidden = graphic.hidden; int textCharacterLength = graphic.text.length; if (graphic.textHidden != YES) { graphic.text = @"Hello"; } graphic.bounds = NSMakeRect(10.0, 10.0, 20.0, 120.0); Accessing a property property calls the get method associated with the property (by default, property) and setting it calls the set method associated with the property (by default, setProperty:). In fact, as long as there is an appropriate getter and setter available, you can use the dot-syntax invoke those methods even if you haven’t declared a property (if you haven’t declared a property, the getter and setter must be named property and setProperty: respectively). Despite appearances to the contrary, the dot syntax therefore preserves encapsulation—you are not accessing an instance variable directly. The following statements are equivalent to the code shown in Listing 4-5 (page 55), but use custom accessor methods (that you either implemented yourself or that were synthesized): Listing 4-6 Accessing properties using accessor methods Graphic *graphic = [[Graphic alloc] init]; NSColor *color = [graphic color]; CGFloat xLoc = [graphic xLoc]; BOOL hidden = [graphic hidden]; int textCharacterLength = [[graphic text] length]; if ([graphic isTextHidden] != YES) { [graphic setText:@"Hello"]; } [graphic setBounds:NSMakeRect(10.0, 10.0, 20.0, 120.0)]; Although property expressions are functionally equivalent to message sends, the compiler can signal an error when it detects a write to a read-only property, whereas at best it can only generate an undeclared method warning you invoke a non-existent setProperty: method, which will fail at runtime. For properties of the appropriate C language type, the meaning of compound assignments is well-defined. For example, you could update the length property of an instance of NSMutableData using compound assignments: NSMutableData *data = [NSMutableData dataWithLength:1024]; data.length += 1024; data.length *= 2; data.length /= 4; which is equivalent to: [data setLength:[data length] + 1024]; The Dot Syntax 55 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 4 Properties [data setLength:[data length] * 2]; [data setLength:[data length] / 4]; There is one case where properties cannot be used. Consider the following code fragment: id y; x = y.z; // z is an undeclared property Note that y is untyped and the z property is undeclared. There are several ways in which this could be interpreted. Since this is ambiguous, the statement is treated as an undeclared property error. If z is declared, then it is not ambiguous if there's only one declaration of a z property in the current compilation unit. If there are multiple declarations of a z property, as long as they all have the same type (such as BOOL) then it is legal. One source of ambiguity would also arise from one of them being declared readonly. nil Values If a nil value is encountered during property traversal, the result is the same as sending the equivalent message to nil. For example, the following pairs are all equivalent: // each member of the path is an object x =; x = [[[person address] street] name]; // the path contains a C struct // will crash if window is nil or -contentView returns nil y = window.contentView.bounds.origin.y; y = [[window contentView] bounds].origin.y; // an example of using a setter.... = @"Oxford Road"; [[[person address] street] setName: @"Oxford Road"]; self If you want to access a property of self using accessor methods, you must explicitly call out self as illustrated in this example: self.age = 10; If you do not use self., you access the instance variable directly. In the following example, the set accessor method for the age property is not invoked: age = 10; Performance and Threading The dot syntax generates code equivalent to the standard method invocation syntax. As a result, code using the dot syntax performs exactly the same as code written directly using the accessor methods. Since the dot syntax simply invokes methods, no additional thread dependencies are introduced as a result of its use. 56 The Dot Syntax 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 4 Properties Usage Summary aVariable = anObject.aProperty; Invokes the aProperty method and assigns the return value to aVariable. The type of the property aProperty and the type of aVariable must be compatible, otherwise you get a compiler warning. = @"New Name"; Invokes the setName: method on anObject, passing @"New Name" as the argument. You get a compiler warning if setName: does not exist, if the property name does not exist, or if setName: returns anything but void. xOrigin = aView.bounds.origin.x; Invokes the bounds method and assigns xOrigin to be the value of the origin.x structure element of the NSRect returned by bounds. NSInteger i = 10; anObject.intProperty = anotherObject.floatProperty = ++i; Assigns 11 to both anObject.intProperty and anotherObject.floatProperty. That is, the right hand side of the assignment is pre-evaluated and the result is passed to setIntProperty: and setFloatProperty:. The pre-evaluated result is coerced, as required, at each point of assignment. Abusage Summary The following patterns are strongly discouraged. anObject.retain; Generates a compiler warning (warning: value returned from property not used.). /* method declaration */ - (BOOL) setFooIfYouCan: (MyClass *)newFoo; /* code fragment */ anObject.fooIfYouCan = myInstance; Generates a compiler warning that setFooIfYouCan: does not appear to be a setter method because it does not return (void). flag = aView.lockFocusIfCanDraw; Invokes lockFocusIfCanDraw and assigns the return value to flag. This does not generate a compiler warning unless flag’s type mismatches the method’s return type. /* property declaration */ @property(readonly) NSInteger readonlyProperty; /* method declaration */ - (void) setReadonlyProperty: (NSInteger)newValue; /* code fragment */ self.readonlyProperty = 5; The Dot Syntax 57 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 4 Properties Since the property is declared readonly, this code generates a compiler warning (warning: assignment to readonly property 'readonlyProperty'). It will work at runtime, but simply adding a setter for a property does not imply readwrite. Properties and Key-Value Coding In Cocoa, there are two primary ways to access an object's properties: statically using direct invocations of accessor methods and dynamically using key-value coding (KVC). KVC defines generic property accessor methods—valueForKey: and setValue:forKey:—which identify properties with string-based keys. KVC is not meant as a general alternative to using accessor methods—it is for use by code that has no other option, because the code cannot know the names of the relevant properties at compile time. Key-value coding and declared properties are orthogonal technologies. You can use KVC whether or not you use properties, and you can use properties whether or not you use KVC. Both, though, make use of a “dot syntax”. In the case of KVC, the syntax is used to delimit elements in a key path. It is important to remember that when you directly access a property using the dot syntax, you invoke the receiver’s standard accessor methods (as a corollary, to emphasize, the dot syntax does not result in invocation of KVC methods valueForKey: or setValue:forKey:). You can use KVC methods to access a property, for example, given a class defined as follows: @interface MyClass @property NSString *stringProperty; @property NSInteger integerProperty; @property MyClass *linkedInstance; @end you could access the properties in an instance using KVC: MyClass *myInstance = [[MyClass alloc] init]; NSString *string = [myInstance valueForKey:@"stringProperty"]; [myInstance setValue:[NSNumber numberWithInt:2] forKey:@"integerProperty"]; To illustrate the difference between the properties dot syntax and KVC key paths, consider the following. MyClass *anotherInstance = [[MyClass alloc] init]; myInstance.linkedInstance = anotherInstance; myInstance.linkedInstance.integerProperty = 2; This has the same result as: MyClass *anotherInstance = [[MyClass alloc] init]; myInstance.linkedInstance = anotherInstance; [myInstance setValue:[NSNumber numberWithInt:2] forKeyPath:@"linkedInstance.integerProperty"]; 58 Properties and Key-Value Coding 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 4 Properties Subclassing with Properties You can override a readonly property to make it writable. For example, you could define a class MyInteger with a readonly property, value: @interface MyInteger : NSObject { NSInteger value; } @property(readonly) NSInteger value; @end @implementation MyInteger @synthesize value; @end You could then implement a subclass, MyMutableInteger, which redefines the property to make it writable: @interface MyMutableInteger : MyInteger @property(readwrite) NSInteger value; @end @implementation MyMutableInteger @dynamic value; - (void)setValue:(NSInteger)newX { value = newX; } @end Property Introspection When the compiler encounters property declarations, it generates descriptive metadata that is associated with the enclosing class, category or protocol. This metadata is accessible through functions that support looking up a property by name on a class or protocol, obtaining the type of a property as an @encode string, and copying a list of a property's attributes as an array of C strings. A list of declared properties is available for each class and protocol. The Property structure defines an opaque handle to a property descriptor. typedef struct objc_property *Property; The functions class_getProperty and protocol_getProperty look up a named property in a class and protocol respectively. Property class_getProperty(Class cls, const char *name); Property protocol_getProperty(Protocol *proto, const char *name); Subclassing with Properties 59 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 4 Properties The class_copyPropertyList and protocol_copyPropertyList functions return a malloc'd array of pointers to all of the properties declared in a class (including loaded categories) and protocol respectively. Property* class_copyPropertyList(Class cls, uint32_t *count); Property* protocol_copyPropertyList(Protocol *protocol, uint32_t *count); The property_getInfo function returns the name and @encode type string of a property. void property_getInfo(Property *property, const char **name, const char **type); The property_copyAttributeList function returns a malloc'd array of pointers to C strings which represent a property's compile-time attribute list. const char **property_copyAttributeList(Property *property, unint32_t *count); Runtime Differences In general the behavior of properties is identical on all runtimes. There is one key difference, modern (64-bit) runtimes supports non-fragile instance variables whereas the legacy runtime does not. Using the legacy (32-bit) runtime, you must fully declare instance variables in the @interface declaration block of a class since all clients of the class including—of course—subclasses must know the full storage details of the class. Since the storage definition of a class cannot change without incurring binary compatibility issues, synthesis of instance variables in the legacy runtime is not supported. For @synthesize to work in the legacy runtime, you must either provide an instance variable with the same name and compatible type of the property or specify another existing instance variable in the @synthesize statement. For example: @interface My32BitClass : NSObject { CGFloat gratuitousFloat; } @property CGFloat gratuitousFloat; @end @implementation My32BitClass @synthesize gratuitousFloat; // uses the instance variable "gratuitousFloat" for storage @end @interface My64BitClass : NSObject { } @property CGFloat gratuitousFloat; @end @implementation My64BitClass @synthesize gratuitousFloat; // synthesizes the instance variable "gratuitousFloat" for storage @end 60 Runtime Differences 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 4 Properties Protocols declare methods that can be implemented by any class. Protocols are useful in at least three situations: ■ To declare methods that others are expected to implement ■ To declare the interface to an object while concealing its class ■ To capture similarities among classes that are not hierarchically related Declaring Interfaces for Others to Implement Class and category interfaces declare methods that are associated with a particular class—mainly methods that the class implements. Informal and formal protocols, on the other hand, declare methods not associated with a class, but which any class, and perhaps many classes, might implement. A protocol is simply a list of method declarations, unattached to a class definition. For example, these methods that report user actions on the mouse could be gathered into a protocol: - (void)mouseDown:(NSEvent *)theEvent; - (void)mouseDragged:(NSEvent *)theEvent; - (void)mouseUp:(NSEvent *)theEvent; Any class that wanted to respond to mouse events could adopt the protocol and implement its methods. Protocols free method declarations from dependency on the class hierarchy, so they can be used in ways that classes and categories cannot. Protocols list methods that are (or may be) implemented somewhere, but the identity of the class that implements them is not of interest. What is of interest is whether or not a particular class conforms to the protocol—whether it has implementations of the methods the protocol declares. Thus objects can be grouped into types not just on the basis of similarities due to the fact that they inherit from the same class, but also on the basis of their similarity in conforming to the same protocol. Classes in unrelated branches of the inheritance hierarchy might be typed alike because they conform to the same protocol. Protocols can play a significant role in object-oriented design, especially where a project is divided among many implementors or it incorporates objects developed in other projects. Cocoa software uses them heavily to support interprocess communication through Objective-C messages. Declaring Interfaces for Others to Implement 61 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 5 Protocols However, an Objective-C program doesn’t need to use protocols. Unlike class definitions and message expressions, they’re optional. Some Cocoa frameworks use them; some don’t. It all depends on the task at hand. Methods for Others to Implement If you know the class of an object, you can look at its interface declaration (and the interface declarations of the classes it inherits from) to find what messages it responds to. These declarations advertise the messages it can receive. Protocols provide a way for it to also advertise the messages it sends. Communication works both ways; objects send messages as well as receive them. For example, an object might delegate responsibility for a certain operation to another object, or it may on occasion simply need to ask another object for information. In some cases, an object might be willing to notify other objects of its actions so that they can take whatever collateral measures might be required. If you develop the class of the sender and the class of the receiver as part of the same project (or if someone else has supplied you with the receiver and its interface file), this communication is easily coordinated. The sender simply imports the interface file of the receiver. The imported file declares the method selectors the sender uses in the messages it sends. However, if you develop an object that sends messages to objects that aren’t yet defined—objects that you’re leaving for others to implement—you won’t have the receiver’s interface file. You need another way to declare the methods you use in messages but don’t implement. A protocol serves this purpose. It informs the compiler about methods the class uses and also informs other implementors of the methods they need to define to have their objects work with yours. Suppose, for example, that you develop an object that asks for the assistance of another object by sending it helpOut: and other messages. You provide an assistant instance variable to record the outlet for these messages and define a companion method to set the instance variable. This method lets other objects register themselves as potential recipients of your object’s messages: - setAssistant:anObject { assistant = anObject; } Then, whenever a message is to be sent to the assistant, a check is made to be sure that the receiver implements a method that can respond: - (BOOL)doWork { ... if ( [assistant respondsTo:@selector(helpOut:)] ) { [assistant helpOut:self]; return YES; } return NO; } Since, at the time you write this code, you can’t know what kind of object might register itself as the assistant, you can only declare a protocol for the helpOut: method; you can’t import the interface file of the class that implements it. 62 Methods for Others to Implement 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 5 Protocols Declaring Interfaces for Anonymous Objects A protocol can be used to declare the methods of an anonymous object, an object of unknown class. An anonymous object may represent a service or handle a limited set of functions, especially where only one object of its kind is needed. (Objects that play a fundamental role in defining an application’s architecture and objects that you must initialize before using are not good candidates for anonymity.) Objects are not anonymous to their developers, of course, but they are anonymous when the developer supplies them to someone else. For example, consider the following situations: ■ Someone who supplies a framework or a suite of objects for others to use can include objects that are not identified by a class name or an interface file. Lacking the name and class interface, users have no way of creating instances of the class. Instead, the supplier must provide a ready-made instance. Typically, a method in another class returns a usable object: id formatter = [receiver formattingService]; The object returned by the method is an object without a class identity, at least not one the supplier is willing to reveal. For it to be of any use at all, the supplier must be willing to identify at least some of the messages that it can respond to. This is done by associating the object with a list of methods declared in a protocol. ■ You can send Objective-C messages to remote objects—objects in other applications. (“Remote Messaging” (page 117), discusses this possibility in more detail.) Each application has its own structure, classes, and internal logic. But you don’t need to know how another application works or what its components are to communicate with it. As an outsider, all you need to know is what messages you can send (the protocol) and where to send them (the receiver). An application that publishes one of its objects as a potential receiver of remote messages must also publish a protocol declaring the methods the object will use to respond to those messages. It doesn’t have to disclose anything else about the object. The sending application doesn’t need to know the class of the object or use the class in its own design. All it needs is the protocol. Protocols make anonymous objects possible. Without a protocol, there would be no way to declare an interface to an object without identifying its class. Note: Even though the supplier of an anonymous object doesn’t reveal its class, the object itself reveals it at runtime. A class message returns the anonymous object’s class. However, there’s usually little point in discovering this extra information; the information in the protocol is sufficient. Non-Hierarchical Similarities If more than one class implements a set of methods, those classes are often grouped under an abstract class that declares the methods they have in common. Each subclass may re-implement the methods in its own way, but the inheritance hierarchy and the common declaration in the abstract class captures the essential similarity between the subclasses. Declaring Interfaces for Anonymous Objects 63 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 5 Protocols However, sometimes it’s not possible to group common methods in an abstract class. Classes that are unrelated in most respects might nevertheless need to implement some similar methods. This limited similarity may not justify a hierarchical relationship. For example, you might want to add support for creating XML representations of objects in your application and for initializing objects from an XML representation: - (NSXMLElement *)XMLRepresentation; - initFromXMLRepresentation:(NSXMLElement *)xmlString; These methods could be grouped into a protocol and the similarity between implementing classes accounted for by noting that they all conform to the same protocol. Objects can be typed by this similarity (the protocols they conform to), rather than by their class. For example, an NSMatrix instance must communicate with the objects that represent its cells. The matrix could require each of these objects to be a kind of NSCell (a type based on class) and rely on the fact that all objects that inherit from the NSCell class have the methods needed to respond to NSMatrix messages. Alternatively, the NSMatrix object could require objects representing cells to have methods that can respond to a particular set of messages (a type based on protocol). In this case, the NSMatrix object wouldn’t care what class a cell object belonged to, just that it implemented the methods. Formal Protocols The Objective-C language provides a way to formally declare a list of methods as a protocol. Formal protocols are supported by the language and the runtime system. For example, the compiler can check for types based on protocols, and objects can introspect at runtime to report whether or not they conform to a protocol. Declaring a Protocol You declare formal protocols with the @protocol directive: @protocol ProtocolName method declarations @end For example, you could declare the XML representation protocol like this: @protocol MyXMLSupport - (NSXMLElement *)XMLRepresentation; - initFromXMLRepresentation:(NSXMLElement *)XMLElement; @end Unlike class names, protocol names don’t have global visibility. They live in their own namespace. 