COEN244: Polymorphism

Transcription

1 COEN244: Polymorphism Aishy Amer Electrical & Computer Engineering Polymorphism means variable behavior / ability to appear in many forms Outline Casting between objects Using pointers to access objects Static polymorphism: compile-time type and function checking Dynamic polymorphism: run-time type and function checking Virtual functions: can be over-ridden at run time Pure Virtual functions: must be over-ridden Abstract Classes: classes with at least one pure virtual function Virtual destructor c A. Amer Polymorphism 1

2 Casting Casting: conversion from one data type to another type c o n v e r t t o t y p e ( expression ) ; // Example : t = i n t ( x ) ; q=double ( y ) ; c A. Amer Polymorphism 2

3 Casting Casting: conversion from one data type to another type c o n v e r t t o t y p e ( expression ) ; // Example : t = i n t ( x ) ; q=double ( y ) ; Standard numerical conversions are used for integral promotions (e.g., enum to int) integral conversions (e.g., int to unsigned int) floating point conversions (e.g., float to double) floating-integral conversions (e.g., int to float) arithmetic conversions (e.g., converting operands to the type of the widest operand before evaluation) c A. Amer Polymorphism 2

4 Casting Implicit conversions: Some conversions are performed automatically by the compiler without intervention by the programmer Standard C++ conversions and user-defined conversions are performed implicitly by the compiler where needed Examples? Explicit conversions: Conversions which must be explicitly specified by the programmer c A. Amer Polymorphism 3

5 C++ explicit casting operators static cast: convert one type to another type can be used to reverse any of the implicit conversions (e.g., int to float) rely on static (compile-time) type information fast and safe c A. Amer Polymorphism 4

6 C++ explicit casting operators static cast: convert one type to another type can be used to reverse any of the implicit conversions (e.g., int to float) rely on static (compile-time) type information fast and safe dynamic cast: careful, type-sensitive casting at run time for safe navigation of an inheritance hierarchy reinterpret cast: type conversions on un-related types produces something that has the same bit pattern as the original const cast : (UNSAFE!!) cast away the const-ness or volatility of a type c A. Amer Polymorphism 4

7 C++ explicit casting operators All casting operators have the same syntax: static_cast convert_to_type ( expr ) ; / / ==> Convert expr to type convert_to_type static_cast <DerivedClass > ( PointerToBaseObj ) Use the least dangerous (most specific) casting alternative c A. Amer Polymorphism 5

8 C++ casting: examples const i n t x =5; const i n t px ; / / changeable p o i n t e r to constant i n t px = 3; / / i l l e g a l can t use px to modify an i n t px = &someotherintvar ; / / l e g a l px can p o i n t somewhere else i n t const py ; / / constant p o i n t e r to changeable i n t py = 4; / / l e g a l can use py to modify an i n t py = &someotherintvar ; / / i l l e g a l can t make py p o i n t anywhere else const i n t const pz ; / / const p o i n t e r to const i n t pz = 5; / / i l l e g a l can t use pz to modify an i n t pz = &someotherintvar ; / / i l l e g a l can t make pz p o i n t anywhere else class Person { public : / / r e t u r n a const char const ==> can t change the p o i n t e r to p o i n t / / somewhere else, and you can t modify what the p o i n t e r p o i n t s to const char const GetName ( ) { return m_szname ; } ;... c A. Amer Polymorphism 6

9 C++ casting: examples i n t x = 37; i n t y = 8; double q = x / y ; / / c l a s s i c mistake, r e s u l t i s rounded to an i n t cout << q ; / / p r i n t s " " double q = ( double ) x / y ; / / cast r e s u l t as double so i t s not rounded cout << q ; / / p r i n t s " " / / Bas c a s t i n g : f o r c e the compiler to put the address of / / a const i n t v a r i a b l e i n t o a normal i n t const i n t x = 4; / / x i s const, i t can t be modified const i n t px = &x ; / / you can t modify x through the px p o i n t e r cout << x << endl ; / / p r i n t s " 4 " i n t px2 = ( i n t )px ; / / e x p l i c i t l y cast px as an i n t px2 = 3; / / r e s u l t i s undefined cout << x << endl ; / / who knows what i t p r i n t s? / / The code compiles and runs w i t h o u t crashing, but who knows what i s x c A. Amer Polymorphism 7

10 C++ casting: examples / / The Const_cast Operator const i n t x = 4; / / x i s const, i t can t be modified const i n t px = &x ; / / you can t modify x through the px p o i n t e r cout << x << endl ; / / p r i n t s " 4 " i n t px2 = const_cast < i n t > ( px ) ; / / e x p l i c i t l y cast px as non const px2 = 3; / / r e s u l t i s undefined cout << x << endl ; / / who knows what i t p r i n t s? / / With const_cast : change the const ness of a v a r i a b l e, and never i t s type c A. Amer Polymorphism 8

11 Object casting Object casting: conversion of the type of an object to that of another type s t a t i c c a s t <DerivedClass > ( PointerToBaseObj ) / / Example c i r c l e P t r = s t a t i c c a s t < C i r c l e >( p o i n t P t r ) ; An object of a publicly derived class can also be treated as an object of its base class c A. Amer Polymorphism 9

