aishy amer - encsstatic polymorphism: compile-time type and function checking dynamic polymorphism:...

COEN244: Polymorphism

Aishy Amer

Electrical & Computer Engineering

Polymorphism means “variable behavior”/“ability to appear in many forms”

OutlineCasting between objects

Using pointers to access objects

Static polymorphism: compile-time type andfunction checking

Dynamic polymorphism: run-time type andfunction checking

Virtual functions: can be over-ridden at run

time

Pure Virtual functions: must be over-ridden

Abstract Classes: classes with at least onepure virtual function

Virtual destructor

c© A. Amer Polymorphism 1

Casting

Casting: conversion from one data type to another typec o n v e r t t o t y p e ( express ion ) ;// Example :t = i n t ( x ) ; q=double ( y ) ;


Casting

Casting: conversion from one data type to another typec o n v e r t t o t y p e ( express ion ) ;// Example :t = i n t ( x ) ; q=double ( y ) ;

Standard numerical conversions are used forintegral 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 widestoperand before evaluation)


Casting

Implicit conversions:

Some conversions are performed automatically by thecompiler without intervention by the programmerStandard C++ conversions and user-defined conversionsare performed implicitly by the compiler where neededExamples?

Explicit conversions:

Conversions which must be explicitly specified by theprogrammer


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 informationfast and safe



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 informationfast 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 theoriginal

const cast : (UNSAFE!!)cast away the “const-ness” or “volatility” of a type



All casting operators have the same syntax:stat ic_cast conver t_ to_type ( expr ) ;/ / ==> Convert expr to type conver t_ to_typestat ic_cast <DerivedClass ∗> ( PointerToBaseObj )

Use the least dangerous (most specific) casting alternative


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 tpX = &someOtherIntVar ; / / l e g a l − pX can po in t somewhere e lse

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 tpY = &someOtherIntVar ; / / i l l e g a l − can ’ t make pY po in t anywhere e lse

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 tpZ = &someOtherIntVar ; / / i l l e g a l − can ’ t make pZ po in t anywhere e lse

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 po in t/ / somewhere else , and you can ’ t modify what the p o i n t e r po in t s toconst char∗ const GetName ( ) { return m_szName; } ;. . .c© A. Amer Polymorphism 6


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 tcout << q ; / / p r i n t s " 4.000000"double q = ( double ) x / y ; / / cast r e s u l t as double so i t ’ s not roundedcout << q ; / / p r i n t s "4.625000"

/ / Bas cas t ing : fo rce the compi ler to put the address o f/ / 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 modi f iedconst i n t ∗ pX = &x ; / / you can ’ t modify x through the pX p o i n t e rcout << 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 undef ined

cout << x << endl ; / / who knows what i t p r i n t s ?/ / The code compiles and runs w i thou t crashing , but who knows what i s x



/ / The Const_cast Operatorconst i n t x = 4; / / x i s const , i t can ’ t be modi f iedconst 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 undef inedcout << x << endl ; / / who knows what i t p r i n t s ?

/ / With const_cast : change the const−ness of a va r iab le , and never i t s type


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 )/ / Examplec 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 asan object of its base class


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 )/ / Examplec 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 asan object of its base class

A base class object cannot automatically be treated as aderived class object!

We can use an explicit castto convert a base-class pointer to a derived-class pointer

Results of such conversion cannot be guaranteed ingeneral


Object casting

Object casting is used forpointer 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 tomember 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


Object pointer casting: static_cast

static_cast: no run-time type checking

Down-casting:convert a base class pointer to a derived class pointeronly if the conversion is unambiguous(& base class is non-polymorphic)



static_cast: no run-time type checking

Down-casting:convert a base class pointer to a derived class pointeronly 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 ) ;}



static cast : no run-time type checking→ if acct does not refer to an actual SavingsAcctthen the result of the cast is undefined



static cast : no run-time type checking→ if acct does not refer to an actual SavingsAcctthen the result of the cast is undefined

Applied to pointers to objects:Cast a pointer of a derived class to its base classCast 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 thedestination 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 ) ;


static_cast examples

Circle is derived from Point:class Poin t {

f r iend ostream& operator <<(ostream&, Po in t & ) ;public :