64 Formal Protocols 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 5 Protocols Optional Protocol Methods Protocol methods can be marked as optional using the @optional keyword. Corresponding to the @optional modal keyword, there is a @required keyword to formally denote the semantics of the default behavior. You can use @optional and @required to partition your protocol into sections as you see fit. If you do not specify any keyword, the default is @required. @protocol MyProtocol - (void)requiredMethod; @optional - (void)anOptionalMethod; - (void)anotherOptionalMethod; @required - (void)anotherRequiredMethod; @end Informal Protocols In addition to formal protocols, you can also define an informal protocol by grouping the methods in a category declaration: @interface NSObject ( MyXMLSupport ) - (NSXMLElement *) XMLRepresentation; - initFromXMLRepresentation:(NSXMLElement *)XMLElement; @end Informal protocols are typically declared as categories of the NSObject class, since that broadly associates the method names with any class that inherits from NSObject. Because all classes inherit from the root class, the methods aren’t restricted to any part of the inheritance hierarchy. (It would also be possible to declare an informal protocol as a category of another class to limit it to a certain branch of the inheritance hierarchy, but there is little reason to do so.) When used to declare a protocol, a category interface doesn’t have a corresponding implementation. Instead, classes that implement the protocol declare the methods again in their own interface files and define them along with other methods in their implementation files. An informal protocol bends the rules of category declarations to list a group of methods but not associate them with any particular class or implementation. Being informal, protocols declared in categories don’t receive much language support. There’s no type checking at compile time nor a check at runtime to see whether an object conforms to the protocol. To get these benefits, you must use a formal protocol. An informal protocol may be useful when implementing all the methods is optional, such as for a delegate, but (on Mac OS X v10.5 and later) it is typically better to use a formal protocol with optional methods. Informal Protocols 65 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 5 Protocols Protocol Objects Just as classes are represented at runtime by class objects and methods by selector codes, formal protocols are represented by a special data type—instances of the Protocol class. Source code that deals with a protocol (other than to use it in a type specification) must refer to the Protocol object. In many ways, protocols are similar to class definitions. They both declare methods, and at runtime they’re both represented by objects—classes by class objects and protocols by Protocol objects. Like class objects, Protocol objects are created automatically from the definitions and declarations found in source code and are used by the runtime system. They’re not allocated and initialized in program source code. Source code can refer to a Protocol object using the @protocol() directive—the same directive that declares a protocol, except that here it has a set of trailing parentheses. The parentheses enclose the protocol name: Protocol *myXMLSupportProtocol = @protocol(MyXMLSupport); This is the only way that source code can conjure up a Protocol object. Unlike a class name, a protocol name doesn’t designate the object—except inside @protocol(). The compiler creates a Protocol object for each protocol declaration it encounters, but only if the protocol is also: ■ Adopted by a class, or ■ Referred to somewhere in source code (using @protocol()) Protocols that are declared but not used (except for type checking as described below) aren’t represented by Protocol objects at runtime. Adopting a Protocol Adopting a protocol is similar in some ways to declaring a superclass. Both assign methods to the class. The superclass declaration assigns it inherited methods; the protocol assigns it methods declared in the protocol list. A class is said to adopt a formal protocol if in its declaration it lists the protocol within angle brackets after the superclass name: @interface ClassName : ItsSuperclass < protocol list > Categories adopt protocols in much the same way: @interface ClassName ( CategoryName ) < protocol list > A class can adopt more than one protocol; names in the protocol list are separated by commas. @interface Formatter : NSObject < Formatting, Prettifying > 66 Protocol Objects 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 5 Protocols A class or category that adopts a protocol must implement all the required methods the protocol declares, otherwise the compiler issues a warning. The Formatter class above would define all the required methods declared in the two protocols it adopts, in addition to any it might have declared itself. A class or category that adopts a protocol must import the header file where the protocol is declared. The methods declared in the adopted protocol are not declared elsewhere in the class or category interface. It’s possible for a class to simply adopt protocols and declare no other methods. For example, the following class declaration adopts the Formatting and Prettifying protocols, but declares no instance variables or methods of its own: @interface Formatter : NSObject < Formatting, Prettifying > @end Conforming to a Protocol A class is said to conform to a formal protocol if it adopts the protocol or inherits from another class that adopts it. An instance of a class is said to conform to the same set of protocols its class conforms to. Since a class must implement all the required methods declared in the protocols it adopts, saying that a class or an instance conforms to a protocol is equivalent to saying that it has in its repertoire all the methods the protocol declares. It’s possible to check whether an object conforms to a protocol by sending it a conformsToProtocol: message. if ( ! [receiver conformsToProtocol:@protocol(MyXMLSupport)] ) { // Object does not conform to MyXMLSupport protocol // If you are expecting receiver to implement methods declared in the // MyXMLSupport protocol, this is probably an error } (Note that there is also a class method with the same name—conformsToProtocol:.) The conformsToProtocol: test is like the respondsToSelector: test for a single method, except that it tests whether a protocol has been adopted (and presumably all the methods it declares implemented) rather than just whether one particular method has been implemented. Because it checks for a all the methods in the protocol, conformsToProtocol: can be more efficient than respondsToSelector:. The conformsToProtocol: test is also like the isKindOfClass: test, except that it tests for a type based on a protocol rather than a type based on the inheritance hierarchy. Conforming to a Protocol 67 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 5 Protocols Type Checking Type declarations for objects can be extended to include formal protocols. Protocols thus offer the possibility of another level of type checking by the compiler, one that’s more abstract since it’s not tied to particular implementations. In a type declaration, protocol names are listed between angle brackets after the type name: - (id )formattingService; id anObject; Just as static typing permits the compiler to test for a type based on the class hierarchy, this syntax permits the compiler to test for a type based on conformance to a protocol. For example, if Formatter is an abstract class, this declaration Formatter *anObject; groups all objects that inherit from Formatter into a type and permits the compiler to check assignments against that type. Similarly, this declaration, id anObject; groups all objects that conform to the Formatting protocol into a type, regardless of their positions in the class hierarchy. The compiler can make sure only objects that conform to the protocol are assigned to the type. In each case, the type groups similar objects—either because they share a common inheritance, or because they converge on a common set of methods. The two types can be combined in a single declaration: Formatter *anObject; Protocols can’t be used to type class objects. Only instances can be statically typed to a protocol, just as only instances can be statically typed to a class. (However, at runtime, both classes and instances will respond to a conformsToProtocol: message.) Protocols Within Protocols One protocol can incorporate other protocols using the same syntax that classes use to adopt a protocol: @protocol ProtocolName < protocol list > All the protocols listed between angle brackets are considered part of the ProtocolName protocol. For example, if the Paging protocol incorporates the Formatting protocol, @protocol Paging < Formatting > any object that conforms to the Paging protocol also conforms to Formatting. Type declarations 68 Type Checking 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 5 Protocols id someObject; and conformsToProtocol: messages if ( [anotherObject conformsToProtocol:@protocol(Paging)] ) ... need to mention only the Paging protocol to test for conformance to Formatting as well. When a class adopts a protocol, it must implement the required methods the protocol declares, as mentioned earlier. In addition, it must conform to any protocols the adopted protocol incorporates. If an incorporated protocol incorporates still other protocols, the class must also conform to them. A class can conform to an incorporated protocol by either: ■ Implementing the methods the protocol declares, or ■ Inheriting from a class that adopts the protocol and implements the methods. Suppose, for example, that the Pager class adopts the Paging protocol. If Pager is a subclass of NSObject, @interface Pager : NSObject < Paging > it must implement all the Paging methods, including those declared in the incorporated Formatting protocol. It adopts the Formatting protocol along with Paging. On the other hand, if Pager is a subclass of Formatter (a class that independently adopts the Formatting protocol), @interface Pager : Formatter < Paging > it must implement all the methods declared in the Paging protocol proper, but not those declared in Formatting. Pager inherits conformance to the Formatting protocol from Formatter. Note that a class can conform to a protocol without formally adopting it simply by implementing the methods declared in the protocol. Referring to Other Protocols When working on complex applications, you occasionally find yourself writing code that looks like this: #import "B.h" @protocol A - foo:(id )anObject; @end where protocol B is declared like this: #import "A.h" @protocol B - bar:(id )anObject; Referring to Other Protocols 69 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 5 Protocols @end In such a situation, circularity results and neither file will compile correctly. To break this recursive cycle, you must use the @protocol directive to make a forward reference to the needed protocol instead of importing the interface file where the protocol is defined. The following code excerpt illustrates how you would do this: @protocol B; @protocol A - foo:(id )anObject; @end Note that using the @protocol directive in this manner simply informs the compiler that “B” is a protocol to be defined later. It doesn’t import the interface file where protocol B is defined. 70 Referring to Other Protocols 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 5 Protocols Objective-C 2.0 provides a language feature that allows you to efficiently and safely enumerate over the contents of a collection using a concise syntax. The for…in Feature Objective-C 2.0 provides a language feature that allows you to enumerate over the contents of a collection. The syntax is defined as follows: for ( Type newVariable in expression ) { stmts } or Type existingItem; for ( existingItem in expression ) { stmts } In both cases, expression yields an object that conforms to the NSFastEnumeration protocol, typically an array or enumerator (the Cocoa collection classes—NSArray, NSDictionary, and NSSet—adopt this protocol, as does NSEnumerator). Any class whose instances provide access to a collection of other objects can adopt the NSFastEnumeration protocol (see “Implementing the NSFastEnumeration Protocol” (page 73)). It should be obvious that in the cases of NSArray and NSSet the enumeration is over their contents. Other classes should make clear what property is iterated over—for example, NSDictionary and the Core Data class NSManagedObjectModel provide support for fast enumeration; NSDictionary enumerates its keys, and NSManagedObjectModel enumerates its entities. There are several advantages to using fast enumeration: ■ The enumeration is considerably more efficient than, for example, using NSEnumerator directly. ■ The syntax is concise. ■ Enumeration is “safe”—the enumerator has a mutation guard so that if you attempt to modify the collection during enumeration, an exception is raised. Since mutation of the object during iteration is forbidden, you can perform multiple enumerations concurrently. The for…in Feature 71 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 6 Fast Enumeration Using Fast Enumeration The following code example illustrates using fast enumeration with NSArray and NSDictionary objects. NSArray *array = [NSArray arrayWithObjects: @"One", @"Two", @"Three", @"Four", nil]; for (NSString *element in array) { NSLog(@"element: %@", element); } NSDictionary *dictionary = [NSDictionary dictionaryWithObjectsAndKeys: @"quattuor", @"four", @"quinque", @"five", @"sex", @"six", nil]; NSString *key; for (key in dictionary) { NSLog(@"English: %@, Latin: %@", key, [dictionary valueForKey:key]); } You can also use NSEnumerator objects with fast enumeration, as illustrated in the following example: NSArray *array = [NSArray arrayWithObjects: @"One", @"Two", @"Three", @"Four", nil]; NSEnumerator *enumerator = [array reverseObjectEnumerator]; for (NSString *element in enumerator) { if ([element isEqualToString:@"Three"]) { break; } } NSString *next = [enumerator nextObject]; // next = "Four" For collections or enumerators that have a well-defined order—such as NSArray or NSEnumerator instance derived from an array—the enumeration proceeds in that order, so simply counting iterations will give you the proper index into the collection if you need it. NSArray *array = /* assume this exists */; NSInteger index = 0; BOOL ok = NO; for (id element in array) { if (/* some test for element */) { ok = YES; break; } index++; } if (ok) { NSLog(@"Test passed by element at index %d", index"); } 72 The for…in Feature 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 6 Fast Enumeration Implementing the NSFastEnumeration Protocol To implement a custom class that supports fast enumeration, you must implement the NSFastEnumeration protocol. The protocol comprises a single method—countByEnumeratingWithState:objects:count:. @protocol NSFastEnumeration - (NSUInteger)countByEnumeratingWithState:(NSFastEnumerationState *)state objects:(id *)stackbuf count:(NSUInteger)len; @end The first parameter is an enumeration structure (NSFastEnumerationState): typedef struct { unsigned long state; id *itemsPtr; unsigned long *mutationsPtr; unsigned long extra[5]; } NSFastEnumerationState; The following rules must be obeyed by any implementation: 1. On first entry, the "state" field will be 0, otherwise the client is neither allowed or required to alter state. State must be set to a non-zero before returning a non-zero count (otherwise the client will go into an infinite loop expecting a zero return, and the object will always be starting a new enumeration). For complicated enumerations "extra" is supplied to hold extra state. 2. On exit, a count of 0 means that there are no objects available for enumeration. This may reflect a bad or inconsistent internal state, or more normally it may simply reflect that no more objects are available. But if the value is zero, the client can make no assumptions about the content of state. 3. A non-zero count return implies that itemsPtr has been set to point to the first of the non-zero count objects. This is encouraged to be a pointer to internal object state where feasible. If not feasible, the client has supplied a stack buffer at "objects" and the length in the "count" parameter, and the object pointers to be iterated should be copied there (no GC primitives required). In addition, mutationsPtr is set to a valid address that should track some form of change counter for this object. If the object is immutable, this pointer could point to the object itself. 4. The enumeration must traverse in order collections or enumerators that have a well-defined order, such as NSArray. 5. The protocol is designed to work when sent to nil objects. When countByEnumeratingWithState:objects:count: is invoked, the receiver is passed an enumeration structure with the first field state set to zero. When enumeration is ended, this method returns 0. Items are returned indirectly by setting the itemsPtr field. The mutationsPtr field must be set to point to a location that changes if your object is mutated. If mutations are not possible, this pointer can be set to the object itself, whose first field will never change. If the stackbuf argument is supplied, you can use it as a buffer to hold several copied items, or the “extra” state fields may be used to hold the returned values. You can use the state fields in any way necessary to perform the iteration. The state structure is assumed to be of stack local memory and—from a garbage collection perspective—does not require write-barriers on stores, so you can recast the passed-in state structure to one more suitable for your iteration. The for…in Feature 73 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 6 Fast Enumeration The compiler transforms the following code: for ( id elem in collection ) { stmts } into: { id elem; NSEnumerationState __enumState = { 0 }; id __items[16]; unsigned long __limit = [collection countByEnumeratingWithState:&__enumState objects:__items count:16]; unsigned __counter = 0; unsigned long __startMutations = * __enumState.mutationsPtr; if (__limit) do { __counter = 0 do { if (__startMutations != * __enumState.mutationsPtr) objc_enumerationMutation(); elem = __enumState.itemsPtr[__counter++]; stmts; } while (__counter0 < __limit); } while (__limit = [collection countByEnumeratingWithState:&__enumState objects:__items count:16]); }; First, the state field is set to 0 to indicate the first time entry. Next, countByEnumeratingWithState:objects:count: is invoked which returns the count of the first batch of items to iterate. If this is zero, the enumeration is finished, otherwise the program enters an inner loop to iterate through the items ready at the end of which it exits to the outer loop to get another batch. For each item, the compiler first checks to see if the collection has been mutated before attempting to dereference the indirect pointer. If the collection has been mutated, an exception is raised. If it has not, the item is extracted and control passes to the stmts provided. 74 The for…in Feature 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 6 Fast Enumeration This chapter describes how the message expressions are converted into objc_msgSend function calls, and how you can refer to methods by name. It then explains how you can take advantage of objc_msgSend, and the roles of self and super. The objc_msgSend Function In Objective-C, messages aren’t bound to method implementations until runtime. The compiler converts a message expression, [receiver message] into a call on a messaging function, objc_msgSend. This function takes the receiver and the name of the method mentioned in the message—that is, the method selector—as its two principal parameters: objc_msgSend(receiver, selector) Any arguments passed in the message are also handed to objc_msgSend: objc_msgSend(receiver, selector, arg1, arg2, ...) The messaging function does everything necessary for dynamic binding: ■ It first finds the procedure (method implementation) that the selector refers to. Since the same method can be implemented differently by separate classes, the precise procedure that it finds depends on the class of the receiver. ■ It then calls the procedure, passing it the receiving object (a pointer to its data), along with any arguments that were specified for the method. ■ Finally, it passes on the return value of the procedure as its own return value. Note: The compiler generates calls to the messaging function. You should never call it directly in the code you write. The key to messaging lies in the structures that the compiler builds for each class and object. Every class structure includes these two essential elements: ■ A pointer to the superclass. The objc_msgSend Function 75 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 7 How Messaging Works ■ A class dispatch table. This table has entries that associate method selectors with the class-specific addresses of the methods they identify. The selector for the setOrigin:: method is associated with the address of (the procedure that implements) setOrigin::, the selector for the display method is associated with display’s address, and so on. When a new object is created, memory for it is allocated, and its instance variables are initialized. First among the object’s variables is a pointer to its class structure. This pointer, called isa, gives the object access to its class and, through the class, to all the classes it inherits from. Note: While not strictly a part of the language, the isa pointer is required for an object to work with the Objective-C runtime system. An object needs to be “equivalent” to a struct objc_object (defined in objc/objc.h) in whatever fields the structure defines. However, you rarely, if ever, need to create your own root object, and objects that inherit from NSObject or NSProxy automatically have the isa variable. These elements of class and object structure are illustrated in Figure 7-1. 76 The objc_msgSend Function 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 7 How Messaging Works Figure 7-1 Messaging Framework . . . superclass selector...address selector...address selector...address . . . superclass selector...address selector...address selector...address . . . superclass selector...address selector...address selector...address isa instance variable instance variable . . . The object’s superclass The root class (NSObject) The object’s class When a message is sent to an object, the messaging function follows the object’s isa pointer to the class structure where it looks up the method selector in the dispatch table. If it can’t find the selector there, objc_msgSend follows the pointer to the superclass and tries to find the selector in its dispatch table. Successive failures cause objc_msgSend to climb the class hierarchy until it reaches the NSObject class. Once it locates the selector, the function calls the method entered in the table and passes it the receiving object’s data structure. This is the way that method implementations are chosen at runtime—or, in the jargon of object-oriented programming, that methods are dynamically bound to messages. To speed the messaging process, the runtime system caches the selectors and addresses of methods as they are used. There’s a separate cache for each class, and it can contain selectors for inherited methods as well as for methods defined in the class. Before searching the dispatch tables, the messaging routine first checks the cache of the receiving object’s class (on the theory that a method that was used The objc_msgSend Function 77 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 7 How Messaging Works once may likely be used again). If the method selector is in the cache, messaging is only slightly slower than a function call. Once a program has been running long enough to “warm up” its caches, almost all the messages it sends find a cached method. Caches grow dynamically to accommodate new messages as the program runs. Selectors For efficiency, full ASCII names are not used as method selectors in compiled code. Instead, the compiler writes each method name into a table, then pairs the name with a unique identifier that represents the method at runtime. The runtime system makes sure each identifier is unique: No two selectors are the same, and all methods with the same name have the same selector. Compiled selectors are assigned to a special type, SEL, to distinguish them from other data. Valid selectors are never 0. You must let the system assign SEL identifiers to methods; it’s futile to assign them arbitrarily. The @selector() directive lets Objective-C source code refer to the compiled selector, rather than to the full method name. Here, the selector for setWidth:height: is assigned to the setWidthHeight variable: SEL setWidthHeight; setWidthHeight = @selector(setWidth:height:); It’s most efficient to assign values to SEL variables at compile time with the @selector() directive. However, in some cases, a program may need to convert a character string to a selector at runtime. This can be done with the NSSelectorFromString function: setWidthHeight = NSSelectorFromString(aBuffer); Conversion in the opposite direction is also possible. The NSStringFromSelector function returns a method name for a selector: NSString *method; method = NSStringFromSelector(setWidthHeight); These and other runtime functions are described in the Cocoa framework reference documentation. Methods and Selectors Compiled selectors identify method names, not method implementations. Rectangle’s display method, for example, has the same selector as display methods defined in other classes. This is essential for polymorphism and dynamic binding; it lets you send the same message to receivers belonging to different classes. If there were one selector per method implementation, a message would be no different than a function call. A class method and an instance method with the same name are assigned the same selector. However, because of their separate domains, there’s no confusion between the two. A class could define a display class method in addition to a display instance method. 