12 Object casting Object casting: conversion of the type of an object to that of another type s t a t i c c a s t <DerivedClass > ( PointerToBaseObj ) / / Example c i r c l e P t r = s t a t i c c a s t < C i r c l e >( p o i n t P t r ) ; An object of a publicly derived class can also be treated as an object of its base class A base class object cannot automatically be treated as a derived class object! We can use an explicit cast to convert a base-class pointer to a derived-class pointer Results of such conversion cannot be guaranteed in general c A. Amer Polymorphism 9

13 Object casting Object casting is used for pointer conversion (e.g., derived class pointer to base class pointer) reference conversion (e.g., derived class reference to base class reference) pointer-to-member conversion (e.g., from pointer to member of a base class to pointer to member of a derived class) A conversion from type SS to TT can ONLY be done if the conversion from type TT to SS is an implicit conversion c A. Amer Polymorphism 10

14 Object pointer casting: static_cast static_cast: no run-time type checking Down-casting: convert a base class pointer to a derived class pointer only if the conversion is unambiguous (& base class is non-polymorphic) c A. Amer Polymorphism 11

15 Object pointer casting: static_cast static_cast: no run-time type checking Down-casting: convert a base class pointer to a derived class pointer only if the conversion is unambiguous (& base class is non-polymorphic) Example: c l a s s BankAcct {... } ; c l a s s SavingsAcct : public BankAcct {... } ; / / Given a b a s e c l a s s p o i n t e r, / / we can c a s t i t t o a d e r i v e d c l a s s p o i n t e r : void foo ( BankAcct a c c t ) { SavingsAcct d1 = s t a t i c c a s t <SavingsAcct > ( a c c t ) ; } c A. Amer Polymorphism 11

16 Object pointer casting: static_cast static cast : no run-time type checking if acct does not refer to an actual SavingsAcct then the result of the cast is undefined c A. Amer Polymorphism 12

17 Object pointer casting: static_cast static cast : no run-time type checking if acct does not refer to an actual SavingsAcct then the result of the cast is undefined Applied to pointers to objects: Cast a pointer of a derived class to its base class Cast a pointer of a base class to its derived class The base class that is being casted is not checked to determine whether this is a complete class of the destination type or not: c l a s s Base {... } ; c l a s s Derived : public Base {... } ; Base a = new Base ; Derived b = s t a t i c c a s t <Derived > ( a ) ; c A. Amer Polymorphism 12

18 static_cast examples Circle is derived from Point: class Point { friend ostream& operator <<( ostream &, Point & ) ; public : Point ( i n t = 0, i n t = 0 ) ; void s e t P o i n t ( int, i n t ) ; / / access only base p a r t s i n t getx ( ) const { return x } ; protected : i n t x ; i n t y ; } ; / / Constructor Point : : Point ( i n t a, i n t b ) { s e t P o i n t ( a, b ) ; } c A. Amer Polymorphism 13

19 static_cast examples const double PI = ; class C i r c l e : public Point { friend ostream& operator <<( ostream &, C i r c l e & ) ; public : C i r c l e ( f l o a t = 0.0, i n t = 0, i n t = 0 ) ; void setradius ( f l o a t ) ; / / access d e r i v e p a r t s only i n t getradius ( ) const { return r ; } f l o a t area ( ) const { return PI r r ; } protected : f l o a t r ; } ; C i r c l e : : C i r c l e ( f l o a t rad, i n t a, i n t b ) : Point ( a, b ) { setradius ( rad ) ; } c A. Amer Polymorphism 14

20 static_cast examples / / Normal Use Point p ( 4, 5 ), p o i n t P t r ; C i r c l e c (0.707, 1, 2 ), c i r c l e P t r ; / / Assign p o i n t e r s normally p o i n t P t r = &p ; c i r c l e P t r = &c ; cout << p o i n t P t r ; / / C a l l operator << f o r Point / / Output : ( x, y )= 4 5 cout << c i r c l e P t r ; / / C a l l operator << f o r C i r c l e / / Output : ( x, y, r )= c A. Amer Polymorphism 15

21 static_cast examples / / Up c a s t i n g : using C i r c l e o b j e c t as a Point o b j e c t Point p ( 4, 5 ), p o i n t P t r ; C i r c l e c (0.707, 1, 2 ), c i r c l e P t r ; / / Assign C i r c l e o b j e c t to Point o b j e c t p o i n t e r / / Can only access the Point i n f o! p o i n t P t r = &c ; cout << p o i n t P t r ; / / C a l l operator << f o r Point / / Output : ( x, y )= 1 2 c A. Amer Polymorphism 16

22 static_cast examples / / Down c a s t i n g : Point to C i r c l e Point p ( 4, 5 ), p o i n t P t r ; C i r c l e c (0.707, 1, 2 ), c i r c l e P t r ; / / Assign C i r c l e o b j e c t to Point o b j e c t p o i n t e r / / Can only access the Point i n f o! p o i n t P t r = &c ; / / Since p o i n t P t r r e a l l y p o i n t s to a C i r c l e, we can / / cast i t back to a C i r c l e c i r c l e P t r = static_cast < C i r c l e >( p o i n t P t r ) ; cout << p o i n t P t r ; / / C a l l operator << f o r Point / / Output : ( x, y )= 1 2 cout << c i r c l e P t r ; / / C a l l operator << f o r C i r c l e / / Output : ( x, y, r )= c A. Amer Polymorphism 17

