Post on 16-Mar-2016
description
CHAPTER 10 Inheritance 继承
IntroductionA key feature of an object-oriented language in inheritance. Inheritance is t
he ability to define new classes using existing classes as a basis. The new c
lass inherits the attributes and behaviors of the classes on which it is based,
and it can also have attributes and behaviors that are specific to it.
继承是面向对象语言的重要特征之一。继承是把已有类作为基类定义新的类。这个新的类继承了基类的属性和操作，而且还可以具有自己特定的属性和操作。
Three relationships are commonly used in object-oriented design :
• is-a relationships
Hierarchy relationship. It specifies that one abstraction is a specialization
of another.
• has-a relationships
Contain relationship. Some object is part of another.
• uses-a relationships
One object uses another object in some way.
10.1 OBJECT-ORIENTED DESIGN USING INHERITANCE
is-a relationships
AA
B1B1 B2B2
C1C1 C2C2 C3C3
Base class of B1 and B2
Derived class of ABase class of C1, C2 and C3
Derived class of ABase class of C3
Derived class of B1 and B2Multiple inheritance
Derived class of B1
has-a relationships
class A { … B DMiB DMi … FM} ;
class BB { … DM … FM} ;
uses-a relationships
class A { … DM … FMj ( B &B & … )} ;
class BB { … DM … FM} ;
class DerivedClass : public BaseClass {
public :
// public section
…
private :
// private section
…
} ;
10.3.1 Declaring a derived class
class DerivedClass : public BaseClass {
public :
// public section
…
private :
// private section
…
} ;
10.3.1 Declaring a derived class
Derived class name
class DerivedClass : public BaseClass {
public :
// public section
…
private :
// private section
…
} ;
10.3.1 Declaring a derived class
Access specifier(usually public)
class DerivedClass : public BaseClass {
public :
// public section
…
private :
// private section
…
} ;
10.3.1 Declaring a derived class
Class name of base class
Declare vehicle class and derived class car ：class vehiclevehicle { public : void initialize (int in_wheels , float in_weight ) ; int get_wheels ( ) ; float get_weight ( ) ; float get_loading ( ) ; private : int wheels ; float weight ; float loading ;} ；class carcar : public vehiclevehicle { public : void initialize ( int in_wheels , float in_weight , int people = 4 ) ; int passengers ( ) ; private : int passenger_load ;} ；
Example
The syntax for the constructor of derived class isThe syntax for the constructor of derived class is
10.3.2 Implementing a derived class
DClass :: DClass ( Plist ) : BClass ( Plist ) , DMbrList {
// Body of derived class constructor…
}
The syntax for the constructor of derived class isThe syntax for the constructor of derived class is
10.3.2 Implementing a derived class
DClass :: DClass ( Plist ) : BClass ( Plist ) , DMbrList {
// Body of derived class constructor…
}Derived class name
The syntax for the constructor of derived class isThe syntax for the constructor of derived class is
10.3.2 Implementing a derived class
DClass :: DClass ( Plist ) : BClass ( Plist ) , DMbrList {
// Body of derived class constructor…
}Derived class constructor
parameter list
The syntax for the constructor of derived class isThe syntax for the constructor of derived class is
10.3.2 Implementing a derived class
DClass :: DClass ( Plist ) : BClass ( Plist ) , DMbrList {
// Body of derived class constructor…
}Base class name
The syntax for the constructor of derived class isThe syntax for the constructor of derived class is
10.3.2 Implementing a derived class
DClass :: DClass ( Plist ) : BClass ( Plist ) , DMbrList {
// Body of derived class constructor…
}Base class constructor
parameter list
The syntax for the constructor of derived class isThe syntax for the constructor of derived class is
10.3.2 Implementing a derived class
DClass :: DClass ( Plist ) : BClass ( Plist ) , DMbrList {
// Body of derived class constructor…
} Class data member initialization list
The syntax for the constructor of derived class isThe syntax for the constructor of derived class is
10.3.2 Implementing a derived class
DClass :: DClass ( Plist ) : BClass ( Plist ) , DMbrList {
// Body of derived class constructor…
}
• First base classFirst base class
• Second class data memberSecond class data member
• Last derived classLast derived class
// Example13-1#include < iostream.h >class Base{ public : Base ( ) { cout << "\nBase created.\n" ; }} ;class D_class : public Base{ public : D_class ( ) { cout << "D_class created.\n" ; }} ;void main (){ D_class d1 ; }
Output Base created.
D_class created.