78 Selectors 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 7 How Messaging Works Method Return and Argument Types The messaging routine has access to method implementations only through selectors, so it treats all methods with the same selector alike. It discovers the return type of a method, and the data types of its arguments, from the selector. Therefore, except for messages sent to statically typed receivers, dynamic binding requires all implementations of identically named methods to have the same return type and the same argument types. (Statically typed receivers are an exception to this rule, since the compiler can learn about the method implementation from the class type.) Although identically named class methods and instance methods are represented by the same selector, they can have different argument and return types. Varying the Message at Runtime The performSelector:, performSelector:withObject:, and performSelector:withObject:withObject: methods, defined in the NSObject protocol, take SEL identifiers as their initial arguments. All three methods map directly into the messaging function. For example, [friend performSelector:@selector(gossipAbout:) withObject:aNeighbor]; is equivalent to: [friend gossipAbout:aNeighbor]; These methods make it possible to vary a message at runtime, just as it’s possible to vary the object that receives the message. Variable names can be used in both halves of a message expression: id helper = getTheReceiver(); SEL request = getTheSelector(); [helper performSelector:request]; In this example, the receiver (helper) is chosen at runtime (by the fictitious getTheReceiver function), and the method the receiver is asked to perform (request) is also determined at runtime (by the equally fictitious getTheSelector function). Note: performSelector: and its companion methods return an id. If the method that’s performed returns a different type, it should be cast to the proper type. (However, casting doesn’t work for all types; the method should return a pointer or a type compatible with a pointer.) The Target-Action Design Pattern In its treatment of user-interface controls, the Application Kit makes good use of the ability to vary both the receiver and the message. NSControl objects are graphical devices that can be used to give instructions to an application. Most resemble real-world control devices such as buttons, switches, knobs, text fields, dials, menu items, and the like. In software, these devices stand between the application and the user. They interpret Selectors 79 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 7 How Messaging Works events coming from hardware devices like the keyboard and mouse and translate them into application-specific instructions. For example, a button labeled “Find” would translate a mouse click into an instruction for the application to start searching for something. The Application Kit defines a template for creating control devices and defines a few “off-the-shelf” devices of its own. For example, the NSButtonCell class defines an object that you can assign to an NSMatrix instance and initialize with a size, a label, a picture, a font, and a keyboard alternative. When the user clicks the button (or uses the keyboard alternative), the NSButtonCell object sends a message instructing the application to do something. To do this, an NSButtonCell object must be initialized not just with an image, a size, and a label, but with directions on what message to send and who to send it to. Accordingly, an NSButtonCell instance can be initialized for an action message, the method selector it should use in the message it sends, and a target, the object that should receive the message. [myButtonCell setAction:@selector(reapTheWind:)]; [myButtonCell setTarget:anObject]; The button cell sends the message using NSObject’s performSelector:withObject: method. All action messages take a single argument, the id of the control device sending the message. If Objective-C didn’t allow the message to be varied, all NSButtonCell objects would have to send the same message; the name of the method would be frozen in the NSButtonCell source code. Instead of simply implementing a mechanism for translating user actions into action messages, button cells and other controls would have to constrain the content of the message. This would make it difficult for any object to respond to more than one button cell. There would either have to be one target for each button, or the target object would have to discover which button the message came from and act accordingly. Each time you rearranged the user interface, you would also have to re-implement the method that responds to the action message. This would be an unnecessary complication that Objective-C happily avoids. Avoiding Messaging Errors If an object receives a message to perform a method that isn’t in its repertoire, an error results. It’s the same sort of error as calling a nonexistent function. But because messaging occurs at runtime, the error often isn’t evident until the program executes. It’s relatively easy to avoid this error when the message selector is constant and the class of the receiving object is known. As you write your programs, you can make sure that the receiver is able to respond. If the receiver is statically typed, the compiler performs this test for you. However, if the message selector or the class of the receiver varies, it may be necessary to postpone this test until runtime. The respondsToSelector: method, defined in the NSObject class, determines whether a receiver can respond to a message. It takes the method selector as an argument and returns whether the receiver has access to a method matching the selector: if ( [anObject respondsToSelector:@selector(setOrigin::)] ) [anObject setOrigin:0.0 :0.0]; else fprintf(stderr, "%s can’t be placed\n", [NSStringFromClass([anObject class]) UTF8String]); 80 Selectors 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 7 How Messaging Works The respondsToSelector: test is especially important when sending messages to objects that you don’t have control over at compile time. For example, if you write code that sends a message to an object represented by a variable that others can set, you should make sure the receiver implements a method that can respond to the message. Note: An object can also arrange to have the messages it receives forwarded to other objects if it can’t respond to them directly itself. In that case, it appears that the object can handle the message, even though the object responds to the message indirectly by assigning it to another object. See “Forwarding” (page 112) for more information. Dynamic Method Resolution You can provide implementations of class and instance methods at runtime using dynamic method resolution. See “Dynamic Method Resolution” (page 116) in “The Runtime System” (page 103) for more details. Using Hidden Arguments When the objc_msgSend finds the procedure that implements a method, it calls the procedure and passes it all the arguments in the message. It also passes the procedure two hidden arguments: ■ The receiving object ■ The selector for the method These arguments give every method implementation explicit information about the two halves of the message expression that invoked it. They’re said to be “hidden” because they aren’t declared in the source code that defines the method. They’re inserted into the implementation when the code is compiled. Although these arguments aren’t explicitly declared, source code can still refer to them (just as it can refer to the receiving object’s instance variables). A method refers to the receiving object as self, and to its own selector as _cmd. In the example below, _cmd refers to the selector for the strange method and self to the object that receives a strange message. - strange { id target = getTheReceiver(); SEL method = getTheMethod(); if ( target == self || method == _cmd ) return nil; return [target performSelector:method]; } self is the more useful of the two arguments. It is, in fact, the way the receiving object’s instance variables are made available to the method definition. Using Hidden Arguments 81 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 7 How Messaging Works Messages to self and super Objective-C provides two terms that can be used within a method definition to refer to the object that performs the method—self and super. Suppose, for example, that you define a reposition method that needs to change the coordinates of whatever object it acts on. It can invoke the setOrigin:: method to make the change. All it needs to do is send a setOrigin:: message to the same object that the reposition message itself was sent to. When you’re writing the reposition code, you can refer to that object as either self or super. The reposition method could read either: - reposition { ... [self setOrigin:someX :someY]; ... } or: - reposition { ... [super setOrigin:someX :someY]; ... } Here, self and super both refer to the object receiving a reposition message, whatever object that may happen to be. The two terms are quite different, however. self is one of the hidden arguments that the messaging routine passes to every method; it’s a local variable that can be used freely within a method implementation, just as the names of instance variables can be. super is a term that substitutes for self only as the receiver in a message expression. As receivers, the two terms differ principally in how they affect the messaging process: ■ self searches for the method implementation in the usual manner, starting in the dispatch table of the receiving object’s class. In the example above, it would begin with the class of the object receiving the reposition message. ■ super starts the search for the method implementation in a very different place. It begins in the superclass of the class that defines the method where super appears. In the example above, it would begin with the superclass of the class where reposition is defined. Wherever super receives a message, the compiler substitutes another messaging routine for the objc_msgSend function. The substitute routine looks directly to the superclass of the defining class—that is, to the superclass of the class sending the message to super—rather than to the class of the object receiving the message. 82 Messages to self and super 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 7 How Messaging Works An Example The difference between self and super becomes clear in a hierarchy of three classes. Suppose, for example, that we create an object belonging to a class called Low. Low’s superclass is Mid; Mid’s superclass is High. All three classes define a method called negotiate, which they use for a variety of purposes. In addition, Mid defines an ambitious method called makeLastingPeace, which also has need of the negotiate method. This is illustrated in Figure 7-2: Figure 7-2 High, Mid, Low Mid High Low superclass – negotiate superclass – negotiate superclass – negotiate – makeLastingPeace We now send a message to our Low object to perform the makeLastingPeace method, and makeLastingPeace, in turn, sends a negotiate message to the same Low object. If source code calls this object self, - makeLastingPeace { [self negotiate]; ... } the messaging routine finds the version of negotiate defined in Low, self’s class. However, if Mid’s source code calls this object super, Messages to self and super 83 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 7 How Messaging Works - makeLastingPeace { [super negotiate]; ... } the messaging routine will find the version of negotiate defined in High. It ignores the receiving object’s class (Low) and skips to the superclass of Mid, since Mid is where makeLastingPeace is defined. Neither message finds Mid’s version of negotiate. As this example illustrates, super provides a way to bypass a method that overrides another method. Here it enabled makeLastingPeace to avoid the Mid version of negotiate that redefined the original High version. Not being able to reach Mid’s version of negotiate may seem like a flaw, but, under the circumstances, it’s right to avoid it: ■ The author of the Low class intentionally overrode Mid’s version of negotiate so that instances of the Low class (and its subclasses) would invoke the redefined version of the method instead. The designer of Low didn’t want Low objects to perform the inherited method. ■ In sending the message to super, the author of Mid’s makeLastingPeace method intentionally skipped over Mid’s version of negotiate (and over any versions that might be defined in classes like Low that inherit from Mid) to perform the version defined in the High class. Mid’s designer wanted to use the High version of negotiate and no other. Mid’s version of negotiate could still be used, but it would take a direct message to a Mid instance to do it. Using super Messages to super allow method implementations to be distributed over more than one class. You can override an existing method to modify or add to it, and still incorporate the original method in the modification: - negotiate { ... return [super negotiate]; } For some tasks, each class in the inheritance hierarchy can implement a method that does part of the job and passes the message on to super for the rest. The init method, which initializes a newly allocated instance, is designed to work like this. Each init method has responsibility for initializing the instance variables defined in its class. But before doing so, it sends an init message to super to have the classes it inherits from initialize their instance variables. Each version of init follows this procedure, so classes initialize their instance variables in the order of inheritance: - (id)init { [super init]; ... } 84 Messages to self and super 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 7 How Messaging Works It’s also possible to concentrate core functionality in one method defined in a superclass, and have subclasses incorporate the method through messages to super. For example, every class method that creates an instance must allocate storage for the new object and initialize its isa pointer to the class structure. This is typically left to the alloc and allocWithZone: methods defined in the NSObject class. If another class overrides these methods (a rare case), it can still get the basic functionality by sending a message to super. Redefining self super is simply a flag to the compiler telling it where to begin searching for the method to perform; it’s used only as the receiver of a message. But self is a variable name that can be used in any number of ways, even assigned a new value. There’s a tendency to do just that in definitions of class methods. Class methods are often concerned not with the class object, but with instances of the class. For example, many class methods combine allocation and initialization of an instance, often setting up instance variable values at the same time. In such a method, it might be tempting to send messages to the newly allocated instance and to call the instance self, just as in an instance method. But that would be an error. self and super both refer to the receiving object—the object that gets a message telling it to perform the method. Inside an instance method, self refers to the instance; but inside a class method, self refers to the class object. This is an example of what not to do: + (Rectangle *)rectangleOfColor:(NSColor *) color { self = [[Rectangle alloc] init]; // BAD [self setColor:color]; return [self autorelease]; } To avoid confusion, it’s usually better to use a variable other than self to refer to an instance inside a class method: + (id)rectangleOfColor:(NSColor *)color { id newInstance = [[Rectangle alloc] init]; // GOOD [newInstance setColor:color]; return [newInstance autorelease]; } In fact, rather than sending the alloc message to the class in a class method, it’s often better to send alloc to self. This way, if the class is subclassed, and the rectangleOfColor: message is received by a subclass, the instance returned will be the same type as the subclass (for example, the array method of NSArray is inherited by NSMutableArray). + (id)rectangleOfColor:(NSColor *)color { id newInstance = [[self alloc] init]; // EXCELLENT [newInstance setColor:color]; return [newInstance autorelease]; } See “Allocating and Initializing Objects” (page 104) for more information about object allocation. Messages to self and super 85 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 7 How Messaging Works 86 Messages to self and super 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 7 How Messaging Works This chapter explains how static typing works and discusses some other features of Objective-C, including ways to temporarily overcome its inherent dynamism. Objective-C objects are dynamic entities. As many decisions about them as possible are pushed from compile time to runtime: ■ The memory for objects is dynamically allocated at runtime by class methods that create new instances. ■ Objects are dynamically typed. In source code (at compile time), any object pointer can be of type id no matter what the object’s class is. The exact class of an id variable (and therefore its particular methods and data structure) isn’t determined until the program runs. ■ Messages and methods are dynamically bound, as described under “How Messaging Works” (page 75). A runtime procedure matches the method selector in the message to a method implementation that “belongs to” the receiver. These features give object-oriented programs a great deal of flexibility and power, but there’s a price to pay. Messages are somewhat slower than function calls, for example, (though not much slower due to the efficiency of the runtime system) and the compiler can’t check the exact types (classes) of id variables. To permit better compile-time type checking, and to make code more self-documenting, Objective-C allows objects to be statically typed with a class name rather than generically typed as id. It also lets you turn some of its object-oriented features off in order to shift operations from runtime back to compile time. Static Typing If a pointer to a class name is used in place of id in an object declaration, Rectangle *thisObject; the compiler restricts the value of the declared variable to be either an instance of the class named in the declaration or an instance of a class that inherits from the named class. In the example above, thisObject can only be a Rectangle of some kind. Static Typing 87 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 8 Enabling Static Behavior Statically typed objects have the same internal data structures as objects declared to be ids. The type doesn’t affect the object; it affects only the amount of information given to the compiler about the object and the amount of information available to those reading the source code. Static typing also doesn’t affect how the object is treated at runtime. Statically typed objects are dynamically allocated by the same class methods that create instances of type id. If Square is a subclass of Rectangle, the following code would still produce an object with all the instance variables of a Square, not just those of a Rectangle: Rectangle *thisObject = [[Square alloc] init]; Messages sent to statically typed objects are dynamically bound, just as objects typed id are. The exact type of a statically typed receiver is still determined at runtime as part of the messaging process. A display message sent to thisObject [thisObject display]; performs the version of the method defined in the Square class, not the one in its Rectangle superclass. By giving the compiler more information about an object, static typing opens up possibilities that are absent for objects typed id: ■ In certain situations, it allows for compile-time type checking. ■ It can free objects from the restriction that identically named methods must have identical return and argument types. ■ It permits you to use the structure pointer operator to directly access an object’s instance variables. The first two topics are discussed in the sections that follow. The third is covered in “Defining a Class” (page 31). Type Checking With the additional information provided by static typing, the compiler can deliver better type-checking services in two situations: ■ When a message is sent to a statically typed receiver, the compiler can make sure the receiver can respond. A warning is issued if the receiver doesn’t have access to the method named in the message. ■ When a statically typed object is assigned to a statically typed variable, the compiler makes sure the types are compatible. A warning is issued if they’re not. An assignment can be made without warning, provided the class of the object being assigned is identical to, or inherits from, the class of the variable receiving the assignment. The following example illustrates this: Shape *aShape; Rectangle *aRect; aRect = [[Rectangle alloc] init]; aShape = aRect; 88 Static Typing 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 8 Enabling Static Behavior Here aRect can be assigned to aShape because a Rectangle is a kind of Shape—the Rectangle class inherits from Shape. However, if the roles of the two variables are reversed and aShape is assigned to aRect, the compiler generates a warning; not every Shape is a Rectangle. (For reference, see Figure 1-2 (page 22), which shows the class hierarchy including Shape and Rectangle.) There’s no check when the expression on either side of the assignment operator is an id. A statically typed object can be freely assigned to an id, or an id to a statically typed object. Because methods like