23 static_cast examples / / Wrong / Unsafe use Point p ( 4, 5 ), p o i n t P t r ; C i r c l e c (0.707, 1, 2 ), c i r c l e P t r ; / / Assign C i r c l e o b j e c t to Point o b j e c t p o i n t e r / / Can only access the Point i n f o! p o i n t P t r = &p ; / /!!!!!! Can I do t h i s? c i r c l e P t r = static_cast < C i r c l e >( p o i n t P t r ) ; cout << p o i n t P t r ; / / C a l l operator << f o r Point / / Output : ( x, y )= 4 5 cout << c i r c l e P t r ; / / C a l l operator << f o r C i r c l e / / Output : ( x, y, r )= 4 5 garbage c A. Amer Polymorphism 18

24 static_cast summary Address 0x1000 0x1001 0x1002 Memory map. 4 x 5 x p r. 0x30A8 0x30A9 0x30AA x x r c Point p(4, 5), *pointptr; Circle c(0.707, 1, 2), *circleptr; 0x1000 0x30A8. pointptr circleptr p o i n t P t r = &p ; p o i n t P t r = &c ; c i r c l e P t r = s t a t i c c a s t <C i r c l e > ( p o i n t P t r ) ; c A. Amer Polymorphism 19

25 static_cast summary Address 0x1000 0x1001 0x1002 Memory map. 4 x 5 x p r. 0x30A8 0x30A9 0x30AA x x r c Point p(4, 5), *pointptr; Circle c(0.707, 1, 2), *circleptr; 0x1000 0x30A8. pointptr circleptr p o i n t P t r = &p ; p o i n t P t r = &c ; c i r c l e P t r = s t a t i c c a s t <C i r c l e > ( p o i n t P t r ) ; Safe casting: from a derived object to a base object Be careful when casting from a base class to a derived class c A. Amer Polymorphism 19

26 Object pointer casting: Example / / L i s t i n g 15.1 Using p o i n t e r s to access o b j e c t s of base and derived classes class Base { / / base class protected : i n t x ; public : Base ( i n t a ) { x = a ; } / / to be used by Derived void set ( i n t a ) { x = a ; } / / to be i n h e r i t e d i n t show ( ) const { return x ; } / / to be i n h e r i t e d } ; class Derived : public Base { / / derived class private : i n t y ; public : Derived ( i n t a, i n t b ) : Base ( a ), y ( b ) { } / / empty c o n s t r u c t o r body void access ( i n t &a, i n t &b ) const / / added i n derived class { a = Base : : x ; b = y ; } } ; c A. Amer Polymorphism 20

27 Object pointer casting: Example i n t x, y ; Derived pd = new Derived ( 5 0, 8 0 ) ; / / unnamed derived o b j e c t cout << " 1. Derived p o i n t e r, object, and derived method \ n " ; pd >access ( x, y ) ; / / no problem : type match cout << " x = " <<x << " y = " <<y <<endl <<endl ; / / x=50 y=80 cout << " 2. Derived p o i n t e r, derived object, base method \ n " ; cout << " x = " << pd >show ( ) << endl << endl ; / / x = 50 Base pb = pd ; / / p o i n t e r to same o b j e c t cout << " 3. Base p o i n t e r, derived object, base method \ n " ; cout << " x = " << pb >show ( ) << endl << endl ; / / x = 50 / / pb >access ( x, y ) ; / / e r r o r : no access to derived method c A. Amer Polymorphism 21

28 Object pointer casting: Example cout << " 4. Converted p o i n t e r, derived o b j e c t and method \ n " ; ( ( Derived ) pb) >access ( x, y ) ; / / we know i t i s there cout << " x = " <<x << " y = " <<y <<endl <<endl ; / / x=50 y=80 pb = new Base ( 6 0 ) ; / / unnamed base o b j e c t cout << " 5. Base p o i n t e r, base object, base method \ n " ; cout << " x = " << pb >show ( ) << endl <<endl ; / / x = 60 cout << " 6. Converted p o i n t e r, base object, derived method \ n " ; ( ( Derived ) pb) >access ( x, y ) ; / / pass on your own r i s k cout << " x = " <<x << " y = " <<y <<endl <<endl ; / / junk!! delete pd ; delete pb ; / / necessary t i d i n e s s c A. Amer Polymorphism 22

29 Compile time versus Run time Static type checking / function checking Also called compile-time checking Examples?: d.show(); checked at compile time Dynamic type checking / function checking Also called run-time checking, i.e., at execution time Examples?: BasePtr->show(); c A. Amer Polymorphism 23

30 Polymorphism An object is denoted by its name (identifier) and the type associated with this identifier c A. Amer Polymorphism 24

31 Polymorphism An object is denoted by its name (identifier) and the type associated with this identifier Polymorphism (dynamic binding, virtual function): The principle that behavior of a function can vary depending on the actual type of an object With polymorphism: Objects of different classes related by inheritance may respond differently to the same member function call This allows a programmer to manipulate an object without knowing what kind of object it is c A. Amer Polymorphism 24

32 Types of polymorphism Static polymorphism (static binding or early binding): At compile time (at the earliest possible time): the compiler selects a method from several candidates Selection is based on the type of the pointer to the object c A. Amer Polymorphism 25

33 Types of polymorphism Static polymorphism (static binding or early binding): At compile time (at the earliest possible time): the compiler selects a method from several candidates Selection is based on the type of the pointer to the object Dynamic polymorphism (dynamic binding or late binding): At run time (at the latest possible time): the compiler selects a method from several candidates Based on the type of the object This allows a programmer to manipulate an object without knowing what kind of object it is c A. Amer Polymorphism 25