Po in t ( i n t = 0 , i n t = 0 ) ;void se tPo in t ( int , i n t ) ; / / access only base par t si n t getX ( ) const { return x } ;

protected :i n t x ; i n t y ;

} ;

/ / Const ruc torPoin t : : Po in t ( i n t a , i n t b ) { se tPo in t ( a , b ) ; }



const double PI = 3.14159;

class C i r c l e : public Poin t {f r iend 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 der i ve par t s on lyi 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 ) : Po in t ( a , b ) { setRadius ( rad ) ; }



/ / Normal UsePoin t 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 po in te r s normal lyp 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 ; / / Ca l l operator << f o r Po in t/ / Output : ( x , y )= 4 5

cout << ∗ c i r c l e P t r ; / / Ca l l operator << f o r C i r c l e/ / Output : ( x , y , r )= 1 2 0.707



/ / Up−cas t ing : using C i r c l e ob jec t as a Po in t ob jec t

Poin t 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 ob jec t to Po in t ob jec t p o i n t e r/ / Can only access the Po in t i n f o !p o i n t P t r = &c ;

cout << ∗ p o i n t P t r ; / / Ca l l operator << f o r Po in t/ / Output : ( x , y )= 1 2



/ / Down−cas t ing : Po in t to C i r c l e


/ / Assign C i r c l e ob jec t to Po in t ob jec t p o i n t e r/ / Can only access the Po in t 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 po in t s to a C i rc le , we can/ / cast i t back to a C i r c l ec i r c l e P t r = stat ic_cast < C i r c l e ∗ >( p o i n t P t r ) ;cout << ∗ p o i n t P t r ; / / Ca l l operator << f o r Po in t/ / Output : ( x , y )= 1 2

cout << ∗ c i r c l e P t r ; / / Ca l l operator << f o r C i r c l e/ / Output : ( x , y , r )= 1 2 0.707



/ / Wrong / Unsafe use


/ / Assign C i r c l e ob jec t to Po in t ob jec t p o i n t e r/ / Can only access the Po in t 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 = stat ic_cast < C i r c l e ∗ >( p o i n t P t r ) ;cout << ∗ p o i n t P t r ; / / Ca l l operator << f o r Po in t/ / Output : ( x , y )= 4 5

cout << ∗ c i r c l e P t r ; / / Ca l l operator << f o r C i r c l e/ / Output : ( x , y , r )= 4 5 ’ garbage ’


static_cast summary

Point p(4, 5), *pointPtr;

..

..

.

..

.

.

Memory mapAddress

0x10000x10010x1002

120.707

c

45 p

0x30AA0x30A90x30A8

circlePtrpointPtr

0x30A80x1000

xx

r

xx

r

Circle c(0.707, 1, 2), *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 ) ;


static_cast summary

Point p(4, 5), *pointPtr;

..

..

.

..

.

.

Memory mapAddress

0x10000x10010x1002

120.707

c

45 p

0x30AA0x30A90x30A8

circlePtrpointPtr

0x30A80x1000

xx

r

xx

r

Circle c(0.707, 1, 2), *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 classc© A. Amer Polymorphism 19

Object pointer casting: Example

/ / L i s t i n g 15.1 Using po in te r s to access ob jec ts o f base and der ived classesclass Base { / / base c lass

protected : i n t x ;public :Base ( i n t a ) { x = a ; } / / to be used by Derivedvoid set ( i n t a ) { x = a ; } / / to be i n h e r i t e di n t show ( ) const { return x ; } / / to be i n h e r i t e d

} ;

class Derived : public Base { / / der ived c lassprivate : i n t y ;public :

Derived ( i n t a , i n t b ) : Base ( a ) , y ( b ){ } / / empty cons t ruc to r body

void access ( i n t &a , i n t &b ) const / / added i n der ived c lass{ a = Base : : x ; b = y ; }