alloc and init return ids, the compiler doesn’t ensure that a compatible object is returned to a statically typed variable. The following code is error-prone, but is allowed nonetheless: Rectangle *aRect; aRect = [[Shape alloc] init]; Return and Argument Types In general, methods in different classes that have the same selector (the same name) must also share the same return and argument types. This constraint is imposed by the compiler to allow dynamic binding. Because the class of a message receiver (and therefore class-specific details about the method it’s asked to perform), can’t be known at compile time, the compiler must treat all methods with the same name alike. When it prepares information on method return and argument types for the runtime system, it creates just one method description for each method selector. However, when a message is sent to a statically typed object, the class of the receiver is known by the compiler. The compiler has access to class-specific information about the methods. Therefore, the message is freed from the restrictions on its return and argument types. Static Typing to an Inherited Class An instance can be statically typed to its own class or to any class that it inherits from. All instances, for example, can be statically typed as NSObject. However, the compiler understands the class of a statically typed object only from the class name in the type designation, and it does its type checking accordingly. Typing an instance to an inherited class can therefore result in discrepancies between what the compiler thinks would happen at runtime and what actually happens. For example, if you statically type a Rectangle instance as a Shape, Shape *myRect = [[Rectangle alloc] init]; the compiler will treat it as a Shape. If you send the object a message to perform a Rectangle method, BOOL solid = [myRect isFilled]; the compiler will complain. The isFilled method is defined in the Rectangle class, not in Shape. However, if you send it a message to perform a method that the Shape class knows about, [myRect display]; the compiler won’t complain, even though Rectangle overrides the method. At runtime, Rectangle’s version of the method is performed. Static Typing 89 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 8 Enabling Static Behavior Similarly, suppose that the Upper class declares a worry method that returns a double, - (double)worry; and the Middle subclass of Upper overrides the method and declares a new return type: - (int)worry; If an instance is statically typed to the Upper class, the compiler will think that its worry method returns a double, and if an instance is typed to the Middle class, it will think that worry returns an int. Errors will obviously result if a Middle instance is typed to the Upper class. The compiler will inform the runtime system that a worry message sent to the object returns a double, but at runtime it actually returns an int and generates an error. Static typing can free identically named methods from the restriction that they must have identical return and argument types, but it can do so reliably only if the methods are declared in different branches of the class hierarchy. Getting a Method Address The only way to circumvent dynamic binding is to get the address of a method and call it directly as if it were a function. This might be appropriate on the rare occasions when a particular method will be performed many times in succession and you want to avoid the overhead of messaging each time the method is performed. With a method defined in the NSObject class, methodForSelector:, you can ask for a pointer to the procedure that implements a method, then use the pointer to call the procedure. The pointer that methodForSelector: returns must be carefully cast to the proper function type. Both return and argument types should be included in the cast. The example below shows how the procedure that implements the setFilled: method might be called: void (*setter)(id, SEL, BOOL); int i; setter = (void (*)(id, SEL, BOOL))[target methodForSelector:@selector(setFilled:)]; for ( i = 0; i < 1000, i++ ) setter(targetList[i], @selector(setFilled:), YES); The first two arguments passed to the procedure are the receiving object (self) and the method selector (_cmd). These arguments are hidden in method syntax but must be made explicit when the method is called as a function. Using methodForSelector: to circumvent dynamic binding saves most of the time required by messaging. However, the savings will be significant only where a particular message is repeated many times, as in the for loop shown above. Note that methodForSelector: is provided by the Cocoa runtime system; it’s not a feature of the Objective-C language itself. 90 Getting a Method Address 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 8 Enabling Static Behavior Objective-C provides support for exception handling and thread synchronization, which are explained in this article and “Threading” (page 95). To turn on support for these features, use the -fobjc-exceptions switch of the GNU Compiler Collection (GCC) version 3.3 and later. Note: Using either of these features in a program, renders the application runnable only in Mac OS X v10.3 and later because runtime support for exception handling and synchronization is not present in earlier versions of the software. Handling Exceptions The Objective-C language has an exception-handling syntax similar to that of Java and C++. Coupled with the use of the NSException, NSError, or custom classes, you can add robust error-handling to your programs. The exception support revolves around four compiler directives: @try, @catch, @throw, and @finally. Code that can potentially throw an exception is enclosed in a @try block. @catch()blocks contain the exception-handling logic for exceptions thrown in a @tryblock. A @finallyblock contains code that must be executed whether an exception is thrown or not. You use the @throwdirective to throw an exception, which is essentially a pointer to an Objective-C object. You can use NSException objects but are not limited to them. The example below depicts a simple exception-handling algorithm: Cup *cup = [[Cup alloc] init]; @try { [cup fill]; } @catch (NSException *exception) { NSLog(@"main: Caught %@: %@", [exception name], [exception reason]); } @finally { [cup release]; } Handling Exceptions 91 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 9 Exception Handling Throwing Exceptions To throw an exception you must instantiate an object with the appropriate information, such as the exception name and the reason it was thrown. NSException *exception = [NSException exceptionWithName:@"HotTeaException" reason:@"The tea is too hot" userInfo:nil]; @throw exception; Inside a @catch() block, you can re-throw the caught exception using the @throw directive without an argument. This can help make your code more readable. You can subclass NSException to implement specialized types of exceptions, such as file-system exceptions or communications exceptions. Note: You are not limited to throwing NSException objects. You can throw any Objective-C object as an exception object. The NSException class provides methods that help in exception processing, but you can implement your own if you so desire. Processing Exceptions To catch an exception thrown in a @try block, use one or more @catch()blocks following the @try block. The @catch() blocks should be ordered from most-specific to the least-specific. That way you can tailor the processing of exceptions as groups, as shown in Listing 9-1. Listing 9-1 An exception handler @try { ... } // 1@catch (CustomException *ce) { ... } // 2@catch (NSException *ne) { // Perform processing necessary at this level. ... // Rethrow the exception so that it's handled at a higher level. // 3@throw; } @catch (id ue) { ... } // 4@finally { // Perform processing necessary whether an exception occurred or not. ... } The following list describes the numbered code-lines: 1. Catches the most specific exception type. 2. Catches a more general exception type. 92 Handling Exceptions 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 9 Exception Handling 3. Re-throws the exception caught. To compartmentalize exception processing, you can nest exception handlers in a program. That way if a method or function catches an exception that it cannot process, it can re-throw it to the next exception handler. 4. Performs any clean-up processing that must always be performed, whether exceptions were thrown or not. Handling Exceptions 93 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 9 Exception Handling 94 Handling Exceptions 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 9 Exception Handling Objective-C provides support for thread synchronization and exception handling, which are explained in this article and “Exception Handling” (page 91). To turn on support for these features, use the -fobjc-exceptions switch of the GNU Compiler Collection (GCC) version 3.3 and later. Note: Using either of these features in a program, renders the application runnable only in Mac OS X v10.3 and later because runtime support for exception handling and synchronization is not present in earlier versions of the software. Synchronizing Thread Execution Objective-C supports multithreading in applications. This means that two threads can try to modify the same object at the same time, a situation that can cause serious problems in a program. To protect sections of code from being executed by more than one thread at a time, Objective-C provides the @synchronized() directive. The @synchronized()directive locks a section of code for use by a single thread. Other threads are blocked until the thread exits the protected code; that is, when execution continues past the last statement in the @synchronized() block. The @synchronized() directive takes as its only argument any Objective-C object, including self. This object is known as a mutual exclusion semaphore or mutex. It allows a thread to lock a section of code to prevent its use by other threads. You should use separate semaphores to protect different critical sections of a program. It’s safest to create all the mutual exclusion objects before the application becomes multithreaded to avoid race conditions. Listing 10-1 shows an example of code that uses self as the mutex to synchronize access to the instance methods of the current object. You can take a similar approach to synchronize the class methods of the associated class, using the Class object instead of self. In the latter case, of course, only one thread at a time is allowed to execute a class method because there is only one class object that is shared by all callers. Listing 10-1 Locking a method using self - (void)criticalMethod { @synchronized(self) { // Critical code. Synchronizing Thread Execution 95 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 10 Threading ... } } Listing 10-2 uses the current selector, _cmd, as the mutex. This kind of synchronization is beneficial only when the method being synchronized has a unique name. This is because no other object or class would be allowed to execute a different method with the same name until the current method ends. Listing 10-2 Locking a method using _cmd - (void)criticalMethod { @synchronized(NSStringFromSelector(_cmd)) { // Critical code. ... } } Listing 10-3 shows a general approach. Before executing a critical process, the code obtains a semaphore from the Account class and uses it to lock the critical section. The Account class could create the semaphore in its initialize method. Listing 10-3 Locking a method using a custom semaphore Account *account = [Account accountFromString:[accountField stringValue]]; // Get the semaphore. id accountSemaphore = [Account semaphore]; @synchronized(accountSemaphore) { // Critical code. ... } The Objective-C synchronization feature supports recursive and reentrant code. A thread can use a single semaphore several times in a recursive manner; other threads are blocked from using it until the thread releases all the locks obtained with it; that is, every @synchronized() block is exited normally or through an exception. When code in an @synchronized() block throws an exception, the Objective-C runtime catches the exception, releases the semaphore (so that the protected code can be executed by other threads), and re-throws the exception to the next exception handler. 96 Synchronizing Thread Execution 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 10 Threading Overview Apple’s Objective-C compiler allows you to freely mix C++ and Objective-C code in the same source file. This Objective-C/C++ language hybrid is called Objective-C++. With it you can make use of existing C++ libraries from your Objective-C applications. Note that Xcode requires that file names have a “.mm” extension for the Objective-C++ extensions to be enabled by the compiler. Objective-C++ does not add C++ features to Objective-C classes, nor does it add Objective-C features to C++ classes. For example, you cannot use Objective-C syntax to call a C++ object, you cannot add constructors or destructors to an Objective-C object, and you cannot use the keywords this and self interchangeably. The class hierarchies are separate; a C++ class cannot inherit from an Objective-C class, and an Objective-C class cannot inherit from a C++ class. In addition, multi-language exception handling is not supported. That is, an exception thrown in Objective-C code cannot be caught in C++ code and, conversely, an exception thrown in C++ code cannot be caught in Objective-C code. For more information on exceptions in Objective-C, see “Exception Handling” (page 91). The next section discusses what you can do with Objective-C++. Mixing Objective-C and C++ Language Features In Objective-C++, you can call methods from either language in C++ code and in Objective-C methods. Pointers to objects in either language are just pointers, and as such can be used anywhere. For example, you can include pointers to Objective-C objects as data members of C++ classes, and you can include pointers to C++ objects as instance variables of Objective-C classes. Listing 11-1 illustrates this. Listing 11-1 Using C++ and Objective-C instances as instance variables /* * Compile with: g++ -x objective-c++ -framework Foundation -o hello */ #import class Hello { private: id greeting_text; // holds an NSString public: Overview 97 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 11 Using C++ With Objective-C Hello() { greeting_text = @"Hello, world!"; } Hello(const char* greeting_text) { greeting_text = [NSString stringWithUTF8String:greeting_text]; } void say_hello() { printf("%s\n", [greeting_text UTF8String]); } }; @interface Greeting : NSObject { @private Hello *hello; } - (id)init; - (void)dealloc; - (void)sayGreeting; - (void)sayGreeting:(Hello*)greeting; @end @implementation Greeting - (id)init { if (self = [super init]) { hello = new Hello(); } return self; } - (void)dealloc { delete hello; [super dealloc]; } - (void)sayGreeting { hello->say_hello(); } - (void)sayGreeting:(Hello*)greeting { greeting->say_hello(); } @end int main() { NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init]; Greeting *greeting = [[Greeting alloc] init]; [greeting sayGreeting]; // > Hello, world! Hello *hello = new Hello("Bonjour, monde!"); [greeting sayGreeting:hello]; // > Bonjour, monde! delete hello; [greeting release]; [pool release]; return 0; } 98 Mixing Objective-C and C++ Language Features 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 11 Using C++ With Objective-C As you can declare C structs in Objective-C interfaces, you can also declare C++ classes in Objective-C interfaces. As with C structs, C++ classes defined within an Objective-C interface are globally-scoped, not nested within the Objective-C class. (This is consistent with the way in which standard C—though not C++—promotes nested struct definitions to file scope.) To allow you to conditionalize your code based on the language variant, the Objective-C++ compiler defines both the __cplusplus and the __OBJC__ preprocessor constants, as specified by (respectively) the C++ and Objective-C language standards. As previously noted, Objective-C++ does not allow you to inherit C++ classes from Objective-C objects, nor does it allow you to inherit Objective-C classes from C++ objects. class Base { /* ... */ }; @interface ObjCClass: Base ... @end // ERROR! class Derived: public ObjCClass ... // ERROR! Unlike Objective-C, objects in C++ are statically typed, with runtime polymorphism available as an exceptional case. The object models of the two languages are thus not directly compatible. More fundamentally, the layout of Objective-C and C++ objects in memory is mutually incompatible, meaning that it is generally impossible to create an object instance that would be valid from the perspective of both languages. Hence, the two type hierarchies cannot be intermixed. You can declare a C++ class within an Objective-C class declaration. The compiler treats such classes as having been declared in the global namespace, as follows: @interface Foo { class Bar { ... } // OK } @end Bar *barPtr; // OK Objective-C allows C structures (whether declared inside of an Objective-C declaration or not) to be used as instance variables. @interface Foo { struct CStruct { ... }; struct CStruct bigIvar; // OK } ... @end Objective-C++ similarly strives to allow C++ class instances to serve as instance variables. This is possible as long as the C++ class in question (along with all of its superclasses) does not have any virtual member functions defined. If any virtual member functions are present, the C++ class may not serve as an Objective-C instance variable. #import struct Class0 { void foo(); }; struct Class1 { virtual void foo(); }; struct Class2 { Class2(int i, int j); }; @interface Foo : NSObject { Class0 class0; // OK Class1 class1; // ERROR! Class1 *ptr; // OK—call 'ptr = new Class1()' from Foo's init, // 'delete ptr' from Foo's dealloc Class2 class2; // WARNING - constructor not called! Mixing Objective-C and C++ Language Features 99 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 11 Using C++ With Objective-C ... @end C++ requires each instance of a class containing virtual functions to contain a suitable virtual function table pointer. However, the Objective-C runtime cannot initialize the virtual function table pointer, because it is not familiar with the C++ object model. Similarly, the Objective-C runtime cannot dispatch calls to C++ constructors or destructors for those objects. If a C++ class has any user-defined constructors or destructors, they are not called. The compiler emits a warning in such cases. Objective-C does not have a notion of nested namespaces. You cannot declare Objective-C classes within C++ namespaces, nor can you declare namespaces within Objective-C classes. Objective-C classes, protocols, and categories cannot be declared inside a C++ template, nor can a C++ template be declared inside the scope of an Objective-C interface, protocol, or category. However, Objective-C classes may serve as C++ template parameters. C++ template parameters can also be used as receivers or parameters (though not as selectors) in Objective-C message expressions. C++ Lexical Ambiguities and Conflicts There are a few identifiers that are defined in the Objective-C header files that every Objective-C program must include. These identifiers are id, Class, SEL, IMP, and BOOL. Inside an Objective-C method, the compiler pre-declares the identifiers self and super, similarly to the keyword this in C++. However, unlike the C++ this keyword, self and super are context-sensitive; they may be used as ordinary identifiers outside of Objective-C methods. In the parameter list of methods within a protocol, there are five more context-sensitive keywords (oneway, in, out, inout, and bycopy). These are not keywords in any other contexts. From an Objective-C programmer's point of view, C++ adds quite a few new keywords. You can still use C++ keywords as a part of an Objective-C selector, so the impact isn’t too severe, but you cannot use them for naming Objective-C classes or instance variables. For example, even though class is a C++ keyword, you can still use the NSObject method class: [foo class]; // OK However, because it is a keyword, you cannot use class as the name of a variable: NSObject *class; // Error In Objective-C, the names for classes and categories live in separate namespaces. That is, both @interface foo and @interface(foo) can exist in the same source code. In Objective-C++, you can also have a category whose name matches that of a C++ class or structure. Protocol and template specifiers use the same syntax for different purposes: id foo; TemplateType bar; To avoid this ambiguity, the compiler doesn’t permit id to be used as a template name. 100 C++ Lexical Ambiguities and Conflicts 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 11 Using C++ With Objective-C Finally, there is a lexical ambiguity in C++ when a label is followed by an expression that mentions a global name, as in: label: ::global_name = 3; The space after the first colon is required. Objective-C++ adds a similar case, which also requires a space: receiver selector: ::global_c++_name; C++ Lexical Ambiguities and Conflicts 101 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 11 Using C++ With Objective-C 102 C++ Lexical Ambiguities and Conflicts 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 11 Using C++ With Objective-C The Objective-C language defers as many decisions as it can from compile time and link time to runtime. Whenever possible, it does things dynamically. This means that the language requires not just a compiler, but also a runtime system to execute the compiled code. The runtime system acts as a kind of operating system for the Objective-C language; it’s what makes the language work. The following sections look in particular at three areas where the NSObject class provides a framework and defines conventions: ■ Allocating and initializing new instances of a class, and deallocating instances when they’re no longer needed ■ Forwarding messages to another object ■ Dynamically loading new modules into a running program Additional conventions of the NSObject class are described in the NSObject class specification in the Foundation framework reference. Other sections look at how you interact with the runtime at an abstract level; how you can use the Distributed Objects system for sending messages between objects in different address spaces; and how the compiler encodes the return and argument types for each method. Interacting with the Runtime System Objective-C programs interact with the runtime system at three distinct levels: 1. Through Objective-C source code. For the most part, the runtime system works automatically and behind the scenes. You use it just by writing and compiling Objective-C source code. When you compile code containing Objective-C classes and methods, the compiler creates the data structures and function calls that implement the dynamic characteristics of the language. The data structures capture information found in class and category definitions and in protocol declarations; they include the class and protocol objects discussed in “Defining a Class” (page 31) and “Protocols” (page 61), as well as method selectors, instance variable templates, and other Interacting with the Runtime System 103 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 12 The Runtime System information distilled from source code. The principal runtime function is the one that sends messages, as described in “How Messaging Works” (page 75). It’s invoked by source-code message expressions. 