34 Static polymorphism Recall: In a derived class, you can redefine (this is?) functions of the base class compile-time solution c A. Amer Polymorphism 26

35 Static polymorphism Recall: In a derived class, you can redefine (this is?) functions of the base class compile-time solution Static polymorphism (early binding): When the compiler selects a method from several possible candidates at compile time The legality of the implementation (e.g., a member function invocation) is checked at compile time Example: if Vehicle has a certain member function, Car also has that member function c A. Amer Polymorphism 26

36 Dynamic polymorphism Dynamic polymorphism (late binding): When the compiler selects a method from several possible candidates at run time The legality of some types and functions is determined based on the dynamic type of the object at run time Dynamic polymorphism is implemented through virtual functions c A. Amer Polymorphism 27

37 Dynamic polymorphism Dynamic polymorphism (late binding): When the compiler selects a method from several possible candidates at run time The legality of some types and functions is determined based on the dynamic type of the object at run time Also called "dynamic binding": the binding to the code that gets called is accomplished dynamically (at run time) Dynamic polymorphism is implemented through virtual functions c A. Amer Polymorphism 27

38 Dynamic polymorphism: When? Suppose: We have a base class "Shape" with various derived classes "Triangle", "Rectangle", etc. Each derived class with its own print() member function Collect objects of these derived classes into an array of pointers to these objects CLASS arr[10] c A. Amer Polymorphism 28

39 Dynamic polymorphism: When? Suppose: We have a base class "Shape" with various derived classes "Triangle", "Rectangle", etc. Each derived class with its own print() member function Collect objects of these derived classes into an array of pointers to these objects CLASS arr[10] But what sort of pointers could we use? Recall: A derived pointer can be assigned a base pointer Shape arr[10]: an array of pointers to the base class Rectangle rec; arr[5] = &rec;: rec is the actual object; arr[5] is a pointer of type Shape c A. Amer Polymorphism 28

40 Dynamic polymorphism: When? Assume we want to print the content of each object in the array arr[i]->print() Since the pointer points to a Shape object, the print() function of the Shape (base) class is called NOT the appropriate derived class function!! Solution? Dynamic binding (using virtual functions) c A. Amer Polymorphism 29

41 Dynamic polymorphism: When? So: when you have a pointer to an object, this actual object may be of a derived class A pointer to an object of type Vehicle* maybe actually be pointing to a Car object A pointer to an object of type Shape* maybe actually be pointing to a Rectangle object c A. Amer Polymorphism 30

42 Dynamic polymorphism: When? So: when you have a pointer to an object, this actual object may be of a derived class A pointer to an object of type Vehicle* maybe actually be pointing to a Car object A pointer to an object of type Shape* maybe actually be pointing to a Rectangle object Dynamic binding implies that a base class pointer figures out which derived class it really points to at run-time c A. Amer Polymorphism 30

43 Dynamic binding Create a group of related derived classes under a common public base class Equip each derived class with a function that performs processing specific to this derived class Make sure that each function has the same name and signature (interface) Call this function through a base class pointer c A. Amer Polymorphism 31

44 Dynamic binding Create a group of related derived classes under a common public base class Equip each derived class with a function that performs processing specific to this derived class Make sure that each function has the same name and signature (interface) Call this function through a base class pointer The called function does not depend on the type of the pointer that points to the object It depends on the type of the object pointed to by the pointer c A. Amer Polymorphism 31

45 Dynamic binding: Useful? Before OOP: software reuse when new code call old code Dynamic binding can improve reuse by letting old code call new code c A. Amer Polymorphism 32

46 Dynamic binding: Useful? Before OOP: software reuse when new code call old code Dynamic binding can improve reuse by letting old code call new code Dynamic binding permits software extensibility: With OOP: a new code may be created, compiled, and tested Later, this code can be called by a framework that was written long time ago There is no need to change the old code: it does not even need to be recompiled c A. Amer Polymorphism 32

47 Dynamic binding: Useful? Dynamic binding does not make static binding irrelevant It introduces many additional dimension of complexity c A. Amer Polymorphism 33

48 Dynamic binding: Useful? Dynamic binding does not make static binding irrelevant It introduces many additional dimension of complexity poly.method(...); OR poly->method(...); Check if the target (here poly) is an object or a pointer If it is an object: Only static binding possible Check the function signature and Verify if the function call is correct If the target is a pointer: consider dynamic binding In most cases: the method being called depends on the type of the pointer not on the type of the object c A. Amer Polymorphism 33

49 Dynamic binding: a scenario If the target is a pointer: consider dynamic binding Define at which place in the inheritance hierarchy the pointer belongs If the pointer is of the derived type: only static binding is possible If the pointer is of the base type: dynamic binding is possible c A. Amer Polymorphism 34

50 Dynamic binding: a scenario If the pointer is of the base type: dynamic binding is possible if the object the pointer points to is of the base type: no dynamic binding if the object is of a derived type: dynamic binding is possible If the function called is redefined in the appropriate derived class and has the same name and signature as the (virtual) function in the base class, Apply dynamic binding implemented through virtual functions c A. Amer Polymorphism 35

51 Types of member functions Defined in the base class and inherited in a derived class without redefinition Defined in a derived class without a counterpart in the base class c A. Amer Polymorphism 36

52 Types of member functions Defined in the base class and inherited in a derived class without redefinition Defined in a derived class without a counterpart in the base class Defined in the base class and redefined in a derived class with the same name and same signature Defined in the base class and redefined in a derived class with the same name and different signature Defined in the base class and redefined in a derived class with the same name and same signature as virtual In which case you can apply dynamic binding? c A. Amer Polymorphism 36