} ;



i n t x , y ;Derived ∗pD = new Derived (50 ,80 ) ; / / unnamed der ived ob jec t

cout << " 1 . Derived po in te r , ob jec t , and der ived method \ n " ;pD−>access ( x , y ) ; / / no problem : type matchcout <<" x = " <<x <<" y = " <<y <<endl <<endl ; / / x=50 y=80

cout << " 2 . Derived po in te r , der ived ob jec t , base method \ n " ;cout << " x = " << pD−>show ( ) << endl << endl ; / / x = 50Base ∗pB = pD; / / p o i n t e r to same ob jec t

cout << " 3 . Base po in te r , der ived ob jec t , base method \ n " ;cout << " x = " << pB−>show ( ) << endl << endl ; / / x = 50/ / pB−>access ( x , y ) ; / / e r r o r : no access to der ived method



cout << " 4 . Converted po in te r , der ived ob jec t and method \ n " ;( ( Derived ∗ ) pB)−>access ( x , y ) ; / / we know i t i s therecout <<" x = " <<x <<" y = " <<y <<endl <<endl ; / / x=50 y=80pB = new Base ( 6 0 ) ; / / unnamed base ob jec t

cout << " 5 . Base po in te r , base ob jec t , base method \ n " ;cout << " x = " << pB−>show ( ) << endl <<endl ; / / x = 60

cout << " 6 . Converted po in te r , base ob jec t , der ived method \ n " ;( ( Derived ∗ ) pB)−>access ( x , y ) ; / / pass on your own r i s kcout <<" x = " <<x <<" y = " <<y <<endl <<endl ; / / junk ! !

delete pD; delete pB ; / / necessary t i d i n e s s


Compile time versus Run time

Static type checking / function checkingAlso called compile-time checkingExamples?: d.show(); checked at compile time

Dynamic type checking / function checkingAlso called run-time checking, i.e., at execution timeExamples?: BasePtr->show();


Polymorphism

An object is denoted by its name (identifier) and the typeassociated with this identifier


Polymorphism

An object is denoted by its name (identifier) and the typeassociated with this identifier

Polymorphism (dynamic binding, virtual function):The principle that behavior of a function can varydepending on the actual type of an object

With polymorphism:Objects of different classes related by inheritance mayrespond differently to the same member function callThis allows a programmer to manipulate an object withoutknowing what kind of object it is


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 candidatesSelection is based on the type of the pointer to the object


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 candidatesSelection 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 candidatesBased on the type of the objectThis allows a programmer to manipulate an object withoutknowing what kind of object it is


Static polymorphism

Recall: In a derived class,you can redefine (this is?) functions of the base class⇒ compile-time solution


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 severalpossible candidates at compile timeThe legality of the implementation(e.g., a member function invocation)is checked at compile timeExample: if Vehicle has a certain member function,Car also has that member function


Dynamic polymorphism

Dynamic polymorphism (late binding):When the compiler selects a method from severalpossible candidates at run timeThe legality of some types and functions is determinedbased on the dynamic type of the object at run time

Dynamic polymorphism is implemented through virtual functions


Dynamic polymorphism

Dynamic polymorphism (late binding):When the compiler selects a method from severalpossible candidates at run timeThe legality of some types and functions is determinedbased on the dynamic type of the object at run time

Also called "dynamic binding":the binding to the code that gets called is accomplisheddynamically (at run time)

Dynamic polymorphism is implemented through virtual functions


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 ofpointers to these objects CLASS ∗ arr[10]



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 ofpointers 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



Assume we want to print the content of each object in thearray arr[i]->print()

Since the pointer points to a Shape object,the print() function of the Shape (base) class is calledNOT the appropriate derived class function!!

Solution? Dynamic binding (using virtual functions)



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 objectA pointer to an object of type Shape*maybe actually be pointing to a Rectangle object



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 objectA pointer to an object of type Shape*maybe actually be pointing to a Rectangle object

Dynamic binding implies that a base class pointer figures outwhich derived class it really points to at run-time


Dynamic binding

Create a group of related derived classes under a commonpublic base class

Equip each derived class with a function that performsprocessing specific to this derived class

Make sure that each function has the same name andsignature (interface)

Call this function through a base class pointer


Dynamic binding

Create a group of related derived classes under a commonpublic base class

Equip each derived class with a function that performsprocessing specific to this derived class

Make sure that each function has the same name andsignature (interface)

Call this function through a base class pointer

⇒ The called function does not depend on the type of the pointerthat points to the object

⇒ It depends on the type of the object pointed to by the pointer


Dynamic binding: Useful?