// Example13-2
#include <iostream.h>
class parent_class
{ int private1 , private2 ;
public :
parent_class ( int p1 , int p2int p1 , int p2 ) { private1 = p1; private2 = p2; }
int inc1 ( ) { return ++private1; }
int inc2 ( ) { return ++private2 ; }
void display ( )
{ cout << "private1=" << private1 << " , private2=" << private2 << endl ; }
};
Base class constructor parameter list
class derived_class : private parent_class{ int private3 ; parent_class private4 ; // Class data member public: derived_class ( int p1 , int p2 , int p3 , int p4 , int p5 )
: parent_class ( p1 , p2 ) , private4 ( p3 , p4 ) { private3 = p5 ; } int inc1 ( ) { return parent_class :: inc1 ( ) ; } int inc3 ( ) { return ++private3 ; } void display ( ) { parent_class :: display ( ) ; private4 . display ( ) ; cout << "private3=" << private3 << endl ; }} ;void main ( ){ derived_class d1 ( 17 , 18 , 1 , 2 , -5 ) ; d1 . inc1 ( ) ; d1 . display ( ) ;}
Five parameters of derived class constructor
class derived_class : private parent_class{ int private3 ; parent_class private4 ; // Class data member public: derived_class ( ( int p1 , int p2int p1 , int p2 , int p3 , int p4 , int p5 )
: parent_class ( p1 , p2 )parent_class ( p1 , p2 ) , private4 ( p3 , p4 ) { private3 = p5 ; } int inc1 ( ) { return parent_class :: inc1 ( ) ; } int inc3 ( ) { return ++private3 ; } void display ( ) { parent_class :: display ( ) ; private4 . display ( ) ; cout << "private3=" << private3 << endl ; }} ;void main ( ){ derived_class d1 ( 17 , 18 , 1 , 2 , -5 ) ; d1 . inc1 ( ) ; d1 . display ( ) ;}
For base classparent_class(p1,p2)
Initialize private1 and private2
class derived_class : private parent_class{ int private3 ; parent_class private4 ; // Class data member public: derived_class ( ( int p1 , int p2int p1 , int p2 , int p3 , int p4 ,int p3 , int p4 , int p5 )
: parent_class ( p1 , p2 )parent_class ( p1 , p2 ) , private4 ( p3 , p4 )private4 ( p3 , p4 ) { private3 = p5 ; } int inc1 ( ) { return parent_class :: inc1 ( ) ; } int inc3 ( ) { return ++private3 ; } void display ( ) { parent_class :: display ( ) ; private4 . display ( ) ; cout << "private3=" << private3 << endl ; }} ;void main ( ){ derived_class d1 ( 17 , 18 , 1 , 2 , -5 ) ; d1 . inc1 ( ) ; d1 . display ( ) ;}
For class member private4 parent_class(p1,p2) : initialize
private4.private1 and private4.private2
class derived_class : private parent_class{ int private3 ; parent_class private4 ; // Class data member public: derived_class ( ( int p1 , int p2int p1 , int p2 , int p3 , int p4 ,int p3 , int p4 , int p5int p5 )
: parent_class ( p1 , p2 )parent_class ( p1 , p2 ) , private4 ( p3 , p4 )private4 ( p3 , p4 ) { private3 = p5private3 = p5 ; } int inc1 ( ) { return parent_class :: inc1 ( ) ; } int inc3 ( ) { return ++private3 ; } void display ( ) { parent_class :: display ( ) ; private4 . parent_class :: display ( ) ; cout << "private3=" << private3 << endl ; }} ;void main ( ){ derived_class d1 ( 17 , 18 , 1 , 2 , -5 ) ; d1 . inc1 ( ) ; d1 . display ( ) ;}
For derived class Initialize data member private3
class derived_class : private parent_class{ int private3 ; parent_class private4 ; // Class data member public: derived_class ( ( int p1 , int p2int p1 , int p2 , int p3 , int p4 ,int p3 , int p4 , int p5int p5 )
: parent_class ( p1 , p2 )parent_class ( p1 , p2 ) , private4 ( p3 , p4 )private4 ( p3 , p4 ) { private3 = p5private3 = p5 ; } int inc1 ( ) { return parent_class :: inc1 ( ) ; } int inc3 ( ) { return ++private3 ; } void display ( ) { parent_class :: display ( ) ; private4 . parent_class :: display ( ) ; cout << "private3=" << private3 << endl ; }} ;void main ( ){ derived_class d1 ( 17 , 18 , 1 , 2 , -5 ) ; d1 . inc1 ( ) ; d1 . display ( ) ;}
Output
private1 = 18 , private2 = 18
private1 = 1 , private2 = 2
private3 = -5
• Data members and member functions can be cover in the derived class.