2. Through the methods defined in the NSObject class of the Foundation framework. Most objects in Cocoa are subclasses of the NSObject class, so most objects inherit the methods it defines. (The notable exception is the NSProxy class; see “Forwarding” (page 112) for more information.) Some of the NSObject methods simply query the runtime system for information. These methods allow objects to perform introspection. Examples of such methods are the class method, which asks an object to identify its class; isKindOfClass: and isMemberOfClass:, which test an object’s position in the inheritance hierarchy; respondsToSelector:, which indicates whether an object can accept a particular message; conformsToProtocol:, which indicates whether an object claims to implement the methods defined in a specific protocol; and methodForSelector:, which provides the address of a method’s implementation. Methods like these give an object the ability to introspect about itself. All these methods were mentioned in previous chapters and are described in detail in the NSObject class specification in the Foundation framework reference. 3. Through direct calls to runtime functions. The runtime system is a dynamic shared library with a public interface consisting of a set of functions and data structures in the header files located within the directory /usr/include/objc. Many of these functions allow you to use plain C to replicate what the compiler does when you write Objective-C code. Others form the basis for functionality exported through the methods of the NSObject class. These functions make it possible to develop other interfaces to the runtime system and produce tools that augment the development environment; they’re not needed when programming in Objective-C. However, a few of the runtime functions might on occasion be useful when writing an Objective-C program. All of these functions are documented in Objective-C 2.0 Runtime Reference. Because the NSObject class is at the root of the inheritance hierarchy of the Foundation framework, the methods it defines are usually inherited by all classes. Its methods therefore establish behaviors that are inherent to every instance and every class object. However, in a few cases, the NSObject class merely defines a template for how something should be done; it doesn’t provide all the necessary code itself. For example, the NSObject class defines a description instance method that returns a string describing the contents of the class. This is primarily used for debugging—the GDB print-object command prints the string returned from this method. NSObject’s implementation of this method doesn’t know what the class contains, so it returns a string with the name and address of the object. Subclasses of NSObject can implement this method to return more details. For example, the Foundation class NSArray returns a list of descriptions of the objects it contains. Allocating and Initializing Objects It takes two steps to create an object using Objective-C. You must: 104 Allocating and Initializing Objects 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 12 The Runtime System ■ Dynamically allocate memory for the new object ■ Initialize the newly allocated memory to appropriate values An object isn’t fully functional until both steps have been completed. Each step is accomplished by a separate method but typically in a single line of code: id anObject = [[Rectangle alloc] init]; Separating allocation from initialization gives you individual control over each step so that each can be modified independently of the other. The following sections look first at allocation and then at initialization, and discuss how they are controlled and modified. In Objective-C, memory for new objects is allocated using class methods defined in the NSObject class. NSObject defines two principal methods for this purpose, alloc and allocWithZone:. + (id)alloc; + (id)allocWithZone:(NSZone *)zone; These methods allocate enough memory to hold all the instance variables for an object belonging to the receiving class. They don’t need to be overridden and modified in subclasses. The alloc and allocWithZone: methods initialize a newly allocated object’s isa instance variable so that it points to the object’s class (the class object). All other instance variables are set to 0. Usually, an object needs to be more specifically initialized before it can be safely used. This initialization is the responsibility of class-specific instance methods that, by convention, begin with the abbreviation “init”. If the method takes no arguments, the method name is just those four letters, init. If it takes arguments, labels for the arguments follow the “init” prefix. For example, an NSView object can be initialized with an initWithFrame: method. Every class that declares instance variables must provide an init... method to initialize them. The NSObject class declares the isa variable and defines an init method. However, since isa is initialized when memory for an object is allocated, all NSObject’s init method does is return self. NSObject declares the method mainly to establish the naming convention described earlier. The Returned Object An init... method normally initializes the instance variables of the receiver, then returns it. It’s the responsibility of the method to return an object that can be used without error. However, in some cases, this responsibility can mean returning a different object than the receiver. For example, if a class keeps a list of named objects, it might provide an initWithName: method to initialize new instances. If there can be no more than one object per name, initWithName: might refuse to assign the same name to two objects. When asked to assign a new instance a name that’s already being used by another object, it might free the newly allocated instance and return the other object—thus ensuring the uniqueness of the name while at the same time providing what was asked for, an instance with the requested name. In a few cases, it might be impossible for an init... method to do what it’s asked to do. For example, an initFromFile: method might get the data it needs from a file passed as an argument. If the file name it’s passed doesn’t correspond to an actual file, it won’t be able to complete the initialization. In such a case, the init... method could free the receiver and return nil, indicating that the requested object can’t be created. Allocating and Initializing Objects 105 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 12 The Runtime System Because an init... method might return an object other than the newly allocated receiver, or even return nil, it’s important that programs use the value returned by the initialization method, not just that returned by alloc or allocWithZone:. The following code is very dangerous, since it ignores the return of init. id anObject = [SomeClass alloc]; [anObject init]; [anObject someOtherMessage]; Instead, to safely initialize an object, you should combine allocation and initialization messages in one line of code. id anObject = [[SomeClass alloc] init]; [anObject someOtherMessage]; If there’s a chance that the init... method might return nil, then you should check the return value before proceeding: id anObject = [[SomeClass alloc] init]; if ( anObject ) [anObject someOtherMessage]; else ... Arguments An init... method must ensure that all of an object’s instance variables have reasonable values. This doesn’t mean that it needs to provide an argument for each variable. It can set some to default values or depend on the fact that (except for isa) all bits of memory allocated for a new object are set to 0. For example, if a class requires its instances to have a name and a data source, it might provide an initWithName:fromFile: method, but set nonessential instance variables to arbitrary values or allow them to have the null values set by default. It could then rely on methods like setEnabled:, setFriend:, and setDimensions: to modify default values after the initialization phase had been completed. Any init... method that takes arguments must be prepared to handle cases where an inappropriate value is passed. Coordinating Classes Every class that declares instance variables must provide an init... method to initialize them (unless the variables require no initialization). The init... methods the class defines initialize only those variables declared in the class. Inherited instance variables are initialized by sending a message to super to perform an initialization method defined somewhere farther up the inheritance hierarchy: - initWithName:(char *)string { if ( self = [super init] ) { name = (char *)NSZoneMalloc([self zone], strlen(string) + 1); strcpy(name, string); } return self; 106 Allocating and Initializing Objects 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 12 The Runtime System } The message to super chains together initialization methods in all inherited classes. Because it comes first, it ensures that superclass variables are initialized before those declared in subclasses. For example, a Rectangle object must be initialized as an NSObject, a Graphic, and a Shape before it’s initialized as a Rectangle. The connection between the initWithName: method illustrated above and the inherited init method it incorporates is illustrated in Figure 12-1: Figure 12-1 Incorporating an Inherited Initialization Method Class B Class A – init – initWithName: A class must also make sure that all inherited initialization methods work. For example, if class A defines an init method and its subclass B defines an initWithName: method, as shown in Figure 12-1, B must also make sure that an init message successfully initializes B instances. The easiest way to do that is to replace the inherited init method with a version that invokes initWithName:: - init { return [self initWithName:"default"]; } The initWithName: method would, in turn, invoke the inherited method, as shown earlier. Figure 12-2 includes B’s version of init: Allocating and Initializing Objects 107 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 12 The Runtime System Figure 12-2 Covering an Inherited Initialization Model Class B Class A – init – init – initWithName: Covering inherited initialization methods makes the class you define more portable to other applications. If you leave an inherited method uncovered, someone else may use it to produce incorrectly initialized instances of your class. The Designated Initializer In the example above, initWithName: would be the designated initializer for its class (class B). The designated initializer is the method in each class that guarantees inherited instance variables are initialized (by sending a message to super to perform an inherited method). It’s also the method that does most of the work, and the one that other initialization methods in the same class invoke. It’s a Cocoa convention that the designated initializer is always the method that allows the most freedom to determine the character of a new instance (usually this is the one with the most arguments, but not always). It’s important to know the designated initializer when defining a subclass. For example, suppose we define class C, a subclass of B, and implement an initWithName:fromFile: method. In addition to this method, we have to make sure that the inherited init and initWithName: methods also work for instances of C. This can be done just by covering B’s initWithName: with a version that invokes initWithName:fromFile:. - initWithName:(char *)string { return [self initWithName:string fromFile:NULL]; } For an instance of the C class, the inherited init method invokes this new version of initWithName: which invokes initWithName:fromFile:. The relationship between these methods is shown in Figure 12-3: 108 Allocating and Initializing Objects 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 12 The Runtime System Figure 12-3 Covering the Designated Initializer – initWithName:fromFile: – initWithName: Class B – init Class C – initWithName: This figure omits an important detail. The initWithName:fromFile: method, being the designated initializer for the C class, sends a message to super to invoke an inherited initialization method. But which of B’s methods should it invoke, init or initWithName:? It can’t invoke init, for two reasons: ■ Circularity would result (init invokes C’s initWithName:, which invokes initWithName:fromFile:, which invokes init again). ■ It won’t be able to take advantage of the initialization code in B’s version of initWithName:. Therefore, initWithName:fromFile: must invoke initWithName:: - initWithName:(char *)string fromFile:(char *)pathname { if ( self = [super initWithName:string] ) ... } General Principle: The designated initializer in a class must, through a message to super, invoke the designated initializer in a superclass. Designated initializers are chained to each other through messages to super, while other initialization methods are chained to designated initializers through messages to self. Figure 12-4 shows how all the initialization methods in classes A, B, and C are linked. Messages to self are shown on the left and messages to super are shown on the right. Allocating and Initializing Objects 109 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 12 The Runtime System Figure 12-4 Initialization Chain – initWithName:fromFile: – initWithName: Class B Class A – init – init Class C – initWithName: Note that B’s version of init sends a message to self to invoke the initWithName: method. Therefore, when the receiver is an instance of the B class, it invokes B’s version of initWithName:, and when the receiver is an instance of the C class, it invokes C’s version. Combining Allocation and Initialization In Cocoa, some classes define creation methods that combine the two steps of allocating and initializing to return new, initialized instances of the class. These methods are often referred to as convenience constructors and typically take the form + className... where className is the name of the class. For example, NSString has the following methods (among others): + (NSString *)stringWithCString:(const char *)bytes; + (NSString *)stringWithFormat:(NSString *)format, ...; Similarly, NSArray defines the following class methods that combine allocation and initialization: + (id)array; + (id)arrayWithObject:(id)anObject; 110 Allocating and Initializing Objects 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 12 The Runtime System + (id)arrayWithObjects:(id)firstObj, ...; Important: It is important to understand the memory management implications of using these methods if you do not use garbage collection (see “Memory Management” (page 111)). You must readMemory Management Programming Guide for Cocoa to understand the policy that applies to these convenience constructors. Methods that combine allocation and initialization are particularly valuable if the allocation must somehow be informed by the initialization. For example, if the data for the initialization is taken from a file, and the file might contain enough data to initialize more than one object, it would be impossible to know how many objects to allocate until the file is opened. In this case, you might implement a listFromFile: method that takes the name of the file as an argument. It would open the file, see how many objects to allocate, and create a List object large enough to hold all the new objects. It would then allocate and initialize the objects from data in the file, put them in the List, and finally return the List. It also makes sense to combine allocation and initialization in a single method if you want to avoid the step of blindly allocating memory for a new object that you might not use. As mentioned in “The Returned Object” (page 105), an init... method might sometimes substitute another object for the receiver. For example, when initWithName: is passed a name that’s already taken, it might free the receiver and in its place return the object that was previously assigned the name. This means, of course, that an object is allocated and freed immediately without ever being used. If the code that determines whether the receiver should be initialized is placed inside the method that does the allocation instead of inside init..., you can avoid the step of allocating a new instance when one isn’t needed. In the following example, the soloist method ensures that there’s no more than one instance of the Soloist class. It allocates and initializes an instance only once: + soloist { static Soloist *instance = nil; if ( instance == nil ) { instance = [[self alloc] init]; } return instance; } Memory Management In an Objective-C program, it is important to ensure that objects are deallocated when they are no longer needed—otherwise your application’s memory footprint is larger than necessary. It is also important to ensure that you do not deallocate objects while they’re still being used. Objective-C 2.0 offers two paradigms for memory management that allow you to meet these goals: ■ Managed memory, where you are ultimately responsible for determining the lifetime of objects using a technique called reference counting. Memory Management 111 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 12 The Runtime System Managed memory is described in Memory Management Programming Guide for Cocoa. ■ Garbage collection, where you pass responsibility for determining the lifetime of objects to an automatic “collector.” Garbage collection is described in Garbage Collection Programming Guide. Forwarding Sending a message to an object that does not handle that message is an error. However, before announcing the error, the runtime system gives the receiving object a second chance to handle the message. It sends the object a forwardInvocation: message with an NSInvocation object as its sole argument—the NSInvocation object encapsulates the original message and the arguments that were passed with it. You can implement a forwardInvocation: method to give a default response to the message, or to avoid the error in some other way. As its name implies, forwardInvocation: is commonly used to forward the message to another object. To see the scope and intent of forwarding, imagine the following scenarios: Suppose, first, that you’re designing an object that can respond to a message called negotiate, and you want its response to include the response of another kind of object. You could accomplish this easily by passing a negotiate message to the other object somewhere in the body of the negotiate method you implement. Take this a step further, and suppose that you want your object’s response to a negotiate message to be exactly the response implemented in another class. One way to accomplish this would be to make your class inherit the method from the other class. However, it might not be possible to arrange things this way. There may be good reasons why your class and the class that implements negotiate are in different branches of the inheritance hierarchy. Even if your class can’t inherit the negotiate method, you can still “borrow” it by implementing a version of the method that simply passes the message on to an instance of the other class: - negotiate { if ( [someOtherObject respondsTo:@selector(negotiate)] ) return [someOtherObject negotiate]; return self; } This way of doing things could get a little cumbersome, especially if there were a number of messages you wanted your object to pass on to the other object. You’d have to implement one method to cover each method you wanted to borrow from the other class. Moreover, it would be impossible to handle cases where you didn’t know, at the time you wrote the code, the full set of messages you might want to forward. That set might depend on events at runtime, and it might change as new methods and classes are implemented in the future. The second chance offered by a forwardInvocation: message provides a less ad hoc solution to this problem, and one that’s dynamic rather than static. It works like this: When an object can’t respond to a message because it doesn’t have a method matching the selector in the message, the runtime system informs the object by sending it a forwardInvocation: message. Every object inherits a forwardInvocation: method from the NSObject class. However, NSObject’s version of the method 112 Forwarding 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 12 The Runtime System simply invokes doesNotRecognizeSelector:. By overriding NSObject’s version and implementing your own, you can take advantage of the opportunity that the forwardInvocation: message provides to forward messages to other objects. To forward a message, all a forwardInvocation: method needs to do is: ■ Determine where the message should go, and ■ Send it there with its original arguments. The message can be sent with the invokeWithTarget: method: - (void)forwardInvocation:(NSInvocation *)anInvocation { if ([someOtherObject respondsToSelector: [anInvocation selector]]) [anInvocation invokeWithTarget:someOtherObject]; else [super forwardInvocation:anInvocation]; } The return value of the message that’s forwarded is returned to the original sender. All types of return values can be delivered to the sender, including ids, structures, and double-precision floating-point numbers. A forwardInvocation: method can act as a distribution center for unrecognized messages, parceling them out to different receivers. Or it can be a transfer station, sending all messages to the same destination. It can translate one message into another, or simply “swallow” some messages so there’s no response and no error. A forwardInvocation: method can also consolidate several messages into a single response. What forwardInvocation: does is up to the implementor. However, the opportunity it provides for linking objects in a forwarding chain opens up possibilities for program design. Note: The forwardInvocation: method gets to handle messages only if they don’t invoke an existing method in the nominal receiver. If, for example, you want your object to forward negotiate messages to another object, it can’t have a negotiate method of its own. If it does, the message will never reach forwardInvocation:. For more information on forwarding and invocations, see the NSInvocation class specification in the Foundation framework reference. Forwarding and Multiple Inheritance Forwarding mimics inheritance, and can be used to lend some of the effects of multiple inheritance to Objective-C programs. As shown in Figure 12-5 (page 114), an object that responds to a message by forwarding it appears to borrow or “inherit” a method implementation defined in another class. Forwarding 113 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 12 The Runtime System Figure 12-5 Forwarding isa . . . – forwardInvocation: – negotiate negotiate isa . . . DiplomatWarrior In this illustration, an instance of the Warrior class forwards a negotiate message to an instance of the Diplomat class. The Warrior will appear to negotiate like a Diplomat. It will seem to respond to the negotiate message, and for all practical purposes it does respond (although it’s really a Diplomat that’s doing the work). The object that forwards a message thus “inherits” methods from two branches of the inheritance hierarchy—its own branch and that of the object that responds to the message. In the example above, it appears as if the Warrior class inherits from Diplomat as well as its own superclass. Forwarding provides most of the features that you typically want from multiple inheritance. However, there’s an important difference between the two: Multiple inheritance combines different capabilities in a single object. It tends toward large, multifaceted objects. Forwarding, on the other hand, assigns separate responsibilities to disparate objects. It decomposes problems into smaller objects, but associates those objects in a way that’s transparent to the message sender. Surrogate Objects Forwarding not only mimics multiple inheritance, it also makes it possible to develop lightweight objects that represent or “cover” more substantial objects. The surrogate stands in for the other object and funnels messages to it. The proxy discussed in “Remote Messaging” (page 117) is such a surrogate. A proxy takes care of the administrative details of forwarding messages to a remote receiver, making sure argument values are copied and retrieved across the connection, and so on. But it doesn’t attempt to do much else; it doesn’t duplicate the functionality of the remote object but simply gives the remote object a local address, a place where it can receive messages in another application. Other kinds of surrogate objects are also possible. Suppose, for example, that you have an object that manipulates a lot of data—perhaps it creates a complicated image or reads the contents of a file on disk. Setting this object up could be time-consuming, so you prefer to do it lazily—when it’s really needed or when system resources are temporarily idle. At the same time, you need at least a placeholder for this object in order for the other objects in the application to function properly. 