Before OOP: software reuse when new code call old code

Dynamic binding can improve reuse by letting old code callnew code



Before OOP: software reuse when new code call old code

Dynamic binding can improve reuse by letting old code callnew code

Dynamic binding permits software extensibility:With OOP: a new code may be created, compiled, andtestedLater, this code can be called by a framework that waswritten long time agoThere is no need to change the old code:it does not even need to be recompiled



Dynamic binding does not make static binding irrelevant

It introduces many additional dimension of complexity



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 pointerIf it is an object:• Only static binding possible• Check the function signature and• Verify if the function call is correctIf the target is a pointer: consider dynamic binding

In most cases: the method being called depends on the typeof the pointer not on the type of the object


Dynamic binding: a scenario

If the target is a pointer: consider dynamic bindingDefine at which place in the inheritance hierarchy thepointer belongsIf the pointer is of the derived type:only static binding is possibleIf the pointer is of the base type:dynamic binding is possible


Dynamic binding: a scenario

If the pointer is of the base type: dynamic binding is possibleif the object the pointer points to is of the base type:no dynamic bindingif 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


Types of member functions

Defined in the base class and inherited in a derived classwithout redefinition

Defined in a derived class without a counterpart in the baseclass


Types of member functions

Defined in the base class and inherited in a derived classwithout redefinition

Defined in a derived class without a counterpart in the baseclass

Defined in the base class and redefined in a derived classwith the same name and same signature

Defined in the base class and redefined in a derived classwith the same name and different signature

Defined in the base class and redefined in a derived classwith the same name and same signature as virtualIn which case you can apply dynamic binding?


Virtual functions

A virtual function is a member function of the base class thatcan be redefined at run-time

Even if the object is accessed by a base pointervirtual functions allow derived classesto replace the implementation of a function provided by thebase class at run time


Virtual functions

A virtual function is a member function of the base class thatcan be redefined at run-time

Even if the object is accessed by a base pointervirtual functions allow derived classesto replace the implementation of a function provided by thebase class at run time

The replacement is always called whenever the object inquestion 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 thederived class, even if users don’t know about the derivedclass


Example: without virtual functions

class Shape {protected :i n t width , he igh t ;public :

void set ( i n t a , i n t b ) { width=a ; he igh t=b ; } / / access base pa r t on ly} ;

class Rectangle : public Shape {public :

i n t area ( void ) { return ( width ∗ he igh t ) ; } / / s p e c i f i c to Rect .} ;

class Tr iang le : public Shape {public :

i n t area ( void ) { return ( width ∗ he igh t / 2 ) ; } / / s p e c i f i c to T rg l .} ;



Rectangle r e c t ;T r i ang le t r g l ;

Shape ∗ppoly1 = &r e c t ; / / two po in te r s to ob jec ts o f base c lass ShapeShape ∗ppoly2 = & t r g l ; / / They are assigned the addresses of/ / der ived 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 Tr iang le 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 20cout << t r g l . area ( ) << endl ; / / cannot use ppoly2 ; Output 10



To make it possible for the pointers to the base class toaccess a member, we have to declared it in the base class

But what is the area of an undefined shape?



To make it possible for the pointers to the base class toaccess 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 declareit as virtual so that the use of pointers to base objects can befully applied to derived objects


Example: with virtual functions

class Shape {protected :i n t width , he igh t ;public :

void set ( i n t a , i n t b ) { width=a ; he igh t=b ; }/ / se t ( ) not v i r t u a l : i t does the same f o r a l l c lassesv 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 der ived c l s .