53 Virtual functions A virtual function is a member function of the base class that can be redefined at run-time Even if the object is accessed by a base pointer virtual functions allow derived classes to replace the implementation of a function provided by the base class at run time c A. Amer Polymorphism 37

54 Virtual functions A virtual function is a member function of the base class that can be redefined at run-time Even if the object is accessed by a base pointer virtual functions allow derived classes to replace the implementation of a function provided by the base class at run time The replacement is always called whenever the object in question is of the derived class (even if the object is accessed by a base pointer) This allows algorithms in the base class to be replaced in the derived class, even if users don t know about the derived class c A. Amer Polymorphism 37

55 Example: without virtual functions class Shape { protected : i n t width, h e i g h t ; public : void set ( i n t a, i n t b ) { width=a ; h e i g h t =b ; } / / access base p a r t only } ; class Rectangle : public Shape { public : i n t area ( void ) { return ( width h e i g h t ) ; } / / s p e c i f i c to Rect. } ; class T r i a n g l e : public Shape { public : i n t area ( void ) { return ( width h e i g h t / 2 ) ; } / / s p e c i f i c to T r g l. } ; c A. Amer Polymorphism 38

56 Example: without virtual functions Rectangle r e c t ; T r i a n g l e t r g l ; Shape ppoly1 = &r e c t ; / / two p o i n t e r s to o b j e c t s of base class Shape Shape ppoly2 = & t r g l ; / / They are assigned the addresses of / / derived classes r e c t and t r g l ( v a l i d?) / / ==> we can only r e f e r the members t h a t / / Rectangle and T r i a n g l e i n h e r i t from Shape ppoly1 >set ( 4, 5 ) ; / / can use ppoly1 ; ppoly2 >set ( 4, 5 ) ; cout << r e c t. area ( ) << endl ; / / cannot use ppoly1 ; Output 20 cout << t r g l. area ( ) << endl ; / / cannot use ppoly2 ; Output 10 c A. Amer Polymorphism 39

57 Example: without virtual functions To make it possible for the pointers to the base class to access a member, we have to declared it in the base class But what is the area of an undefined shape? c A. Amer Polymorphism 40

58 Example: without virtual functions To make it possible for the pointers to the base class to access a member, we have to declared it in the base class But what is the area of an undefined shape? To access a (virtual) member of a base class we must declare it as virtual so that the use of pointers to base objects can be fully applied to derived objects c A. Amer Polymorphism 40

59 Example: with virtual functions class Shape { protected : i n t width, h e i g h t ; public : void set ( i n t a, i n t b ) { width=a ; h e i g h t =b ; } / / set ( ) not v i r t u a l : i t does the same f o r a l l classes v i r t u a l i n t area ( void ) / / area ( ) as v i r t u a l f u n c t i o n / / Does not make much sense here, but we need i t l a t e r { return ( 0 ) ; } / / v i r t u a l : works s p e c i f i c i n derived c l s. } ; class Rectangle : public Shape { public : i n t area ( void ) { return ( width h e i g h t ) ; } } ; c A. Amer Polymorphism 41

60 Example: with virtual functions class T r i a n g l e : public Shape { public : i n t area ( void ) { return ( width h e i g h t / 2 ) ; } } ; / / Testing Rectangle r e c t ; T r i a n g l e t r g l ; Shape poly ; Shape ppoly1 = &r e c t ; Shape ppoly2 = & t r g l ; Shape ppoly3 = &poly ; c A. Amer Polymorphism 42

61 Example: with virtual functions ppoly1 >set ( 4, 5 ) ; ppoly2 >set ( 4, 5 ) ; ppoly3 >set ( 4, 5 ) ; / / dynamic binding : / / At run time the implementation of the v i r t u a l f u n c t i o n / / i s replaced by t h a t of the derived class cout << ppoly1 >area ( ) << endl ; / / output : 20 cout << ppoly2 >area ( ) << endl ; / / output : 10 cout << ppoly3 >area ( ) << endl ; / / output : 0 / / despite v i r t u a l i t y we can declare an o b j e c t of type Shape / / and to c a l l i t s area ( ) f u n c t i o n c A. Amer Polymorphism 43

62 Virtual & non-virtual functions Non-virtual member functions are resolved statically it is selected at compile-time based on the type of the pointer to the object c A. Amer Polymorphism 44

63 Virtual & non-virtual functions Non-virtual member functions are resolved statically it is selected at compile-time based on the type of the pointer to the object Virtual member functions are resolved dynamically it is selected at run-time based on the type of the object (not the type of the pointer to that object) Dynamic binding creates extra space and time overhead but... c A. Amer Polymorphism 44

64 Example without virtual functions class Student { friend ostream& operator <<( ostream &, Student & ) ; public : Student ( char pf, char pl ) ; ~Student ( ) ; protected : char f i r s t ; char l a s t ; } ; class COEN244 : public Student { public : COEN244( char pf, char pl ) ; ~COEN244 ( ) { count ; } void setgrade ( f l o a t, f l o a t, f l o a t, f l o a t ) ; f l o a t grade ( ) ; / / <=== s t a t i c i n t getcount ( ) { return count ; } private : f l o a t assignment ; f l o a t quiz ; f l o a t midterm ; f l o a t f i n a l ; s t a t i c i n t count ; } ; c A. Amer Polymorphism 45