114 Forwarding 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 12 The Runtime System In this circumstance, you could initially create, not the full-fledged object, but a lightweight surrogate for it. This object could do some things on its own, such as answer questions about the data, but mostly it would just hold a place for the larger object and, when the time came, forward messages to it. When the surrogate’s forwardInvocation: method first receives a message destined for the other object, it would ensure that the object existed and would create it if it didn’t. All messages for the larger object go through the surrogate, so, as far as the rest of the program is concerned, the surrogate and the larger object would be the same. Forwarding and Inheritance Although forwarding mimics inheritance, the NSObject class never confuses the two. Methods like respondsToSelector: and isKindOfClass: look only at the inheritance hierarchy, never at the forwarding chain. If, for example, a Warrior object is asked whether it responds to a negotiate message, if ( [aWarrior respondsToSelector:@selector(negotiate)] ) ... the answer is NO, even though it can receive negotiate messages without error and respond to them, in a sense, by forwarding them to a Diplomat. (See Figure 12-5 (page 114).) In many cases, NO is the right answer. But it may not be. If you use forwarding to set up a surrogate object or to extend the capabilities of a class, the forwarding mechanism should probably be as transparent as inheritance. If you want your objects to act as if they truly inherited the behavior of the objects they forward messages to, you’ll need to re-implement the respondsToSelector: and isKindOfClass: methods to include your forwarding algorithm: - (BOOL)respondsToSelector:(SEL)aSelector { if ( [super respondsToSelector:aSelector] ) return YES; else { /* Here, test whether the aSelector message can * * be forwarded to another object and whether that * * object can respond to it. Return YES if it can. */ } return NO; } In addition to respondsToSelector: and isKindOfClass:, the instancesRespondToSelector: method should also mirror the forwarding algorithm. If protocols are used, the conformsToProtocol: method should likewise be added to the list. Similarly, if an object forwards any remote messages it receives, it should have a version of methodSignatureForSelector: that can return accurate descriptions of the methods that ultimately respond to the forwarded messages. You might consider putting the forwarding algorithm somewhere in private code and have all these methods, forwardInvocation: included, call it. Forwarding 115 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 12 The Runtime System Note: This is an advanced technique, suitable only for situations where no other solution is possible. It is not intended as a replacement for inheritance. If you must make use of this technique, make sure you fully understand the behavior of the class doing the forwarding and the class you’re forwarding to. The methods mentioned in this section are described in the NSObject class specification in the Foundation framework reference. For information on invokeWithTarget:, see the NSInvocation class specification in the Foundation framework reference. Dynamic Method Resolution There are situations where you might want to provide an implementation of a method dynamically. For example, the Objective-C declared properties feature (see “Properties” (page 45)) includes the @dynamic directive: @dynamic propertyName; which tells the compiler that the methods associated with the property will be provided dynamically. You can implement the methods resolveInstanceMethod: and resolveClassMethod: to dynamically provide an implementation for a given selector for an instance and class method respectively. An Objective-C method is simply a C function that take at least two arguments—self and _cmd. You can add a function to a class as a method using the function class_addMethod. Therefore, given the following function: void dynamicMethodIMP(id self, SEL _cmd) { // implementation .... } you can dynamically add it to a class as a method (called resolveThisMethodDynamically) using resolveInstanceMethod: like this: @implementation MyClass + (BOOL)resolveInstanceMethod:(SEL)aSEL { if (aSEL == @selector(resolveThisMethodDynamically)) { class_addMethod([self class], aSEL, (IMP) dynamicMethodIMP, "v@:"); return YES; } return [super resolveInstanceMethod:aSEL]; } @end Forwarding methods (as described in “Forwarding” (page 112)) and dynamic method resolution are, largely, orthogonal. A class has the opportunity to dynamically resolve a method before the forwarding mechanism kicks in. If respondsToSelector: or instancesRespondToSelector: is invoked, the dynamic method resolver is given the opportunity to provide an IMP for the selector first. If you implement resolveInstanceMethod: but want particular selectors to actually be forwarded via the forwarding mechanism, you return NO for those selectors. 116 Dynamic Method Resolution 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 12 The Runtime System Dynamic Loading An Objective-C program can load and link new classes and categories while it’s running. The new code is incorporated into the program and treated identically to classes and categories loaded at the start. Dynamic loading can be used to do a lot of different things. For example, the various modules in the System Preferences application are dynamically loaded. In the Cocoa environment, dynamic loading is commonly used to allow applications to be customized. Others can write modules that your program loads at runtime—much as Interface Builder loads custom palettes and the Mac OS X System Preferences application loads custom preference modules. The loadable modules extend what your application can do. They contribute to it in ways that you permit but could not have anticipated or defined yourself. You provide the framework, but others provide the code. Although there is a runtime function that performs dynamic loading of Objective-C modules in Mach-O files (objc_loadModules, defined in objc/objc-load.h), Cocoa’s NSBundle class provides a significantly more convenient interface for dynamic loading—one that’s object-oriented and integrated with related services. See the NSBundle class specification in the Foundation framework reference for information on the NSBundle class and its use. See Mac OS X ABI Mach-O File Format Reference for information on Mach-O files. Remote Messaging Like most other programming languages, Objective-C was initially designed for programs that are executed as a single process in a single address space. Nevertheless, the object-oriented model, where communication takes place between relatively self-contained units through messages that are resolved at runtime, would seem well suited for interprocess communication as well. It’s not hard to imagine Objective-C messages between objects that reside in different address spaces (that is, in different tasks) or in different threads of execution of the same task. For example, in a typical server-client interaction, the client task might send its requests to a designated object in the server, and the server might target specific client objects for the notifications and other information it sends. Or imagine an interactive application that needs to do a good deal of computation to carry out a user command. It could simply display a dialog telling the user to wait while it was busy, or it could isolate the processing work in a subordinate task, leaving the main part of the application free to accept user input. Objects in the two tasks would communicate through Objective-C messages. Similarly, several separate processes could cooperate on the editing of a single document. There could be a different editing tool for each type of data in the document. One task might be in charge of presenting a unified onscreen user interface and of sorting out which user instructions are the responsibility of the various editing tools. Each cooperating task could be written in Objective-C, with Objective-C messages being the vehicle of communication between the user interface and the tools and between one tool and another. Dynamic Loading 117 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 12 The Runtime System Distributed Objects Remote messaging in Objective-C requires a runtime system that can establish connections between objects in different address spaces, recognize when a message is intended for an object in a remote address space, and transfer data from one address space to another. It must also mediate between the separate schedules of the two tasks; it has to hold messages until their remote receivers are free to respond to them. Cocoa includes a distributed objects architecture that is essentially this kind of extension to the runtime system. Using distributed objects, you can send Objective-C messages to objects in other tasks or have messages executed in other threads of the same task. (When remote messages are sent between two threads of the same task, the threads are treated exactly like threads in different tasks.) Note that Cocoa’s distributed objects system is built on top of the runtime system; it doesn’t alter the fundamental behavior of your Cocoa objects. To send a remote message, an application must first establish a connection with the remote receiver. Establishing the connection gives the application a proxy for the remote object in its own address space. It then communicates with the remote object through the proxy. The proxy assumes the identity of the remote object; it has no identity of its own. The application is able to regard the proxy as if it were the remote object; for most purposes, it is the remote object. Remote messaging is illustrated in Figure 12-6 (page 118), where object A communicates with object B through a proxy, and messages for B wait in a queue until B is ready to respond to them: Figure 12-6 Remote Messages BA Proxy for B The sender and receiver are in different tasks and are scheduled independently of each other. So there’s no guarantee that the receiver is free to accept a message when the sender is ready to send it. Therefore, arriving messages are placed in a queue and retrieved at the convenience of the receiving application. A proxy doesn’t act on behalf of the remote object or need access to its class. It isn’t a copy of the object, but a lightweight substitute for it. In a sense, it’s transparent; it simply passes the messages it receives on to the remote receiver and manages the interprocess communication. Its main function is to provide a local address for an object that wouldn’t otherwise have one. A proxy isn’t fully transparent, however. For instance, a proxy doesn’t allow you to directly set and get an object’s instance variables. A remote receiver is typically anonymous. Its class is hidden inside the remote application. The sending application doesn’t need to know how that application is designed or what classes it uses. It doesn’t need to use the same classes itself. All it needs to know is what messages the remote object responds to. 118 Remote Messaging 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 12 The Runtime System Because of this, an object that’s designated to receive remote messages advertises its interface in a formal protocol. Both the sending and the receiving application declare the protocol—they both import the same protocol declaration. The receiving application declares it because the remote object must conform to the protocol. The sending application declares it to inform the compiler about the messages it sends and because it may use the conformsToProtocol: method and the @protocol() directive to test the remote receiver. The sending application doesn’t have to implement any of the methods in the protocol; it declares the protocol only because it initiates messages to the remote receiver. The distributed objects architecture, including the NSProxy and NSConnection classes, is documented in the Foundation framework reference and Distributed Objects Programming Topics. Language Support Remote messaging raises not only a number of intriguing possibilities for program design, it also raises some interesting issues for the Objective-C language. Most of the issues are related to the efficiency of remote messaging and the degree of separation that the two tasks should maintain while they’re communicating with each other. So that programmers can give explicit instructions about the intent of a remote message, Objective-C defines six type qualifiers that can be used when declaring methods inside a formal protocol: oneway in out inout bycopy byref These modifiers are restricted to formal protocols; they can’t be used inside class and category declarations. However, if a class or category adopts a protocol, its implementation of the protocol methods can use the same modifiers that are used to declare the methods. The following sections explain how these modifiers are used. Synchronous and Asynchronous Messages Consider first a method with just a simple return value: - (BOOL)canDance; When a canDance message is sent to a receiver in the same application, the method is invoked and the return value provided directly to the sender. But when the receiver is in a remote application, two underlying messages are required—one message to get the remote object to invoke the method, and the other message to send back the result of the remote calculation. This is illustrated in the figure below: Remote Messaging 119 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 12 The Runtime System Figure 12-7 Round-Trip Message initial message return information A Proxy for B B Most remote messages are, at bottom, two-way (or “round trip”) remote procedure calls (RPCs) like this one. The sending application waits for the receiving application to invoke the method, complete its processing, and send back an indication that it has finished, along with any return information requested. Waiting for the receiver to finish, even if no information is returned, has the advantage of coordinating the two communicating applications, of keeping them both “in sync.” For this reason, round-trip messages are often called synchronous. Synchronous messages are the default. However, it’s not always necessary or a good idea to wait for a reply. Sometimes it’s sufficient simply to dispatch the remote message and return, allowing the receiver to get to the task when it can. In the meantime, the sender can go on to other things. Objective-C provides a return type modifier, oneway, to indicate that a method is used only for asynchronous messages: - (oneway void)waltzAtWill; Although oneway is a type qualifier (like const) and can be used in combination with a specific type name, such as oneway float or oneway id, the only such combination that makes any sense is oneway void. An asynchronous message can’t have a valid return value. Pointer Arguments Next, consider methods that take pointer arguments. A pointer can be used to pass information to the receiver by reference. When invoked, the method looks at what’s stored in the address it’s passed. - setTune:(struct tune *)aSong { tune = *aSong; ... } The same sort of argument can also be used to return information by reference. The method uses the pointer to find where it should place information requested in the message. - getTune:(struct tune *)theSong { ... *theSong = tune; } The way the pointer is used makes a difference in how the remote message is carried out. In neither case can the pointer simply be passed to the remote object unchanged; it points to a memory location in the sender’s address space and would not be meaningful in the address space of the remote receiver. The runtime system for remote messaging must make some adjustments behind the scenes. 120 Remote Messaging 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 12 The Runtime System If the argument is used to pass information by reference, the runtime system must dereference the pointer, ship the value it points to over to the remote application, store the value in an address local to that application, and pass that address to the remote receiver. If, on the other hand, the pointer is used to return information by reference, the value it points to doesn’t have to be sent to the other application. Instead, a value from the other application must be sent back and written into the location indicated by the pointer. In the first case, information is passed on the first leg of the round trip. In the second case, information is returned on the second leg of the round trip. Because these cases result in very different actions on the part of the runtime system for remote messaging, Objective-C provides type modifiers that can clarify the programmer’s intention: ■ The type modifierin indicates that information is being passed in a message: - setTune:(in struct tune *)aSong; ■ The modifier out indicates that an argument is being used to return information by reference: - getTune:(out struct tune *)theSong; ■ A third modifier, inout, indicates that an argument is used both to provide information and to get information back: - adjustTune:(inout struct tune *)aSong; The Cocoa distributed objects system takes inout to be the default modifier for all pointer arguments except those declared const, for which in is the default. inout is the safest assumption but also the most time-consuming since it requires passing information in both directions. The only modifier that makes sense for arguments passed by value (non-pointers) is in. While in can be used with any kind of argument, out and inout make sense only for pointers. In C, pointers are sometimes used to represent composite values. For example, a string is represented as a character pointer (char *). Although in notation and implementation there’s a level of indirection here, in concept there’s not. Conceptually, a string is an entity in and of itself, not a pointer to something else. In cases like this, the distributed objects system automatically dereferences the pointer and passes whatever it points to as if by value. Therefore, the out and inout modifiers make no sense with simple character pointers. It takes an additional level of indirection in a remote message to pass or return a string by reference: - getTuneTitle:(out char **)theTitle; The same is true of objects: - adjustRectangle:(inout Rectangle **)theRect; These conventions are enforced at runtime, not by the compiler. Proxies and Copies Finally, consider a method that takes an object as an argument: - danceWith:(id)aPartner; Remote Messaging 121 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 12 The Runtime System A danceWith: message passes an object id to the receiver. If the sender and the receiver are in the same application, they would both be able to refer to the same aPartner object. This is true even if the receiver is in a remote application, except that the receiver needs to refer to the object through a proxy (since the object isn’t in its address space). The pointer that danceWith: delivers to a remote receiver is actually a pointer to the proxy. Messages sent to the proxy would be passed across the connection to the real object and any return information would be passed back to the remote application. There are times when proxies may be unnecessarily inefficient, when it’s better to send a copy of the object to the remote process so that it can interact with it directly in its own address space. To give programmers a way to indicate that this is intended, Objective-C provides a bycopy type modifier: - danceWith:(bycopy id)aClone; bycopy can also be used for return values: - (bycopy)dancer; It can similarly be used with out to indicate that an object returned by reference should be copied rather than delivered in the form of a proxy: - getDancer:(bycopy out id *)theDancer; Note: When a copy of an object is passed to another application, it cannot be anonymous. The application that receives the object must have the class of the object loaded in its address space. bycopy makes so much sense for certain classes—classes that are intended to contain a collection of other objects, for instance—that often these classes are written so that a copy is sent to a remote receiver, instead of the usual reference. You can override this behavior with byref, however, thereby specifying that objects passed to a method or objects returned from a method should be passed or returned by reference. Since passing by reference is the default behavior for the vast majority of Objective-C objects, you will rarely, if ever, make use of the byref keyword. The only type that it makes sense for bycopy or byref to modify is an object, whether dynamically typed id or statically typed by a class name. Although bycopy and byref can’t be used inside class and category declarations, they can be used within formal protocols. For instance, you could write a formal protocol foo as follows: @Protocol foo - (bycopy)array; @end A class or category can then adopt your protocol foo. This allows you to construct protocols so that they provide “hints” as to how objects should be passed and returned by the methods described by the protocol. 