} ;

class Rectangle : public Shape {public :

i n t area ( void ) { return ( width ∗ he igh t ) ; }} ;



class Tr iang le : public Shape {public :

i n t area ( void ) { return ( width ∗ he igh t / 2 ) ; }} ;

/ / Test ingRectangle r e c t ;T r i ang le t r g l ;Shape poly ;Shape ∗ppoly1 = &r e c t ;Shape ∗ppoly2 = & t r g l ;Shape ∗ppoly3 = &poly ;



ppoly1−>set ( 4 , 5 ) ;ppoly2−>set ( 4 , 5 ) ;ppoly3−>set ( 4 , 5 ) ;

/ / dynamic b ind ing :/ / At run−t ime the implementat ion o f the v i r t u a l f u n c t i o n/ / i s replaced by t h a t o f the der ived c lasscout << ppoly1−>area ( ) << endl ; / / ou tput : 20cout << ppoly2−>area ( ) << endl ; / / ou tput : 10cout << ppoly3−>area ( ) << endl ; / / ou tput : 0/ / desp i te v i r t u a l i t y we can dec lare an ob jec t o f type Shape/ / and to c a l l i t s area ( ) f u n c t i o n


Virtual & non-virtual functions

Non-virtual member functions are resolved staticallyit is selected at compile-time based on the type of the pointerto the object


Virtual & non-virtual functions

Non-virtual member functions are resolved staticallyit is selected at compile-time based on the type of the pointerto the object

Virtual member functions are resolved dynamicallyit is selected at run-time based on the type of the object (notthe type of the pointer to that object)

Dynamic binding creates extra space and time overhead but...


Example without virtual functions

class Student {f r iend 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 loa t , f loa t , f loa 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


class ELEC311 : public Student {public :ELEC311( char∗ pF , char∗ pL ) ;~ELEC311 ( ) { count−−; }void setGrade ( f loa t , f loa t , f loa 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 ;

} ;



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 ;

} ;



void p r i n t I n f o ( const Student ∗ ) ;

void main {COEN244 c1 ( " Jane " , "Doe" ) ;c1 . setGrade (50 , 55 , 25 , 95 ) ;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 ia stream i n s e r t i o n over loadcout << ∗pStud << endl ; / / QUESTION??? How do I know what k ind/ / o f Student pStud i s ?cout << pStud−>grade ( ) ; / / We cannot do t h i s ! ! !

}


Example with virtual functions

class Student {f r iend ostream& operator <<(ostream&, Student &) ;public :

Student ( char∗ pF , char∗ pL ) ;~Student ( ) ;v i r t u a l f l o a t grade ( ) ; / / <===


class COEN244 : public Student {public :COEN244( char∗ pF , char∗ pL ) ;~COEN244( ) { count−−; }void setGrade ( f loa t , f loa t , f loa 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

Example with virtual functions

void void main {COEN244 c1 ( " Jane " , "Doe" ) ;c1 . setGrade (50 , 55 , 25 , 95 ) ;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 ia stream i n s e r t i o n over loadcout << ∗pStud << endl ;cout << pStud−>grade ( ) ; / / grade ( ) o f COEN244 c lass w i l l be c a l l e d

}


Virtual functions central to OO

Virtual functions make OO software extensible and adaptableBecause old code (in the base class) calls new code (in thederived class)

Programming with classes but without dynamic binding iscalled "object based," but not "object oriented"


Virtual functions: summary

When a base class member function is declared virtual,it permits run-time (dynamic) selection ofthe equivalent member function of the appropriate derivedclass


Virtual functions: summary

When a base class member function is declared virtual,it permits run-time (dynamic) selection ofthe equivalent member function of the appropriate derivedclass

Base class virtual functions may have their own definitionIt will be called

if the referenced object is a base class object orif the referenced derived class did not define anequivalent member function

Derived classes defining a virtual function need not beexplicitly declared virtual

Polymorphic behavior works with pointers & references ofbase classes


Virtual functions: Advice

If using public inheritance,many base class member functions should be declared virtualto ensure that the derived class customizations will overridethe base class behavioreven in a context where a base class object is expected


Abstract Classes

Some classes represent abstract concepts for which objectscannot exist

(There is no reason to create such objects)

Triangle Rectangle Circle

2DShapePerson

Faculty Student Admine

...