65 Example without virtual functions class ELEC311 : public Student { public : ELEC311( char pf, char pl ) ; ~ELEC311 ( ) { count ; } void setgrade ( f l o a t, f l o a t, f l o a t, f l o a t ) ; f l o a t grade ( ) ; / / <=== s t a t i c i n t getcount ( ) { return count ; } private : f l o a t assignment ; f l o a t l a b o r a t o r y ; f l o a t midterm ; f l o a t f i n a l ; s t a t i c i n t count ; } ; c A. Amer Polymorphism 46

66 Example without virtual functions class ELEC490 : public Student { public : ELEC490( char pf, char pl ) ; ~ELEC490 ( ) { count ; } void setgrade ( f l o a t ) ; f l o a t grade ( ) ; / / <=== s t a t i c i n t getcount ( ) { return count ; } private : f l o a t p r o j e c t ; s t a t i c i n t count ; } ; c A. Amer Polymorphism 47

67 Example without virtual functions void p r i n t I n f o ( const Student ) ; void main { COEN244 c1 ( " Jane ", "Doe" ) ; c1. setgrade (50, 55, 25, 9 5 ) ; p r i n t I n f o (&c1 ) ; } void p r i n t I n f o ( const Student pstud ) { / / Output name v i a stream i n s e r t i o n overload cout << pstud << endl ; / / QUESTION??? How do I know what kind / / of Student pstud i s? cout << pstud >grade ( ) ; / / We cannot do t h i s!!! } c A. Amer Polymorphism 48

68 Example with virtual functions class Student { friend ostream& operator <<( ostream &, Student & ) ; public : Student ( char pf, char pl ) ; ~Student ( ) ; v i r t u a l f l o a t grade ( ) ; / / <=== protected : char f i r s t ; char l a s t ; } ; class COEN244 : public Student { public : COEN244( char pf, char pl ) ; ~COEN244 ( ) { count ; } void setgrade ( f l o a t, f l o a t, f l o a t, f l o a t ) ; v i r t u a l f l o a t grade ( ) ; / / <=== s t a t i c i n t getcount ( ) { return count ; } private : f l o a t assignment ; f l o a t quiz ; f l o a t midterm ; f l o a t f i n a l ; s t a t i c i n t count ; } ; c A. Amer Polymorphism 49

69 Example with virtual functions void void main { COEN244 c1 ( " Jane ", "Doe" ) ; c1. setgrade (50, 55, 25, 9 5 ) ; p r i n t I n f o (&c1 ) ; } void p r i n t I n f o ( const Student pstud ) { / / Output name v i a stream i n s e r t i o n overload cout << pstud << endl ; cout << pstud >grade ( ) ; / / grade ( ) of COEN244 class w i l l be c a l l e d } c A. Amer Polymorphism 50

70 Virtual functions central to OO Virtual functions make OO software extensible and adaptable Because old code (in the base class) calls new code (in the derived class) Programming with classes but without dynamic binding is called "object based," but not "object oriented" c A. Amer Polymorphism 51

71 Virtual functions: summary When a base class member function is declared virtual, it permits run-time (dynamic) selection of the equivalent member function of the appropriate derived class c A. Amer Polymorphism 52

72 Virtual functions: summary When a base class member function is declared virtual, it permits run-time (dynamic) selection of the equivalent member function of the appropriate derived class Base class virtual functions may have their own definition It will be called if the referenced object is a base class object or if the referenced derived class did not define an equivalent member function Derived classes defining a virtual function need not be explicitly declared virtual Polymorphic behavior works with pointers & references of base classes c A. Amer Polymorphism 52

73 Virtual functions: Advice If using public inheritance, many base class member functions should be declared virtual to ensure that the derived class customizations will override the base class behavior even in a context where a base class object is expected c A. Amer Polymorphism 53

74 Abstract Classes Some classes represent abstract concepts for which objects cannot exist (There is no reason to create such objects) Person 2DShape Base Faculty Student Admine Triangle Rectangle Circle Derived Derived/Base Derived Derived Derived Derived... Base c A. Amer Polymorphism 54

75 Abstract Classes In an inheritance hierarchy with an abstract base class The functions have to be defined in the derived classes The equivalent functions in the base classes are called pure virtual functions virtual void print() =0; An object of an abstract base class cannot be instantiated c A. Amer Polymorphism 55

76 Abstract Classes In an inheritance hierarchy with an abstract base class The functions have to be defined in the derived classes The equivalent functions in the base classes are called pure virtual functions virtual void print() =0; An object of an abstract base class cannot be instantiated Abstract Class: a class with at least one pure virtual function Abstract Data Type (ADT): An abstract class c A. Amer Polymorphism 55

77 Virtual destructor You cannot have virtual constructor, but you can have virtual destructor A virtual destructor is a virtual function that is needed if we will delete (using the delete operator) a derived-class object via a base-class pointer c A. Amer Polymorphism 56

78 Virtual destructor You cannot have virtual constructor, but you can have virtual destructor A virtual destructor is a virtual function that is needed if we will delete (using the delete operator) a derived-class object via a base-class pointer void main ( ) { / / pstud p o i n t s t o a Student o b j e c t Student pstud = new COEN244( John, Smith ) ;... delete pstud ; / / w i l l c a l l t h e Student o b j e c t s d e s t r u c t o r } The COEN244 object destructor will not be called Not all memory is deallocated!!! c A. Amer Polymorphism 56

