12 EECS 280 Polymorphism


EECS 280 Programming and Introductory Data Structures Polymorphism Slides by Andrew DeOrio 1 Review: classes and ADTs Recall our Triangle class (simplified here): class Triangle { public: Triangle(); Triangle(double a_in, double b_in,double c_in); double area() const; void print() const; private: double a,b,c; }; 2 Review: classes and ADTs The C++ class mechanism lets us create our own custom types We can use our new, custom type just like built-in types int main() { Triangle t(3,4,5); t.print(); Triangle *t_ptr = &t; t_ptr->print(); } 3 ./a.out Triangle: a=3 b=4 c=5 Triangle: a=3 b=4 c=5 class Triangle { //... void print() const { cout << "Triangle: " << "a=" << a << " b=" << b << " c=" << c << endl; } Review: derived types A derived type is used to represent an is a relationship A derived type inherits the member functions and member variables of its parent class Isosceles : public Triangle { // ... }; class Equilateral : public Isosceles { // ... }; An Iscescles is a Triangle An Equilateral is a Isosceles An Equilateral is a Triangle 4 Triangle Isosceles Equilateral Review: derived types Again, we can use our new, derived type just like built-in types Isosceles i(1,12); i.print(); Isosceles *i_ptr = &i; i_ptr->print(); 5 ./a.out Isosceles: base=1, leg=12 Isosceles: base=1, leg=12 class Isosceles : public Triangle { //... void print() const { cout << "Isosceles: " << base=" << get_a() << " leg=" << get_b() << endl; } } Review: subtypes If S is a subtype of T, then objects of type T may be replaced with objects of type S without altering any of the desirable properties of that program (correctness) Isosceles i(1,12); Isosceles Triangle *ptr = &i; ptr->print(); 6 Static type Often, the compiler can tell which derived type is needed This is called the static type Triangle t(3,4,5); t.print(); Triangle *t_ptr = &t; t_ptr->print(); Isosceles i(1,12); i.print(); Isosceles *i_ptr = &i; i_ptr->print(); 7 ./a.out Triangle: a=3, b=4, c=5 Triangle: a=3, b=4, c=5 Isosceles: base=1, leg=12 Isosceles: base=1, leg=12 Dynamic type Other times, the type is not known until run time This is called the dynamic type //EFFECTS: asks user to select Triangle, // Isosceles or Equilateral // returns a pointer to correct object Triangle * ask_user(); int main() { Triangle *t = ask_user(); //enters "Isosceles" t->print(); } What is the static type of t? What is the dynamic type? 8 Dynamic type static Triangle g_triangle(3,4,5); static Isosceles g_isosceles(1,12); static Equilateral g_equilateral(5); Triangle * ask_user() { cout << "Triangle, Isosceles or Equilateral? "; string s; cin >> s; if (s == "Triangle") return &g_triangle; if (s == "Isosceles") return &g_isosceles; if (s == "Equilateral") return &g_equilateral; cout << "Unrecognized shape `" << s << "'\n"; exit(1); //crash } 10 Returns pointer to global variable I wish we could create as many objects as the user asked for ... Forshadowing I wish we could create as many objects as the user asked for ... We will soon know how! Dynamic memory For now, we will use this global variable work-around static Triangle g_triangle(3,4,5); //... Triangle * ask_user() { //... if (s == "Triangle") return &g_triangle; //... } 11 Dynamic type //EFFECTS: asks user to select Triangle, // Isosceles or Equilateral // returns a pointer to correct object Triangle * ask_user(); int main() { Triangle *t = ask_user(); //enters "Isosceles" t->print(); } What is the output of this program? 12 Dynamic type //... Triangle *t = ask_user(); //"Isosceles" t->print(); Problem: the Triangle::print() function was called Because the static type of t is Triangle But we wanted the Isosceles::print() function instead Because the dynamic type of t is Isosceles 13 ./a.out Triangle, Isosceles or Equilateral? Isosceles Triangle: a=1 b=12 c=12 Polymorphism //... Triangle *t = ask_user(); //"Isosceles" t->print(); t can change types at runtime, in other words it is polymorphic We can use the virtual function mechanism in C++ to check the dynamic type of t at runtime and call the correct version of print() 14 Polymorphism Polymorphism is the ability to associate many behaviors with one function name dynamically (at runtime) A polymorphic type is any type with a virtual function Virtual functions are the C++ mechanism used to implement polymorphism 15 Polymorphism example class Triangle { virtual void print() const {/*...