... ...Base Base

Derived Derived Derived Derived DerivedDerived/Base


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 calledpure virtual functions virtual void print() =0;

An object of an abstract base class cannot be instantiated


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 calledpure 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


Virtual destructor

You cannot have virtual constructor, but you can have virtualdestructor

A virtual destructor is a virtual function that is needed if wewill delete (using the delete operator) a derived-class objectvia a base-class pointer


Virtual destructor

You cannot have virtual constructor, but you can have virtualdestructor

A virtual destructor is a virtual function that is needed if wewill delete (using the delete operator) a derived-class objectvia a base-class pointer

void main ( ) {/ / pStud p o i n t s t o a Student o b j e c tStudent∗ pStud = new COEN244( ” John” , ”Smith” ) ;

. . .delete pStud ; / / w i l l c a l l t h e S tudent o b j e c t ’ s d e s t r u c t o r}

⇒ The COEN244 object destructor will not be calledNot all memory is deallocated!!!


Virtual destructor

c l a s s Student {fr iend 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 ( ) ;



Virtual destructor

c l a s s Student {fr iend 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 ( ) ;


⇒ When a Student pointer, pointing to a COEN244 object, isdeleted, the COEN244 destructor is called which in turn willcall the Student destructor

⇒ Virtual destructor violate the rule of virtual functions? (name?)but memory leaks are very dangerous


Virtual destructor

A class should have a virtual destructor unless that class hasNO virtual functions

If we have any virtual functions, then we will probably going toprocess derived objects via a base pointer

Such processing may include invoking a destructor (normallydone implicitly via delete)


Virtual destructor

A class should have a virtual destructor unless that class hasNO virtual functions

If we have any virtual functions, then we will probably going toprocess derived objects via a base pointer

Such processing may include invoking a destructor (normallydone implicitly via delete)

Recall virtual functions bind to the code associated with theclass of the object, rather than with the class of thepointer/reference

When you say delete basePtr, and the base class has a virtualdestructor, the destructor that gets invoked is the oneassociated with the type of the object *basePtr, rather thanthe one associated with the type of the pointer


P: a summary@

Classes: provides data abstraction and encapsulationa means to hide implementations but document

Inheritance: promotes software reuse

Polymorphism: offers software extensibility


Polymorphism: Employee example

class Employee {public :Employee ( const char ∗ , const char ∗ ) ;~Employee ( ) ; / / d e s t r u c t o r rec la ims memoryconst 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 abs t rac t base c lassv i r t u a l double earnings ( ) const = 0; / / pure v i r t u a lv i r t u a l void p r i n t ( ) const ; / / v i r t u a l

private :char ∗ f i rs tName ;char ∗ lastName ;

} ;



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 ;

} ;



class SalaryWorker : public Employee {public :

SalaryWorker ( const char ∗ , const char ∗ , double = 0 . 0 ) ;void se tSa la ry ( 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 sa la ry ; / / weekly sa la ry

} ;



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 hourdouble hours ; / / hours worked f o r week

} ;



class CommissionWorker : public Employee {public :

CommissionWorker ( const char ∗ , const char ∗ , double = 0.0 , i n t = 0 ) ;void setCommission ( double ) ;void se tQuan 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 i tem soldi n t q u a n t i t y ; / / t o t a l i tems sold f o r week

} ;



/ / Test ingEmployee∗ l i s t [ 10 ] ;

Boss b ( "CCC" , "TTTTT" , 800 .00) ;

CommissionWorker c1 ( "SSSS" , "QQQQ" , 3 .0 , 150) ;CommissionWorker c2 ( " JJJJ " , "DDDD" , 4 .0 , 60 ) ;

SalaryWorker s1 ( "BBBB" , "VVVVV" , 500) ;SalaryWorker s2 ( "RRRR" , "Red" , 500) ;

HourlyWorker h1 ( "GGGG" , "PPPP" , 17.00 , 3 7 . 5 ) ;HourlyWorker h2 ( "KKKK" , " I I I I " , 13.75 , 40 ) ;HourlyWorker h3 ( "MMM" , "TT" , 10.15 , 18 ) ;



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 Bindingcout << "DYNAMIC BINDING" << endl ;

for ( i n t i = 0 ; i < 8 ; i ++) {l i s t [ i ]−> p r i n t ( ) ; / / using po in te r scout << " earned $ " << (∗ l i s t [ i ] ) . earn ings ( ) ; / / using ob jec ts