• The derived class member functions do not access to the private member
of base class.
10.4 PROTECTED MEMBERS AND INHERITANCE
保护成员与继承
Exampleclass base{ public : int a , bb ; };class derived : public base { public : int bb , c ; } ;void f ( ){ derived d ; d . a = 1 ; d . base :: bbase :: b = 2 ; // use member b of base class d . bb = 3 ; // use member b of derived class d . c = 4 ;};
Exampleclass X{ public : void get_ij ( ) ; void put_ij ( ) ; private : int i , jprivate : int i , j ;};class Y : public X{ public : int get_k ( ) ; void make_k ( ) ; private : int k ;};void Y :: make_k ( ) ;{ k = i * ji * j ; // illegal } ;
Exampleclass X{ public : void get_ij ( ) ; void put_ij ( ) ; private : int i , jprivate : int i , j ;};class Y : public X{ public : int get_k ( ) ; void make_k ( ) ; private : int k ;};void Y :: make_k ( ) ;{ k = i * ji * j ; // illegal } ;
protected : int i , jprotected : int i , j ;
k = i * ji * j ; // legal
10.5 CONTROLLING INHERITANCE 控制继承 10.5.1 Public Inheritance 公有继承
class DerivedClass : public BaseClass {
public :
// public section
…
private :
// private section
…
} ;
The public members and protected
members keep their character in
derived class.
// Example13-3#include<iostream.h>class XX { public : void get_ij( ) { cout << "Enter two numbers:" ; cin >> i >> j ; }; void put_ij( ) { cout << i << " " << j << '\n' ; }; protected : int i , j ;};class YY : public X { public : int get_k( ) { return k ; }; void make_k( ) { k = i i * jj ; }; // use base class DM private : int k ;};class ZZ : public Y { public : void f( ) { ii = 2 ; jj = 3; }; // use base class DM};void main ( ){ Y var1Y var1; Z var2Z var2 ; var1var1.get_ij( )get_ij( ) ; var1var1.put_ij( )put_ij( ) ; var1.make_k( ) ; var2.f( ) ; var2var2.put_ij( )put_ij( ) ;}
class X
class Y
class Z
10.5.2 Private inheritance 私有继承
class DerivedClass : private BaseClass {
public :
// public section
…
private :
// private section
…
} ;
The public members and protected
members of base class are private
member in derived class.
// Example13-4#include<iostream.h>class XX { public : void get_ij( ) { cout << "Enter two numbers:" ; cin >> i >> j ; }; void put_ij( ) { cout << i << " " << j << '\n' ; }; protected : int i , j ;};class YY : public X { public : int get_k( ) { return k ; }; void make_k( ) { k = i i * jj ; }; // use base class DM private : int k ;};class ZZ : public Y { public : void f( ) { ii = 2 ; jj = 3; }; // use base class DM};void main ( ){ Y var1Y var1; Z var2Z var2 ; var1var1.get_ij( )get_ij( ) ; var1var1.put_ij( )put_ij( ) ; var1.make_k( ) ; var2.f( ) ; var2var2.put_ij( )put_ij( ) ;}
// Example13-4#include<iostream.h>class XX { public : void get_ij( ) { cout << "Enter two numbers:" ; cin >> i >> j ; }; void put_ij( ) { cout << i << " " << j << '\n' ; }; protected : int i , j ;};class YY : privateprivate X { public : int get_k( ) { return k ; }; void make_k( ) { k = i i * jj ; }; // use base class DM private : int k ;};class ZZ : public Y { public : void f( ) { ii = 2 ; jj = 3; }; // use base class DM};void main ( ){ Y var1Y var1; Z var2Z var2 ; var1var1.get_ij( )get_ij( ) ; var1var1.put_ij( )put_ij( ) ; var1.make_k( ) ; var2.f( ) ; var2var2.put_ij( )put_ij( ) ;}
class X
class Y
class Z
private
public
OK i, j are private member in Y class
// Example13-4#include<iostream.h>class XX { public : void get_ij( ) { cout << "Enter two numbers:" ; cin >> i >> j ; }; void put_ij( ) { cout << i << " " << j << '\n' ; }; protected : int i , j ;};class YY : privateprivate X { public : int get_k( ) { return k ; }; void make_k( ) { k = i i * jj ; }; // use base class DM private : int k ;};class ZZ : public Y { public : void f( ) { ii = 2 ; jj = 3; }; // use base class DM};void main ( ){ Y var1Y var1; Z var2Z var2 ; var1var1.get_ij( )get_ij( ) ; var1var1.put_ij( )put_ij( ) ; var1.make_k( ) ; var2.f( ) ; var2var2.put_ij( )put_ij( ) ;}
class X
class Y
class Z
private
public
Illegal Illegal Can not access private member of
base class
// Example13-4#include<iostream.h>class XX { public : void get_ij( ) { cout << "Enter two numbers:" ; cin >> i >> j ; }; void put_ij( ) { cout << i << " " << j << '\n' ; }; protected : int i , j ;};class YY : privateprivate X { public : int get_k( ) { return k ; }; void make_k( ) { k = i i * jj ; }; // use base class DM private : int k ;};class ZZ : public Y { public : void f( ) { ii = 2 ; jj = 3; }; // use base class DM};void main ( ){ Y var1Y var1; Z var2Z var2 ; var1var1.get_ij( )get_ij( ) ; var1var1.put_ij( )put_ij( ) ; var1.make_k( ) ; var2.f( ) ; var2var2.put_ij( )put_ij( ) ;}
class X
class Y
class Z
private
public
Illegal Illegal main() can not call private MF of
any objects
10.5.3 Protected inheritance 保护继承
class DerivedClass : protected BaseClass {
public :
// public section
…
private :
// private section
…
} ;
The public members and protected
members of base class are protected
member in derived class.