79 Virtual destructor c l a s s Student { friend ostream& operator <<(ostream&, Student &); public : Student ( char pf, char pl ) ; v i r t u a l Student ( ) ; v i r t u a l f l o a t grade ( ) ; protected : char f i r s t ; char l a s t ; } ; c A. Amer Polymorphism 57

80 Virtual destructor c l a s s Student { friend ostream& operator <<(ostream&, Student &); public : Student ( char pf, char pl ) ; v i r t u a l Student ( ) ; v i r t u a l f l o a t grade ( ) ; protected : char f i r s t ; char l a s t ; } ; When a Student pointer, pointing to a COEN244 object, is deleted, the COEN244 destructor is called which in turn will call the Student destructor Virtual destructor violate the rule of virtual functions? (name?) but memory leaks are very dangerous c A. Amer Polymorphism 57

81 Virtual destructor A class should have a virtual destructor unless that class has NO virtual functions If we have any virtual functions, then we will probably going to process derived objects via a base pointer Such processing may include invoking a destructor (normally done implicitly via delete) c A. Amer Polymorphism 58

82 Virtual destructor A class should have a virtual destructor unless that class has NO virtual functions If we have any virtual functions, then we will probably going to process derived objects via a base pointer Such processing may include invoking a destructor (normally done implicitly via delete) Recall virtual functions bind to the code associated with the class of the object, rather than with the class of the pointer/reference When you say delete baseptr, and the base class has a virtual destructor, the destructor that gets invoked is the one associated with the type of the object *baseptr, rather than the one associated with the type of the pointer c A. Amer Polymorphism 58

83 P: a summary@ Classes: provides data abstraction and enca a means to hide implementations but docum Inheritance: promotes software reuse Polymorphism: offers software extensibility c A. Amer Polymorphism 59

84 Polymorphism: Employee example class Employee { public : Employee ( const char, const char ) ; ~Employee ( ) ; / / d e s t r u c t o r reclaims memory const char getfirstname ( ) const ; const char getlastname ( ) const / / Pure v i r t u a l f u n c t i o n makes Employee a b s t r a c t base class v i r t u a l double earnings ( ) const = 0; / / pure v i r t u a l v i r t u a l void p r i n t ( ) const ; / / v i r t u a l } ; private : char firstname ; char lastname ; c A. Amer Polymorphism 60

85 Polymorphism: Employee example class Boss : public Employee { public : Boss ( const char, const char, double = 0. 0 ) ; void setweeklysalary ( double ) ; v i r t u a l double earnings ( ) const ; v i r t u a l void p r i n t ( ) const ; private : double weeklysalary ; } ; c A. Amer Polymorphism 61

86 Polymorphism: Employee example class SalaryWorker : public Employee { public : SalaryWorker ( const char, const char, double = 0. 0 ) ; void setsalary ( double ) ; v i r t u a l double earnings ( ) const ; v i r t u a l void p r i n t ( ) const ; protected : double s a l a r y ; / / weekly s a l a r y } ; c A. Amer Polymorphism 62

87 Polymorphism: Employee example class HourlyWorker : public Employee { public : HourlyWorker ( const char, const char, double = 0. 0, double = 0. 0 ) ; void setwage ( double ) ; void sethours ( double ) ; v i r t u a l double earnings ( ) const ; v i r t u a l void p r i n t ( ) const ; private : double wage ; / / wage per hour double hours ; / / hours worked f o r week } ; c A. Amer Polymorphism 63

88 Polymorphism: Employee example class CommissionWorker : public Employee { public : CommissionWorker ( const char, const char, double = 0. 0, i n t = 0 ) ; void setcommission ( double ) ; void s e t Q u a n t i t y ( i n t ) ; v i r t u a l double earnings ( ) const ; v i r t u a l void p r i n t ( ) const ; protected : double commission ; / / amount per item sold i n t q u a n t i t y ; / / t o t a l items sold f o r week } ; c A. Amer Polymorphism 64

89 Polymorphism: Employee example / / Testing Employee l i s t [ 10 ] ; Boss b ( "CCC", "TTTTT", ) ; CommissionWorker c1 ( "SSSS", "QQQQ", 3.0, 150); CommissionWorker c2 ( " JJJJ ", "DDDD", 4.0, 6 0 ) ; SalaryWorker s1 ( "BBBB", "VVVVV", 500); SalaryWorker s2 ( "RRRR", "Red", 500); HourlyWorker h1 ( "GGGG", "PPPP", 17.00, ) ; HourlyWorker h2 ( "KKKK", " I I I I ", 13.75, 4 0 ) ; HourlyWorker h3 ( "MMM", "TT", 10.15, 1 8 ) ; c A. Amer Polymorphism 65

90 Polymorphism: Employee example l i s t [ 0 ] = &b ; l i s t [ 1 ] = &c1 ; l i s t [ 2 ] = &c2 ; l i s t [ 3 ] = &s1 ; l i s t [ 4 ] = &s2 ; l i s t [ 5 ] = &h1 ; l i s t [ 6 ] = &h2 ; l i s t [ 7 ] = &h3 ; / / Dynamic Binding cout << "DYNAMIC BINDING" << endl ; for ( i n t i = 0; i < 8; i ++) { l i s t [ i ] > p r i n t ( ) ; / / using p o i n t e r s cout << " earned $ " << ( l i s t [ i ] ). earnings ( ) ; / / using o b j e c t s c A. Amer Polymorphism 66