/ / S t a t i c Bindingcout << "STATIC BINDING" << endl ;

/ / access through ob jec ts : no polymorphism / v i r t u a l i t yb . p r i n t ( ) ; cout << " earned $ " << b . earn ings ( ) ;c1 . p r i n t ( ) ; cout << " earned $ " << c1 . earn ings ( ) ;c2 . p r i n t ( ) ; cout << " earned $ " << c2 . earn ings ( ) ;s1 . p r i n t ( ) ; cout << " earned $ " << s1 . earn ings ( ) ;s2 . p r i n t ( ) ; cout << " earned $ " << s2 . earn ings ( ) ;

h1 . p r i n t ( ) ; cout << " earned $ " << h1 . earn ings ( ) ;h2 . p r i n t ( ) ; cout << " earned $ " << h2 . earn ings ( ) ;h3 . p r i n t ( ) ; cout << " earned $ " << h3 . earn ings ( ) ;


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 sclass Person {

protected :char i d [ 1 0 ] ; / / data common to both typeschar∗ name ; / / v a r i a b l e leng th

public :Person ( const char i d [ ] , const char nm [ ] ) / / Kind type{ s t r cpy ( Person : : id , i d ) ; / / copy i d

name = new char [ s t r l e n (nm) + 1 ] ; / / get space f o r namei f (name == 0) { cout << " Out o f memory \ n " ; e x i t ( 0 ) ; }s t r cpy (name,nm) ; / / copy name

}

v i r t u a l void w r i t e ( ) const / / to d i sp lay data{ } / / not much to do at the moment

~Person ( ) / / r e t u r n heap memory{ delete [ ] name ; } / / f o r Person ob jec t on ly

} ;



class Facu l ty : public Person {private :char∗ rank ; / / f o r f a c u l t y on ly

public :Facu l ty ( 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 o f memory \ n " ; e x i t ( 0 ) ; }s t r cpy ( rank , r ) ; }

void w r i t e ( ) const / / d i sp lay 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 on ly

~Facu l ty ( ){ delete [ ] rank ; } / / r e t u r n heap memory

} ;



class Student : public Person {private :char∗ major ; / / f o r s tudent on ly

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 o f memory \ n " ; e x i t ( 0 ) ; }s t r cpy ( major ,m) ;

}void w r i t e ( ) const / / d i sp lay record

{ cout << " i d : " << i d << endl ; / / p r i n t id , namecout << " name : " << name << endl ;cout << " major : " << major <<endl <<endl ; / / s tudent on ly

}~Student ( ) { delete [ ] major ; } / / r e t u r n heap memory

} ;



void read ( i f s t r eam& f , Person∗& person ) { / / read one recordchar k ind [ 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 typef . g e t l i n e ( id , 1 0 ) ; / / read i df . g e t l i n e (name, 8 0 ) ; / / read namef . g e t l i n e ( buf , 8 0 ) ; / / rank or major?i f ( strcmp ( kind , "FACULTY" ) == 0){ person = new Facu l ty ( id , name, buf ) ; } / / ob jec t i s Facu l ty

else i f ( strcmp ( kind , "STUDENT" ) == 0){ person = new Student ( id , name, buf ) ; } / / ob jec 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 sp lay record{ p−>w r i t e ( ) ; } / / Facu l ty or Student?



/ / Test ingcout << endl << endl ;Person∗ data [ 2 0 ] ; i n t cnt = 0 ; / / a r ray o f po in te r s

i f s t r eam from ( " un iv . dat " ) ; / / i npu t data f i l ei f ( ! from ) { cout << " Cannot open f i l e \ n " ; return 0; }while ( ! from . eof ( ) )

{ read ( from , data [ cn t ] ) ; / / read u n t i l eofcnt ++; }

cout << " To ta l records read : " << cnt << endl << endl ;

for ( i n t i =0; i < cnt ; i ++){ w r i t e ( data [ i ] ) ; } / / d i sp lay data

for ( i n t j =0; j < cnt ; j ++){ delete data [ j ] ; } / / de le te the record


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.


aishy amer - encsstatic polymorphism: compile-time type and function checking dynamic polymorphism:...

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