122 Remote Messaging 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 12 The Runtime System Type Encodings To assist the runtime system, the compiler encodes the return and argument types for each method in a character string and associates the string with the method selector. The coding scheme it uses is also useful in other contexts and so is made publicly available with the @encode() compiler directive. When given a type specification, @encode() returns a string encoding that type. The type can be a basic type such as an int, a pointer, a tagged structure or union, or a class name—any type, in fact, that can be used as an argument to the C sizeof() operator. char *buf1 = @encode(int **); char *buf2 = @encode(struct key); char *buf3 = @encode(Rectangle); The table below lists the type codes. Note that many of them overlap with the codes you use when encoding an object for purposes of archiving or distribution. However, there are codes listed here that you can’t use when writing a coder, and there are codes that you may want to use when writing a coder that aren’t generated by @encode(). (See the NSCoder class specification in the Foundation Framework reference for more information on encoding objects for archiving or distribution.) Table 12-1 Objective-C type encodings MeaningCode A charc An inti A shorts A longl A long longq An unsigned charC An unsigned intI An unsigned shortS An unsigned longL An unsigned long longQ A floatf A doubled A C++ bool or a C99 _BoolB A voidv A character string (char *)* An object (whether statically typed or typed id)@ Type Encodings 123 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 12 The Runtime System MeaningCode A class object (Class)# A method selector (SEL): An array[array type] A structure{name=type...} A union(name=type...) A bit field of num bitsbnum A pointer to type^type An unknown type (among other things, this code is used for function pointers)? Important: Objective-C does not support the long double type. @encode(long double) returns d, which is the same encoding as for double. The type code for an array is enclosed within square brackets; the number of elements in the array is specified immediately after the open bracket, before the array type. For example, an array of 12 pointers to floats would be encoded as: [12^f] Structures are specified within braces, and unions within parentheses. The structure tag is listed first, followed by an equal sign and the codes for the fields of the structure listed in sequence. For example, the structure typedef struct example { id anObject; char *aString; int anInt; } Example; would be encoded like this: {example=@*i} The same encoding results whether the defined type name (Example) or the structure tag (example) is passed to @encode(). The encoding for a structure pointer carries the same amount of information about the structure’s fields: ^{example=@*i} However, another level of indirection removes the internal type specification: ^^{example} Objects are treated like structures. For example, passing the NSObject class name to @encode() yields this encoding: {NSObject=#} 124 Type Encodings 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 12 The Runtime System The NSObject class declares just one instance variable, isa, of type Class. Note that although the @encode() directive doesn’t return them, the runtime system uses the additional encodings listed in Table 12-2 for type qualifiers when they’re used to declare methods in a protocol. Table 12-2 Objective-C method encodings MeaningCode constr inn inoutN outo bycopyO byrefR onewayV Type Encodings 125 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 12 The Runtime System 126 Type Encodings 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. CHAPTER 12 The Runtime System Objective-C adds a small number of constructs to the C language and defines a handful of conventions for effectively interacting with the runtime system. This appendix lists all the additions to the language but doesn’t go into great detail. For more information, see the other chapters in this document. For a more formal presentation of Objective-C syntax, see “Grammar” (page 135). Messages Message expressions are enclosed in square brackets: [receiver message] The receiver can be: ■ A variable or expression that evaluates to an object (including the variable self) ■ A class name (indicating the class object) ■ super (indicating an alternative search for the method implementation) The message is the name of a method plus any arguments passed to it. Defined Types The principal types used in Objective-C are defined in objc/objc.h. They are: DefinitionType An object (a pointer to its data structure).id A class object (a pointer to the class data structure).Class A selector, a compiler-assigned code that identifies a method name.SEL A pointer to a method implementation that returns an id.IMP Messages 127 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. APPENDIX A Language Summary DefinitionType A Boolean value, either YES or NO.BOOL id can be used to type any kind of object, class, or instance. In addition, class names can be used as type names to statically type instances of a class. A statically typed instance is declared to be a pointer to its class or to any class it inherits from. The objc.h header file also defines these useful terms: DefinitionType A null object pointer, (id)0.nil A null class pointer, (Class)0.Nil A boolean false value, (BOOL)0.NO A boolean true value, (BOOL)1.YES Preprocessor Directives The preprocessor understands these special notations: DefinitionNotation Imports a header file. This directive is identical to #include, except that it doesn’t include the same file more than once. #import Begins a comment that continues to the end of the line.// Compiler Directives Directives to the compiler begin with “@”. The following directives are used to declare and define classes, categories, and protocols: DefinitionDirective Begins the declaration of a class or category interface.@interface Begins the definition of a class or category.@implementation Begins the declaration of a formal protocol.@protocol Ends the declaration/definition of a class, category, or protocol.@end 128 Preprocessor Directives 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. APPENDIX A Language Summary The following mutually exclusive directives specify the visibility of instance variables: DefinitionDirective Limits the scope of an instance variable to the class that declares it.@private Limits instance variable scope to declaring and inheriting classes.@protected Removes restrictions on the scope of instance variables.@public The default is @protected. These directives support exception handling: DefinitionDirective Defines a block within which exceptions can be thrown.@try Throws an exception object.@throw Catches an exception thrown within the preceding @try block.@catch() Defines a block of code that is executed whether exceptions were thrown or not in a preceding @try block. @finally In addition, there are directives for these particular purposes: DefinitionDirective Declares the names of classes defined elsewhere.@class Returns the compiled selector that identifies method_name.@selector(method_name) Returns the protocol_name protocol (an instance of the Protocol class). (@protocol is also valid without (protocol_name) for forward declarations.) @protocol(protocol_name) Yields a character string that encodes the type structure of type_spec.@encode(type_spec) Defines a constant NSString object in the current module and initializes the object with the specified 7-bit ASCII-encoded string. @"string" Defines a constant NSString object in the current module. The string created is the result of concatenating the strings specified in the two directives. @"string1" @"string2" ... @"stringN" Defines a block of code that must be executed only by one thread at a time. @synchronized() Compiler Directives 129 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. APPENDIX A Language Summary Classes A new class is declared with the @interface directive. The interface file for its superclass must be imported: #import "ItsSuperclass.h" @interface ClassName : ItsSuperclass < protocol_list > { instance variable declarations } method declarations @end Everything but the compiler directives and class name is optional. If the colon and superclass name are omitted, the class is declared to be a new root class. If any protocols are listed, the header files where they’re declared must also be imported. A file containing a class definition imports its own interface: #import "ClassName.h" @implementation ClassName method definitions @end Categories A category is declared in much the same way as a class. The interface file that declares the class must be imported: #import "ClassName.h" @interface ClassName ( CategoryName ) < protocol list > method declarations @end The protocol list and method declarations are optional. If any protocols are listed, the header files where they’re declared must also be imported. Like a class definition, a file containing a category definition imports its own interface: #import "CategoryName.h" @implementation ClassName ( CategoryName ) method definitions @end 130 Classes 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. APPENDIX A Language Summary Deprecation Syntax Syntax is provided to mark methods as deprecated: @interface SomeClass -method __attribute__((deprecated)); @end or: #include @interface SomeClass -method DEPRECATED_ATTRIBUTE; // or some other deployment-target-specific macro @end This syntax is available only in Objective-C 2.0 and later. Formal Protocols Formal protocols are declared using the @protocol directive: @protocol ProtocolName < protocol list > method declarations @end The list of incorporated protocols and the method declarations are optional. The protocol must import the header files that declare any protocols it incorporates. You can create a forward reference to a protocol using the @protocol directive in the following manner: @protocol ProtocolName; Within source code, protocols are referred to using the similar @protocol() directive, where the parentheses enclose the protocol name. Protocol names listed within angle brackets (<...>) are used to do three different things: ■ In a protocol declaration, to incorporate other protocols (as shown earlier) ■ In a class or category declaration, to adopt the protocol (as shown in “Classes” (page 130) and “Categories” (page 130)) ■ In a type specification, to limit the type to objects that conform to the protocol Within protocol declarations, these type qualifiers support remote messaging: DefinitionType Qualifier The method is for asynchronous messages and has no valid return type.oneway The argument passes information to the remote Deprecation Syntax 131 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. APPENDIX A Language Summary DefinitionType Qualifier The argument gets information returned by reference.out The argument both passes information and gets information.inout A copy of the object, not a proxy, should be passed or returned.bycopy A reference to the object, not a copy, should be passed or returned.byref Method Declarations The following conventions are used in method declarations: ■ A “+” precedes declarations of class methods. ■ A “-” precedes declarations of instance methods. ■ Argument and return types are declared using the C syntax for type casting. ■ Arguments are declared after colons (:), for example: - (void)setWidth:(int)newWidth height:(int)newHeight Typically, a label describing the argument precedes the colon—the following example is valid but is considered bad style: - (void)setWidthAndHeight:(int)newWidth :(int)newHeight Both labels and colons are considered part of the method name. ■ The default return and argument type for methods is id, not int as it is for functions. (However, the modifier unsigned when used without a following type always means unsigned int.) Method Implementations Each method implementation is passed two hidden arguments: ■ The receiving object (self). ■ The selector for the method (_cmd). Within the implementation, both self and super refer to the receiving object. super replaces self as the receiver of a message to indicate that only methods inherited by the implementation should be performed in response to the message. Methods with no other valid return typically return void. 132 Method Declarations 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. APPENDIX A Language Summary Naming Conventions The names of files that contain Objective-C source code have the .m extension. Files that declare class and category interfaces or that declare protocols have the .h extension typical of header files. Class, category, and protocol names generally begin with an uppercase letter; the names of methods and instance variables typically begin with a lowercase letter. The names of variables that hold instances usually also begin with lowercase letters. In Objective-C, identical names that serve different purposes don’t clash. Within a class, names can be freely assigned: ■ A class can declare methods with the same names as methods in other classes. ■ A class can declare instance variables with the same names as variables in other classes. ■ An instance method can have the same name as a class method. ■ A method can have the same name as an instance variable. ■ Method names beginning with “_”, a single underscore character, are reserved for use by Apple. Likewise, protocols and categories of the same class have protected name spaces: ■ A protocol can have the same name as a class, a category, or anything else. ■ A category of one class can have the same name as a category of another class. However, class names are in the same name space as global variables and defined types. A program can’t have a global variable with the same name as a class. Naming Conventions 133 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. APPENDIX A Language Summary 134 Naming Conventions 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. APPENDIX A Language Summary This appendix presents a formal grammar for the Objective-C extensions to the C language—as the Objective-C language is implemented for the Cocoa development environment. It adds to the grammar for ANSI standard C found in Appendix A of The C Programming Language (second edition, 1988) by Brian W. Kernighan and Dennis M. Ritchie, published by Prentice Hall, and should be read in conjunction with that book. The Objective-C extensions introduce some new symbols (such as class interface), but also make use of symbols (such as function definition) that are explained in the standard C grammar. The symbols mentioned but not explained here are listed below: compound-statement constant declaration declaration-list enum-specifier expression function-definition identifier parameter-type-list string struct-declaration-list struct-or-union typedef-name type-name Of these, identifier and string are undefined terminal symbols. Objective-C adds no undefined terminal symbols of its own. Two notational conventions used here differ from those used in The C Programming Language: ■ Literal symbols are shown in code font. ■ Brackets enclose optional elements. Otherwise, this appendix attempts to follow the conventions of The C Programming Language. Each part of the grammar consists of a symbol and a colon in bold and a list of mutually-exclusive possibilities for expanding the symbol. For example: 135 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. APPENDIX B Grammar receiver: expression class-name super However, there is an exception: Even though they’re not mutually exclusive, the constituents of classes, categories, protocols, and blocks are listed on separate lines to clearly show the ordering of elements. For example: protocol-declaration: @protocol protocol-name [ protocol-reference-list ] [ interface-declaration-list ] @end This exception to the general rule is easily recognized since each list terminates with @end or the symbol name ends with “-block”. There are six entry points where the Objective-C language modifies the rules defined for standard C: ■ External declarations ■ Type specifiers ■ Type qualifiers ■ Primary expressions ■ Exceptions ■ Synchronization This appendix is therefore divided into six sections corresponding to these points. Where a rule in the standard C grammar is modified by an Objective-C extension, the entire rule is repeated in its modified form. External Declarations external-declaration: function-definition declaration class-interface class-implementation category-interface category-implementation protocol-declaration protocol-declaration-list class-declaration-list class-interface: @interfaceclass-name [ : superclass-name ] 136 External Declarations 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. APPENDIX B Grammar [ protocol-reference-list ] [ instance-variables ] [ interface-declaration-list ] @end class-implementation: @implementation class-name [ : superclass-name ] [ implementation-definition-list ] @end category-interface: @interface class-name ( category-name ) [ protocol-reference-list ] [ interface-declaration-list ] @end category-implementation: @implementation class-name ( category-name ) [ implementation-definition-list ] @end protocol-declaration: @protocol protocol-name [ protocol-reference-list ] [ interface-declaration-list ] @end protocol-declaration-list: @protocol protocol-list ; class-declaration-list: @class class-list ; class-list: class-name class-list, class-name protocol-reference-list: < protocol-list > protocol-list: protocol-name protocol-list, protocol-name class-name: identifier superclass-name: identifier category-name: identifier protocol-name: identifier instance-variables: { instance-variable-declaration } instance-variable-declaration: External Declarations 137 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. APPENDIX B Grammar visibility-specification struct-declaration-list instance-variables instance-variable-declaration visibility-specification instance-variable-declaration struct-declaration-list instance-variables visibility-specification: @private @protected @package @public interface-declaration-list: declaration method-declaration interface-declaration-list declaration interface-declaration-list method-declaration method-declaration: class-method-declaration instance-method-declaration class-method-declaration: + [ method-type ] method-selector; instance-method-declaration: – [ method-type ] method-selector; implementation-definition-list: function-definition declaration method-definition implementation-definition-list function-definition implementation-definition-list declaration implementation-definition-list method-definition method-definition: class-method-definition instance-method-definition class-method-definition: + [ method-type ] method-selector [ declaration-list ] compound-statement instance-method-definition: – [ method-type ] method-selector [ declaration-list ] compound-statement method-selector: unary-selector keyword-selector [ , ... ] keyword-selector [ , parameter-type-list ] unary-selector: selector keyword-selector: keyword-declarator keyword-selector keyword-declarator keyword-declarator: 138 External Declarations 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. APPENDIX B Grammar : [ method-type ] identifier selector: [ [method-type] method-type ] identifier selector: identifier method-type: ( type-name ) Type Specifiers type-specifier: void char short int long float double signed unsigned id [ protocol-reference-list ] class-name [ protocol-reference-list ] struct-or-union-specifier enum-specifier typedef-name struct-or-union-specifier: struct-or-union [ identifier ] { struct-declaration-list } struct-or-union identifier Type Qualifiers type-qualifier: const volatile protocol-qualifier protocol-qualifier: in out inout bycopy byref oneway Type Specifiers 139 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. APPENDIX B Grammar Primary Expressions primary-expression: identifier constant string ( expression ) self message-expression selector-expression protocol-expression encode-expression message-expression: [ receiver message-selector ] receiver: expression class-name super message-selector: selector keyword-argument-list keyword-argument-list: keyword-argument keyword-argument-list keyword-argument keyword-argument: selector : expression : expression selector-expression: @selector ( selector-name ) selector-name: selector keyword-name-list keyword-name-list: keyword-name keyword-name-list keyword-name keyword-name: selector: : protocol-expression: @protocol ( protocol-name ) encode-expression: @encode ( type-name ) 140 Primary Expressions 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. APPENDIX B Grammar Exceptions exception-declarator: declarator try-statement: @trystatement catch-statement: @catch (exception-declarator)statement finally-statement: @finallystatement throw-statement: @throw (identifier) try-block: try-statement catch-statement [ finally-statement ] Synchronization synchronized-statement: @synchronized ( identifier ) statement Exceptions 141 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. APPENDIX B Grammar 142 Synchronization 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. APPENDIX B Grammar abstract class A class that’s defined solely so that other classes can inherit from it. Programs don’t use instances of an abstract class, only of its subclasses. abstract superclass Same as abstract class. adopt In the Objective-C language, a class is said to adopt a protocol if it declares that it implements all the methods in the protocol. Protocols are adopted by listing their names between angle brackets in a class or category declaration. anonymous object An object of unknown class. The interface to an anonymous object is published through a protocol declaration. Application Kit A Cocoa framework that implements an application's user interface. The Application Kit provides a basic program structure for applications that draw on the screen and respond to events. archiving The process of preserving a data structure, especially an object, for later use. An archived data structure is usually stored in a file, but it can also be written to memory, copied to the pasteboard, or sent to another application. In Cocoa, archiving involves writing data to an NSData object. asynchronous message A remote message that returns immediately, without waiting for the application that receives the message to respond. The sending application and the receiving application act independently, and are therefore not “in sync.” See also synchronous message. category In the Objective-C language, a set of method definitions that is segregated from the rest of the class definition. Categories can be used to split a class definition into parts or to add methods to an existing class. class In the Objective-C language, a prototype for a particular kind of object. A class definition declares instance variables and defines methods for all members of the class. Objects that have the same types of instance variables and have access to the same methods belong to the same class. See also class object. class method In the Objective-C language, a method that can operate on class objects rather than instances of the class. class object In the Objective-C language, an object that represents a class and knows how to create new instances of the class. Class objects are created by the compiler, lack instance variables, and can’t be statically typed, but otherwise behave like all other