*/} //... }; virtual means "check the dynamic type at runtime, then select the correct print() member function" virtual is inherited, so the overridden print() in Isosceles and Equilateral will automatically become virtual 16 Triangle Isosceles Equilateral Polymorphism example class Triangle { virtual void print() const {/*...*/} //... }; class Isosceles { virtual void print() const {/*...*/} //... }; class Equilateral {//... virtual void print() const {/*...*/} //... }; Optionally add virtual keyword to derived types This is more clear 17 Triangle Isosceles Equilateral Dynamic type //... Triangle *t = ask_user(); //"Isosceles" t->print(); Now our program works correctly 18 ./a.out Triangle, Isosceles or Equilateral? Isosceles Isosceles: base=1 leg=12 Extending the class hierarchy Recall our Rectangle class (simplified here): class Rectangle { public: Rectangle(); Rectangle(double a_in, double b_in); double area() const; void print() const; private: double a,b; }; 19 Exercise Create a class hierarchy for Triangle, Isosceles, Equilateral and Rectangle. Draw it. 20 Triangle Isosceles Equilateral Rectangle Exercise A Rectangle is a Shape A Triangle is a Shape etc. Now: write the Shape class Member functions? Virtual? Member variables? Think: what if we wanted to add a Circle class? 21 Triangle Isosceles Equilateral Rectangle Shape Exercise class Shape { public: virtual double area() const {/*...*/} virtual void print() const {/*...*/} }; class Triangle : public Shape {/*...*/}; class Rectangle : public Shape {/*...*/}; All shapes have area() and a print() member functions Use virtual, so pointers to derived types will call the right version of area() and print() 22 Exercise class Shape { public: virtual double area() const {/*...*/} virtual void print() const {/*...*/} }; No member variables Not all shapes have common attributes, e.g., circle and triangle 23 Exercise This code is perfectly legal C++, but it makes no sense! Shape s; s.print(); A shape is an abstract idea Our shape should only be an interface to ensure that all shapes behave the same way 24 Abstract base class Problem: an instance of an abstract idea makes no sense Shape s; //what kind of shape??? Solution: abstract base classes Create an interface only class as a base class, from which an implementation can be derived You cannot create an instance of an abstract base class, which is exactly what we want for a Shape 25 Pure virtual functions Because there will be no implementation, we need to declare member functions in a special way: Declare each method as a virtual function “Assign" a 0 to each of these virtual functions. These are called pure virtual functions You can think of them as a set of function pointers, all of which point to NULL (AKA 0) class Shape { public: virtual double area() const = 0; virtual void print() const = 0; }; 26 Pure virtual functions Shape is now an abstract base class You cannot create an instance of an abstract base class Shape s; //compiler error You can create a pointer to an abstract base class, and then assign the pointer to a concrete class derived the base base This is subtyping at work Rectangle r(2,4); //concrete derived type Shape *s = &r; //OK, Rectangle is a Shape s->print(); //virtual, so correct version //of print() is called 27 ./a.out Rectangle: a=2 b=4 Pure virtual functions //EFFECTS: asks user to select a shape // returns a pointer to correct object Shape * ask_user(); int main() { Shape *s = ask_user(); //"Rectangle" s->print(); } Now we can expand the ask_user() function to work with any Shape 28 ./a.out Rectangle, Triangle, Isosceles or Equilateral? Rectangle Rectangle: a=2 b=4 Factory functions //EFFECTS: asks user to select a shape // returns a pointer to correct object Shape * ask_user(); ask_user() is an example of a factory function A factory function creates objects for a client who doesn't need to know their actual types You will use a factory function in project 4 Return blackjack players using different strategies: simple, card- counting, etc. 29 Upcast Shape *s = ask_user(); //"Isosceles" Substitute subtype (Isosceles) for supertype (Shape) This is called an upcast Type conversion is automatic, an implicit cast 30 Shape * ask_user() { cout << "Rectangle, Triangle, Isosceles or Equilateral? "; string s; cin >> s; if (s == "Triangle") return &g_triangle; if (s == "Isosceles") return &g_isosceles; //... } upcast Upcast Shape *s = ask_user(); //"Isosceles" Think of upcast as a cast from one type in the class hierarchy to a higher one. Since a Isosceles is a Shape, this cast can happen automatically. 