91 Polymorphism: Employee example / / S t a t i c Binding cout << " STATIC BINDING" << endl ; / / access through o b j e c t s : no polymorphism / v i r t u a l i t y b. p r i n t ( ) ; cout << " earned $ " << b. earnings ( ) ; c1. p r i n t ( ) ; cout << " earned $ " << c1. earnings ( ) ; c2. p r i n t ( ) ; cout << " earned $ " << c2. earnings ( ) ; s1. p r i n t ( ) ; cout << " earned $ " << s1. earnings ( ) ; s2. p r i n t ( ) ; cout << " earned $ " << s2. earnings ( ) ; h1. p r i n t ( ) ; cout << " earned $ " << h1. earnings ( ) ; h2. p r i n t ( ) ; cout << " earned $ " << h2. earnings ( ) ; h3. p r i n t ( ) ; cout << " earned $ " << h3. earnings ( ) ; c A. Amer Polymorphism 67

92 Polymorphism: Person example / / L i s t i n g 15.5 Heterogeneous l i s t processing using v i r t u a l f u n c t i o n s class Person { protected : char i d [ 1 0 ] ; / / data common to both types char name ; / / v a r i a b l e l e n g t h public : Person ( const char i d [ ], const char nm [ ] ) / / Kind type { s t r c p y ( Person : : id, i d ) ; / / copy i d name = new char [ s t r l e n (nm) + 1 ] ; / / get space f o r name i f (name == 0) { cout << " Out of memory \ n " ; e x i t ( 0 ) ; } s t r c p y (name,nm ) ; / / copy name } v i r t u a l void w r i t e ( ) const / / to d i s p l a y data { } / / not much to do at the moment ~Person ( ) / / r e t u r n heap memory { delete [ ] name ; } / / f o r Person o b j e c t only } ; c A. Amer Polymorphism 68

93 Polymorphism: Person example class Faculty : public Person { private : char rank ; / / f o r f a c u l t y only public : Faculty ( const char i d [ ], const char nm [ ], const char r [ ] ) : Person ( id,nm) / / i n i t i a l i z a t i o n l i s t { rank = new char [ s t r l e n ( r ) + 1 ] ; i f ( rank == 0) { cout << " Out of memory \ n " ; e x i t ( 0 ) ; } s t r c p y ( rank, r ) ; } void w r i t e ( ) const / / d i s p l a y record { cout << " i d : " << i d << endl ; / / p r i n t id, name cout << " name : " << name << endl ; cout << " rank : " << rank <<endl <<endl ; } / / f a c u l t y only } ; ~ Faculty ( ) { delete [ ] rank ; } / / r e t u r n heap memory c A. Amer Polymorphism 69

94 Polymorphism: Person example class Student : public Person { private : char major ; / / f o r student only public : Student ( const char i d [ ], const char nm [ ], const char m[ ] ) : Person ( id,nm) / / i n i t i a l i z a t i o n l i s t { major = new char [ s t r l e n (m) + 1 ] ; i f ( major == 0) { cout << " Out of memory \ n " ; e x i t ( 0 ) ; } s t r c p y ( major,m) ; } void w r i t e ( ) const / / d i s p l a y record { cout << " i d : " << i d << endl ; / / p r i n t id, name cout << " name : " << name << endl ; cout << " major : " << major <<endl <<endl ; / / student only } ~Student ( ) { delete [ ] major ; } / / r e t u r n heap memory } ; c A. Amer Polymorphism 70

95 Polymorphism: Person example void read ( i f s t r e a m& f, Person & person ) { / / read one record char kind [ 8 ], i d [ 1 0 ], name [ 8 0 ], buf [ 8 0 ] ; f. g e t l i n e ( kind, 8 0 ) ; / / recognize the incoming type f. g e t l i n e ( id, 1 0 ) ; / / read i d f. g e t l i n e (name, 8 0 ) ; / / read name f. g e t l i n e ( buf, 8 0 ) ; / / rank or major? i f ( strcmp ( kind, "FACULTY" ) == 0) { person = new Faculty ( id, name, buf ) ; } / / o b j e c t i s Faculty else i f ( strcmp ( kind, "STUDENT" ) == 0) { person = new Student ( id, name, buf ) ; } / / o b j e c t i s Student else { cout << " Corrupted data : unknown type \ n " ; e x i t ( 0 ) ; } } void w r i t e ( const Person p ) / / d i s p l a y record { p >w r i t e ( ) ; } / / Faculty or Student? c A. Amer Polymorphism 71

96 Polymorphism: Person example / / Testing cout << endl << endl ; Person data [ 2 0 ] ; i n t cnt = 0; / / array of p o i n t e r s i f s t r e a m from ( " univ. dat " ) ; / / i n p u t data f i l e i f (! from ) { cout << " Cannot open f i l e \ n " ; return 0; } while (! from. eof ( ) ) { read ( from, data [ cnt ] ) ; / / read u n t i l eof cnt ++; } cout << " T o t a l records read : " << cnt << endl << endl ; for ( i n t i =0; i < cnt ; i ++) { w r i t e ( data [ i ] ) ; } / / d i s p l a y data for ( i n t j =0; j < cnt ; j ++) { delete data [ j ] ; } / / d e l e t e the record c A. Amer Polymorphism 72

97 A sample exam s problem Define a XXXX class that inherits from XXXX class... Declare base pointers each pointing to a derived object Pay attention to the type of the pointer... To be continued in class. c A. Amer Polymorphism 73