objects. As the receiver in a message expression, a class object is represented by the class name. Cocoa An advanced object-oriented development platform on Mac OS X. Cocoa is a set of frameworks with programming interfaces in both Java and Objective-C. compile time The time when source code is compiled. Decisions made at compile time are constrained by the amount and kind of information encoded in source files. conform In the Objective-C language, a class is said to conform to a protocol if it (or a superclass) implements the methods declared in the protocol. An instance conforms to a protocol if its class does. 143 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. Glossary Thus, an instance that conforms to a protocol can perform any of the instance methods declared in the protocol. content view In the Application Kit, the NSView object that’s associated with the content area of a window—all the area in the window excluding the title bar and border. All other views in the window are arranged in a hierarchy beneath the content view. delegate An object that acts on behalf of another object. designated initializer The init... method that has primary responsibility for initializing new instances of a class. Each class defines or inherits its own designated initializer. Through messages to self, other init... methods in the same class directly or indirectly invoke the designated initializer, and the designated initializer, through a message to super, invokes the designated initializer of its superclass. dispatch table Objective-C runtime table that contains entries that associate method selectors with the class-specific addresses of the methods they identify. distributed objects Architecture that facilitates communication between objects in different address spaces. dynamic allocation Technique used in C-based languages where the operating system provides memory to a running application as it needs it, instead of when it launches. dynamic binding Binding a method to a message—that is, finding the method implementation to invoke in response to the message—at runtime, rather than at compile time. dynamic typing Discovering the class of an object at runtime rather than at compile time. encapsulation Programming technique that hides the implementation of an operation from its users behind an abstract interface. This allows the implementation to be updated or changed without impacting the users of the interface. event The direct or indirect report of external activity, especially user activity on the keyboard and mouse. factory Same as class object. factory method Same as class method. factory object Same as class object. formal protocol In the Objective-C language, a protocol that’s declared with the @protocol directive. Classes can adopt formal protocols, objects can respond at runtime when asked if they conform to a formal protocol, and instances can be typed by the formal protocols they conform to. framework A way to package a logically-related set of classes, protocols and functions together with localized strings, on-line documentation, and other pertinent files. Cocoa provides the Foundation framework and the Application Kit framework, among others. Frameworks are sometimes referred to as “kits.” gdb The standard Mac OS X debugging tool. id In the Objective-C language, the general type for any kind of object regardless of class. id is defined as a pointer to an object data structure. It can be used for both class objects and instances of a class. implementation Part of an Objective-C class specification that defines its implementation. This section defines both public methods as well as private methods—methods that are not declared in the class’s interface. informal protocol In the Objective-C language, a protocol declared as a category, usually as a category of the NSObject class. The language gives explicit support to formal protocols, but not to informal ones. inheritance In object-oriented programming, the ability of a superclass to pass its characteristics (methods and instance variables) on to its subclasses. inheritance hierarchy In object-oriented programming, the hierarchy of classes that’s defined by the arrangement of superclasses and 144 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. GLOSSARY subclasses. Every class (except root classes such as NSObject) has a superclass, and any class may have an unlimited number of subclasses. Through its superclass, each class inherits from those above it in the hierarchy. instance In the Objective-C language, an object that belongs to (is a member of) a particular class. Instances are created at runtime according to the specification in the class definition. instance method In the Objective-C language, any method that can be used by an instance of a class rather than by the class object. instance variable In the Objective-C language, any variable that’s part of the internal data structure of an instance. Instance variables are declared in a class definition and become part of all objects that are members of or inherit from the class. interface Part of an Objective-C class specification that declares its public interface, which include its superclass name, instances variables, and public-method prototypes. Interface Builder A tool that lets you graphically specify your application’s user interface. It sets up the corresponding objects for you and makes it easy for you to establish connections between these objects and your own code where needed. introspection The ability of an object to reveal information about itself as an object—such as its class and superclass, the messages it can respond to, and the protocols it conforms to. key window The window in the active application that receives keyboard events and is the focus of user activity. link time The time when files compiled from different source modules are linked into a single program. Decisions made by the linker are constrained by the compiled code and ultimately by the information contained in source code. localize To adapt an application to work under various local conditions—especially to have it use a language selected by the user. Localization entails freeing application code from language-specific and culture-specific references and making it able to import localized resources (such as character strings, images, and sounds). For example, an application localized in Spanish would display “Salir” in the application menu. In Italian, it would be “Esci,” in German “Verlassen,” and in English “Quit.” main event loop The principal control loop for applications that are driven by events. From the time it’s launched until the moment it’s terminated, an application gets one keyboard or mouse event after another from the Window Manager and responds to them, waiting between events if the next event isn’t ready. In the Application Kit, the NSApplication object runs the main event loop. menu A small window that displays a list of commands. Only menus for the active application are visible on-screen. message In object-oriented programming, the method selector (name) and accompanying arguments that tell the receiving object in a message expression what to do. message expression In object-oriented programming, an expression that sends a message to an object. In the Objective-C language, message expressions are enclosed within square brackets and consist of a receiver followed by a message (method selector and arguments). method In object-oriented programming, a procedure that can be executed by an object. multiple inheritance In object-oriented programming, the ability of a class to have more than one superclass—to inherit from different sources and thus combine separately-defined behaviors in a single class. Objective-C doesn’t support multiple inheritance. mutex Also known as mutual exclusion semaphore. Used to synchronize thread execution. name space A logical subdivision of a program within which all names must be unique. Symbols in one name space won’t conflict with identically named symbols in another name space. For example, in Objective-C, the instance methods of each class are in a separate name space, as are the class methods and instance variables. 145 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. GLOSSARY nil In the Objective-C language, an object id with a value of 0. object A programming unit that groups together a data structure (instance variables) and the operations (methods) that can use or affect that data. Objects are the principal building blocks of object-oriented programs. outlet An instance variable that points to another object. Outlet instance variables are a way for an object to keep track of the other objects to which it may need to send messages. polymorphism In object-oriented programming, the ability of different objects to respond, each in its own way, to the same message. procedural programming language A language, like C, that organizes a program as a set of procedures that have definite beginnings and ends. protocol In the Objective-C language, the declaration of a group of methods not associated with any particular class. See also formal protocol and informal protocol. receiver In object-oriented programming, the object that is sent a message. reference counting Memory-management technique in which each entity that claims ownership of an object increments the object’s reference count and later decrements it. When the object’s reference count reaches zero, the object is deallocated. This technique allows one instance of an object to be safely shared among several other objects. remote message A message sent from one application to an object in another application. remote object An object in another application, one that’s a potential receiver for a remote message. runtime The time after a program is launched and while it’s running. Decisions made at runtime can be influenced by choices the user makes. selector In the Objective-C language, the name of a method when it’s used in a source-code message to an object, or the unique identifier that replaces the name when the source code is compiled. Compiled selectors are of type SEL. static typing In the Objective-C language, giving the compiler information about what kind of object an instance is, by typing it as a pointer to a class. subclass In the Objective-C language, any class that’s one step below another class in the inheritance hierarchy. Occasionally used more generally to mean any class that inherits from another class, and sometimes also used as a verb to mean the process of defining a subclass of another class. superclass In the Objective-C language, a class that’s one step above another class in the inheritance hierarchy; the class through which a subclass inherits methods and instance variables. surrogate An object that stands in for and forwards messages to another object. synchronous message A remote message that doesn’t return until the receiving application finishes responding to the message. Because the application that sends the message waits for an acknowledgment or return information from the receiving application, the two applications are kept “in sync.” See also asynchronous message. 146 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. GLOSSARY This table describes the changes to The Objective-C 2.0 Programming Language. NotesDate Extended the discussion of properties to include mutable objects.2008-02-05 Corrected minor errors.2007-12-11 Provided an example of fast enumeration for dictionaries and enhanced the description of properties. 2007-10-31 Added references to documents describing new features in Objective-C 2.2007-07-22 Corrected minor typographical errors.2007-03-26 Clarified the discussion of sending messages to nil.2007-02-08 Clarified the description of Code Listing 3-3.2006-12-05 Moved the discussion of memory management to "Memory Management Programming Guide for Cocoa." 2006-05-23 Corrected minor typographical errors.2006-04-04 Corrected minor typographical errors.2006-02-07 Clarified use of the static specifier for global variables used by a class.2006-01-10 Clarified effect of sending messages to nil; noted use of ".mm" extension to signal Objective-C++ to compiler. 2005-10-04 Corrected typo in language grammar specification and modified a code example. 2005-04-08 Corrected the grammar for the protocol-declaration-list declaration in “External Declarations” (page 136). Clarified example in Listing 11-1 (page 97). 147 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. REVISION HISTORY Document Revision History NotesDate Removed function and data structure reference. Added exception and synchronization grammar. Made technical corrections and minor editorial changes. 2004-08-31 Moved function and data structure reference to Objective-C 2.0 Runtime Reference. Added examples of thread synchronization approaches to “Synchronizing Thread Execution”. Clarified when the initialize method is called and provided a template for its implementation in “Initializing a Class Object”. Added exception and synchronization grammar to “Grammar”. Replaced conformsTo: with conformsToProtocol: throughout document. Corrected typos in “An exception handler”.2004-02-02 Corrected definition of id.2003-09-16 Documented the Objective-C exception and synchronization support available in Mac OS X version 10.3 and later in “Exception Handling and Thread Synchronization”. 2003-08-14 Documented the language support for concatenating constant strings in “Compiler Directives” (page 128). Moved “Memory Management” before “Retaining Objects”. Corrected the descriptions for the Ivar structure and the objc_ivar_list structure. Changed the font of function result in class_getInstanceMethod and class_getClassMethod. Corrected definition of the term conform in the glossary. Corrected definition of method_getArgumentInfo. Renamed from Inside Mac OS X: The Objective-C Programming Language to The Objective-C Programming Language. Documented the language support for declaring constant strings. Fixed several typographical errors. Added an index. 2003-01-01 Mac OS X 10.1 introduces a compiler for Objective-C++, which allows C++ constructs to be called from Objective-C classes, and vice versa. 2002-05-01 Added runtime library reference material. Fixed a bug in the Objective-C language grammar’s description of instance variable declarations. 148 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. REVISION HISTORY Document Revision History NotesDate Updated grammar and section names throughout the book to reduce ambiguities, passive voice, and archaic tone. Restructured some sections to improve cohesiveness. Renamed from Object Oriented Programming and the Objective-C Language to Inside Mac OS X: The Objective-C Programming Language. 149 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. REVISION HISTORY Document Revision History 150 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. REVISION HISTORY Document Revision History Symbols + (plus sign) before method names 32 - (minus sign) before method names 32 // marker comment 128 @"" directive (string declaration) 129 _cmd 81, 90, 132 __cplusplus preprocessor constant 99 __OBJC__ preprocessor constant 99 A abstract classes 23, 63 action messages 80 adaptation 21 adopting a protocol 66, 131 alloc method 26, 105 allocating memory 110 allocWithZone: method 105 anonymous objects 63 argument types and dynamic binding 89 and selectors 89 declaring 32 encoding 123 in declarations 132 arguments during initialization 106 hidden 81, 132 in remote messages 120 specifying 17 type modifiers 121 variable 17 B behaviors inheriting 115 of Cocoa objects 118 of NSObject class 104 overriding 122 BOOL data type 128 bycopy type qualifier 125, 132 byref type qualifier 125, 132 C .c extension 12 C language support 11 C++ language support 97–101 @catch() directive 91, 92, 129 categories 41–43 See also subclasses and informal protocols 65 declaration of 41–42 declaring 130 defining 130 implementation of 41–42 loading dynamically 117 naming conventions 133 of root classes 43 scope of variables 42 uses of 42, 65 Class data type 25, 127 @class directive 33, 129 class method 104 class methods and selectors 78 and static variables 28 declaration of 32, 132 defined 25 of root class 29 using self 85 class object defined 20 initializing 28 class objects 25–29 and root class 29 and root instance methods 29, 43 151 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. Index and static typing 29 as receivers of messages 30 variables and 27 classes 20–30 root. See root classes abstract 23 and inheritance 20, 22 and instances 20 and namespaces 133 declaring 32–34, 130, 133 defining 31–39, 130 designated initializer of 108 examples 15 identifying 16 implementation of 31, 34 instance methods 25 interfaces 31 introspection 16, 24 loading dynamically 117 naming conventions 133 subclasses 20 superclass 20 uses of 29 comment marker (//) 128 compiler directives, summary of 128 conforming to protocols 61 conformsToProtocol: method 104, 119 const type qualifier 125 conventions of this book 13 customization with class objects 26–27 D data members. See instance variables data structures. See instance variables data types defined by Objective-C 127 designated initializer 108–110 development environment 12 directives, summary of 128–129 dispatch tables 76 dispatching messages 75 distributed objects 118 doesNotRecognizeSelector: method 113 dynamic binding 19–20 dynamic loading 117 dynamic typing 16 E @encode() directive 123, 129 encoding methods 123 @end directive 32, 34, 128 exceptions 91–93 catching 92 clean-up processing 93 compiler switch 91, 95 exception handler 92 nesting exception handlers 93 NSException 91, 92 synchronization 96 system requirements 91, 95 throwing 92, 93 F @finally directive 91, 93, 129 formal protocols 64, 131 See also protocols forwarding messages 112–116 forwardInvocation: method 112 G GNU Compiler Collection 12 H .h extension 31, 133 hidden arguments 81, 132 I id data type 127 and method declarations 132 and static typing 24, 88 as default method return type 32 of class objects 25 overview 15 IMP data type 127 @implementation directive 34, 128 implementation files 31, 35 implementation of classes 34–39, 130 of methods 35, 132 #import directive 33, 128 in type qualifier 125, 131 #include directive 33 152 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. INDEX #include directive See #import directive informal protocols 65 See also protocols inheritance 20–23 and forwarding 115 of instance variables 106 of interface files 33 init method 26, 105 initialize method 28 initializing objects 84, 110 inout type qualifier 125, 132 instance methods 25 and selectors 78 declaration of 132 declaring 32 naming conventions 133 syntax 32 instance variables declaring 22, 32, 129 defined 15 encapsulation 37 inheriting 22, 38 initializing 106 naming conventions 133 of the receiver 18 public access to 129 referring to 35 scope of 15, 36–39, 129 instances of a class allocating 104–105 creating 26 defined 20 initializing 26, 104–111 instances of the class See also objects @interface directive 32, 128, 130 interface files 33, 130 introspection 16, 24 isa instance variable 16, 26, 77 isKindOfClass: method 24, 30, 104 isMemberOfClass: method 24, 104 M .m extension 12, 31, 133 memory allocating 105, 110 message expressions 17, 127 message receivers 127 messages 75–81 See also methods and selectors 19 and static typing 88 asynchronous 120 binding 88 defined 17, 127 encoding 123 forwarding 112–116 remote 117 sending 17, 18 synchronous 120 syntax 127 varying at runtime 20, 79 messaging 75–85 avoiding errors 80 to remote objects 117–122 metaclass object 26 method implementations 35, 132 methodForSelector: method 90, 104 methods 15 See also behaviors See also messages adding with categories 41 address of 90 and selectors 19, 78 and variable arguments 33 argument types 79 arguments 89, 106 calling super 106 class methods 25 declaring 32, 132 encoding 123 hidden arguments 132 implementing 35, 132 inheriting 22 instance methods 25 naming conventions 133 overriding 23 return types 79, 89 returning values 16, 17 selecting 19 specifying arguments 17 using instance variables 35 minus sign (-) before method names 32 .mm extension 12 multiple inheritance 113 N name spaces 133 naming conventions 133 Nil constant 128 nil constant 16, 128 NO constant 128 153 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. INDEX NSBundle 117 NSClassFromString function 30 NSException 91, 92 NSInvocation 112 NSObject 20, 21, 104 NSSelectorFromString function 78 NSStringFromSelector function 78 O objc_loadModules function 117 objc_msgSend function 75, 77 objc_object structure 76 object 15 object identifiers 15 Objective-C 11 Objective-C++ 97 objects 15, 20 See also class objects, Protocol objects allocating memory for 105 anonymous 63 creating 26 customizing 26 designated initializer 108 dynamic typing 16, 87 examples 15 initializing 26, 84, 105, 110 initializing a class object 28 instance variables 18 introspection 24 method inheritance 22 Protocol 66 remote 63 static typing 87 surrogate 114–115 oneway type qualifier 125, 131 out type qualifier 125, 132 overriding methods 23 P parameters. See arguments performSelector: method 79 performSelector:withObject: method 79 performSelector:withObject:withObject: method 79 plus sign (+) before method names 32 polymorphism defined 19 precompiled headers 33 preprocessor directives, summary of 128 @private directive 37, 129 procedures. See methods @protected directive 37, 129 @protocol directive 70, 119, 128, 129, 131 Protocol objects 66 protocols 61–70 adopting 68, 131 conforming to 61, 67, 68 declaring 61, 131 formal 64 forward references to 70, 131 incorporating other protocols 68–69, 131 informal 65 naming conventions 133 type checking 68 uses of 61–70 proxy objects 114, 118 @public directive 37, 129 R receivers of messages and class names 30 defined 127 in messaging expression 17 in messaging function 75 instance variables of 18 remote messages 117 remote objects 63 remote procedure calls (RPC) 120 respondsToSelector: method 80, 104 return types and messaging 79 and statically typed objects 89 declaration of 32 encoding 123 root classes See also NSObject and class interface declarations 32 and inheritance 20 categories of 43 declaration of 130 runtime system functions 104 overview 103 S SEL data type 78, 127 154 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. INDEX @selector() directive 78, 129 selectors 78 and hidden arguments 81 and messaging errors 80 defined 19 in messaging function 75 self 81, 82, 85, 90, 132 Smalltalk 11 specialization 21 static type checking 88 static typing 87–90 and instance variables 36 in interface files 34 introduced 24 to inherited classes 89 strings, declaring 129 subclasses 20 super variable 82, 84, 132 superclasses See also abstract classes and inheritance 20 importing 33 surrogate objects 114 synchronization 95–96 compiler switch 91, 95 exceptions 96 mutexes 95 system requirements 91, 95 @synchronized() directive 95, 129 T target-action paradigm 79–80 targets 80 this keyword 100 @throw directive 91, 92, 93, 129 @try directive 91, 129 type checking class types 87, 88 protocol types 68 type encoding 123–125 type introspection 24 types defined by Objective-C 127 U unsigned int data type 132 V variable arguments 33 void data type 132 Y YES constant 128 155 2008-02-05 | © 2008 Apple Inc. All Rights Reserved. INDEX





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