CSC 213 Code Chapter 10, Fall 2007

Virtual Functions and Polymorphism 1 Virtual Function and Polymorphism allow the design and implementation of systems that are easily extensible… Programs written to generically process objects of all existing classes in a hierarchy… This can be achieved by taking the advantage of the “pointer to derived class” compatibility with a “pointer to its base class”… <Pointer-to-base-class> = <Pointer-to-derived-class> <Pointer-to-base-class> = & <Object-of-derived-class> As seen earlier, a pointer to the base-class can point to any object of its derived classes without the need of explicit casting… The same rule applies for a base-class reference to a derived object… Virtual Functions and Polymorphism 2 LLeettss ccoonnssiiddeerr tthhee ffoolllloowwiinngg sscceennaarriioo… … #include <iostream.h> class pet { public: void speak() {cout << "can't speak \n";} }; class dog : public pet { public: void speak() { cout << "woofffff! \n"; } }; class cat : public pet { public: void speak() { cout << "meawwwww! \n"; } }; class bird : public pet { public: void speak() { cout << "squeekkk! \n"; } }; void main() { dog spike; cat tom; bird tweety; pet *cage= &tom ; spike.speak(); tweety.speak(); tom.speak(); cage->speak();[j1] static_cast<bird*>(cage)->speak();[j2] } Virtual Functions and Polymorphism 3 W Whhaatt aabboouutt tthhiiss sscceennaarriioo ??????… … void main() { pet *cage[3]; cage[0] = new dog; cage[1] = new cat; cage[2] = new bird; cage[1]->speak();[j3] static_cast<bird*>(cage[1])->speak();[j4] } Virtual Functions and Polymorphism 4 AA ssoolluuttiioonn !!!!!!… … Virtual Functions #include <iostream.h> class pet { public: virtual[JAN5] void speak() {cout << "can't speak \n";} }; class dog : public pet { public: void speak() { cout << "woofffff! \n"; } }; class cat : public pet { public: void speak() { cout << "meawwwww! \n"; } }; class bird : public pet { public: void speak() { cout << "squeekkk! \n"; } }; void sayIt(pet &pRf, pet *pPr) { pRf.speak(); // Dynamic binding.[JAN6] pPr->speak(); // Dynamic binding.[JAN7] } Virtual Functions and Polymorphism 5 void main() { bird tweety; pet *cage[3], &pRef_1 = tweety; cage[0] = new dog; cage[1] = new cat; cage[2] = new bird; cage[2]->speak(); // Dynamic binding.[JAN8] cage[0]->speak(); cage[1]->speak(); pRef_1.speak(); // Dynamic binding.[JAN9] tweety.speak(); // Static binding.[JAN10] pet &pRef_2 = *cage[1]; pRef_2.speak(); // Dynamic binding.[JAN11] sayIt(*cage[0], cage[2]); } • By declaring function speak as vviirrttuuaall, and by using: ! a base-class pointer that points to a derived-class object… ! a base-class reference that references a derived class object… the program will chose the correct derived class speak dynamically at run-time… Dynamic binding… • When a virtual function is called by the object name using the “dot-operator”… the function called is the one defined for (or inherited by) the class of that particular object… Static binding… Virtual Functions and Polymorphism 6 M Moorree rreeaalliissttiicc sscceennaarriioo… … #include <iostream.h> class Polygon { public: void setPoly (int a, int b) { width = a; height = b; } virtual[JAN12] int area() {return 0;} protected: int width, height; }; class Rectangle: public Polygon { public: int area () { return (width * height); } }; class Triangle: public Polygon { public: int area() {return (width * height / 2);} }; void main () { Rectangle rect; Polygon *polyPtr[2], &polyRef = rect ; polyPtr[0] = new Rectangle; polyPtr[1] = new Triangle; polyPtr[0]->setPoly(4,5); polyPtr[1]->setPoly(2,50); polyRef.setPoly(1,30); cout << polyPtr[0]->area()[JAN13] << endl; cout << polyPtr[1]->area() << endl; cout << polyRef.area()[JAN14] << endl; } Virtual Functions and Polymorphism 7 Abstract and Concrete Classes • Abstract classes: ! Sole purpose is to provide a base class for other classes… Too generic to define real objects… ! No objects of an abstract base class can be instantiated… Syntax error… ! Can have pointers and references… • Concrete classes: ! Classes that can instantiate objects… ! Provide specifics to make real objects… Square, Circle, etc… • A class is made aabbssttrraacctt by declaring one or more of its virtual function to be “pure”… A pure virtual function is one with an initializer of =0 in its declarations… class Polygon { public: void setPoly (int a, int b) { width = a; height = b; } virtual int area() =0; virtual void print() const;[JAN15] protected: int width, height; }; TwoDimensionalShape Circle Square Triangle Virtual Functions and Polymorphism 8 #include <iostream.h> class Polygon { public: void setPoly (int a, int b) { width = a; height = b; } virtual int area() =0; virtual void print()=0; protected: int width, height; }; class Rectangle: public Polygon { public: int area () { return (width * height); } void print() { cout << width << endl; cout << height << endl; } }; class Triangle : public Polygon[JAN16] { public: int area() {return (width * height / 2);} }; void main () { Polygon poly;[JAN17] Rectangle rect; Triangle trgl;[JAN18] } Syntax Error… Virtual Functions and Polymorphism 9 Polymorphism • From the Greek: PPoollyy # M Maannyy M Moorrpphh # SShhaappee • In OOP: ! Ability for objects of different classes (related by inheritance) to respond differently to the same function call… • PPoollyym moorrpphhiissm m is implemented via vviirrttuuaall functions… ! When a request is made trough a base class pointer (or reference) to use a virtual function, C++ chooses the correct overridden function in the appropriate derived class associated with the object… • Needless to say that nnoonn--vviirrttuuaall function behave in the same manner discussed in the inheritance chapter… ! If non-virtual member function defined in multiple classes and called from base-class pointer then the base-class version is used… ! If called from derived-class pointer then derived- class version is used… ! NNoonn--ppoollyym moorrpphhiicc behavior… Virtual Functions and Polymorphism 10 #include <iostream.h> class Polygon { public: void setPoly (int a, int b) { width = a; height = b; } virtual int area() { return 0;} void print() const { cout << "Width:" << width << " - " << "Height:" << height << endl; } protected: int width, height; }; class Rectangle: public Polygon { public: int area() {return (width * height);} }; class Triangle: public Polygon { public: int area() {return (width * height / 2);} void print() const { cout << "Base:" << width << " - " << "Height:" << height << endl; } }; Virtual Functions and Polymorphism 11 void main() { Rectangle rect; Triangle trgl; Polygon *polyPtr, &polyRefR = rect, &polyRefT = trgl; rect.setPoly(10,2); trgl.setPoly(30,3); rect.print(); trgl.print(); cout << rect.area() << endl; cout << trgl.area() << endl; polyPtr = &trgl; polyPtr->print(); cout << polyPtr->area() << endl; polyPtr = &rect; polyPtr->print(); cout << polyPtr->area() << endl; polyRefT.print(); cout << polyRefT.area() << endl; polyRefR.print(); cout << polyRefR.area() << endl; }[JAN19] Virtual Functions and Polymorphism 12 Case Study: A Payroll system using Polymorphism • Perform different payroll calculations for different types of employees (base class Employee): ! Boss - fixed weekly salary ! CommissionWorker - flat base salary + commission ! PieceWorker - paid according to items produced ! HourlyWorker - hourly wage + overtime Employee firstName lastName getFirstName( ) getLastName( ) earnings( ) print( ) Boss WeeklySalary setWeeklySalary( ) earnings( ) print( ) HourlyWorker wage hours setWage( ) setHours( ) earnings( ) print( ) PieceWorker wagePerPiece quantity setWage( ) setQuantity( ) earnings( ) print( ) CommissionWorker salary commission quantity setSalary( ) setCommission( ) setQuatity( ) earnings( ) print( ) Virtual Functions and Polymorphism 13 Virtual destructor • Recall #include <iostream.h> class A { public: A(int x = 0) { valA = x; cout << "Constructor: valA= " << valA <<endl; } ~A() { cout << "Destructor: valA= " << valA << endl; } private: int valA; }; void main() { A *aPtr, aObj(5); aPtr = new A[2]; delete [] aPtr; }[JAN20] • An object allocated using the new operator must be explicitly deallocated using the delete operator… It is at the deletion time that the destructors is called… Virtual Functions and Polymorphism 14 #include <iostream.h> class basA { public: basA (int a) { x = a; } virtual[JAN21] ~basA() { cout << "Base destructor... x = " << x << endl; } protected: int x; }; class drvA: public basA { public: drvA(int a, int b) : basA(a) {y = b;} int getA() {return y;} void setA(int a) { y = a;} ~drvA() { cout << "Derived destructor... y = " << y <<endl; } private: int y; }; void main() { basA *basPtr; drvA *drvPtr; drvPtr=new drvA(10, 20); delete drvPtr; drvPtr=new drvA(100, 200); basPtr = drvPtr; delete basPtr;[JAN22] } Virtual Functions and Polymorphism 15 • Problem: ! If a base-class pointer to a derived object is deleted, the base-class destructor will act on the object… • Solution: ! Declare a virtual base-class destructor to ensure that the appropriate destructor will be called Virtual Functions and Polymorphism 16 Case Study: Inheriting Interface and Implementation Shape virtual area( ) virtual volume( ) virtual printShapeName( ) virtual print( ) Point x y setPoint( ) getX( ) getY( ) printShapeName( ) print( ) Circle radius setRadius( ) getRadius( ) area( ) printShapeName( ) print( ) Cylinder height setheight( ) getheight( ) area( ) volume( ) printShapeName( ) print( ) Virtual Functions and Polymorphism 17 Polymorphism, virtual Functions and Dynamic Binding “Under the Hood” The fundamental idea behind polymorphism is that the compiler does not know which function to call compile-time…the appropriate function will be selected run-time…. A common implementation is the following… • An object containing virtual member functions holds as its first data member a hidden field, pointing to an array of pointers containing the addresses of the virtual member functions … • The hidden data member is usually called the vpointer… • The array of virtual member function addresses is called the vtable… • Final Note: ! Polymorphism requires some overhead… ! Polymorphism is not used in STL (Standard Template Library) to optimize performance… point circle printShapeName print()