31 Triangle Isosceles Equilateral Rectangle Shape Downcast Isosceles *t= ask_user();//enter "Isosceles" Can’t convert supertype (Shape) to subtype (Isosceles) Type conversion is not automatic This is called a downcast 32 error: invalid conversion from ‘Shape*’ to ‘Isosceles*’ Shape * ask_user(); Downcast Isosceles *t= ask_user();//enter "Isosceles" Think of downcast as a cast from one type in the class hierarchy to a lower one. Since a Shape might not be a Isosceles, this cast cannot happen automatically. 33 error: invalid conversion from ‘Shape*’ to ‘Isosceles*’ Shape * ask_user() Triangle Isosceles Equilateral Rectangle Shape Downcast Shape *s = ask_user(); //"Isosceles" Isosceles *i = dynamic_cast(s); if (i != 0){ //check for NULL pointer //something triangular double c = i->get_c(); } dynamic_cast(ptr) downcasts ptr to type T*, if possible. Otherwise, it returns 0. In this example, if the user enters “Isoscles”, the call to  dynamic_cast will cast from Shape* to Isoscles* 34 Shape * ask_user(); Downcast Shape *s = ask_user(); //"Isosceles" Isosceles *i = dynamic_cast(s); if (i != 0){ //check for NULL pointer //something triangular } Always need to check a dynamic_cast for success dynamic_cast only works on polymorphic types. AKA classes with virtual functions 35 Shape * ask_user(); dynamic_cast vs. static_cast We have now seen two different kinds of cast dynamic_cast Checks at runtime if it is OK to cast Cast from a polymorphic base class to a derived class Supertype to subtype, downcast static_cast Does not check at runtime The programmer tells the compiler “trust me” Works with non-polymorphic types as well as polymorphic 36 Constructors and polymorphism Recall how constructors of derived classes work First, the base class constructor runs, then the derived class constructor runs, etc. In other words, instances of a derived class are constructed starting from the base class 37 Note: The constructor that is called automatically is the default constructor. If you want a non-default constructor, you must call it explicitly. Constructors and polymorphism int main () { Triangle t; } 38 class Shape { public: Shape() { cout << "Shape default ctor\n"; } //... }; class Triangle : Shape { public: Triangle() { cout << "Triangle default ctor\n"; } //... }; ./a.out Shape default ctor Triangle default ctor Constructors and polymorphism class Triangle : public Shape { //... Triangle() : a(0), b(0), c(0) { cout << "Triangle default ctor\n"; } Triangle(double a_in, double b_in, double c_in) : a(a_in), b(b_in), c(c_in) { cout << "Triangle double ctor\n"; } //... }; Add the same messages to Isosceles, Equilateral and Rectangle 39 Exercise: What is the output? int main() { Equilateral e; } int main() { Rectangle r; } 40 Exercise: What is the output? int main() { Isosceles i; Triangle *t_ptr = &i; } int main() { Isosceles i(1,12); Shape *s_ptr = &i; } 42 Exercise: What is the output? int main() { Rectangle r; Triangle *t_ptr = &r; } int main() { Isosceles i; Equilateral *e_ptr = &i; } 44 Exercise: What is the output? int main() { Equilateral e1(5); Equilateral e2(6); Shape *array[2]; array[0] = &e1; array[1] = &e2; array[0]->print(); array[1]->print(); } 46 Arrays and polymorphism Polymorphism gives us another cool feature: we can put (pointers to) objects of different types together in the same container Rectangle r(2,4); Isosceles i(1,12); Equilateral e(5); const int SIZE=3; Shape * shapes[SIZE]; shapes[0] = &r; shapes[1] = &i; shapes[2] = &e; 48 Exercise: write a for-loop to call print() on each Shape* in the array. Use traversal by pointer. Project 4 You now have everything you need for project four: blackjack! Code in this project: ADTs representing a deck of cards Three polymorphic blackjack player ADTs that use different strategies A game driver We will grade your code (as usual) We will also grade your testcases 51
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