Protected inheritance is used rarely.
Inheritance Type Base Class Member Access
Derived Class Member Access
public publicprotectedprivate
publicprotectedinaccessible
protected publicprotectedprivate
protectedprotectedinaccessible
private publicprotectedprivate
privateprivateinaccessible
Do you know Do you know how access x, y and zhow access x, y and z
public : x class A protected : y
private : z
Such as : main()
Common functions
class B : public A
Derived
class C : private A
Derived
A derived class can inherit from two or more base class.
10.6 MULTIPLE INHERITANCE 多继承
class C : public A ， public B
class A class B
A derived class can inherit from two or more base class.
10.6 MULTIPLE INHERITANCE 多继承
The syntax for declaring a class derived using multiple inheritance is
class DClass : publicpublic BClassBClass, , publicpublic BClassBClass {
public :// public section …
private :// private section…
} ;
// Example13-4#include<iostream.h>class A { public : void setA ( int x ) { a = x ; } ; void showA( ) { cout << a << endl ; } ; private : int a ;} ;class B { public : void setB ( int x ) { b = x ; } ; void showB ( ) { cout << b << endl ; } ; private : int b ;} ;class C : public Apublic A , private Bprivate B { public : void setC ( int x , int y , int z ) { setA ( x ) ; setB ( y ) ; c = z ; } ; void showC ( ) { showA ( ) ; showB ( ) ; cout << c << endl ; } ; private : int c ;} ;void main ( ) { C obj ; obj . setA ( 5 ) ; obj . showA ( ) ; obj . setC ( 6 , 7 , 9 ) ; obj . showC ( ) ; //obj . setB ( 6 ) ;//obj . setB ( 6 ) ; // error, private inheritance //obj . showB ( ) ;//obj . showB ( ) ; // error}
class A class B
class C
• If a derived class be derived from multiple base classes, and these base
have a common base class, then, when access members of this common
base class may be open to different interpretations.
10.7 VIRTUAL BASE CLASS
Example
class B { public : int b ;} ;
class B1 : public B { private : int b1 ; } ;
class B2 : public B { private : int b2 ; } ;
class C : public B1 , public B2
{ public : int f ( ) ; private : int d ; } ;
…
C c ;
c . bc . b // error
c . B :: bc . B :: b // error ， where from?
c . B1 :: b // ok ， from B1
c . B2 :: b // ok ， from B2
class C { f () , d }
class B1 { b1 } class B2 {b2}
class B { b } class B { b }
// Example13-5
#include<iostream.h>
class B{public: int b;};
class B1:public B {public: int b1;};
class B2:public B {public: int b2;};
class C:public B1, public B2 {public: f(); private: int b1;};
void main()
{ C c;
c.B1::b=5;
c.B2::b=10;
cout<<"path B1==>"<<c.B1::b<<endl;
cout<<"path B2==>"<<c.B2::b<<endl;
}
Output:
path B1==> 5
path B2==> 10
class C { f () , d }
class B1 { b1 } class B2 {b2}
class B { b } class B { b }
b
b1
b
b2
d
B
B
B1
B2C
Store of multiple derived class object C
When create object C, constructor of B is called two times. Once for B1,Others for B2. A C object contains two sub-object of B.
class C { f () , d }
class B1 { b1 } class B2 {b2}
class B { b } class B { b }Example
class B { public : int b ;} ;
class B1 : public B { private : int b1 ; } ;
class B2 : public B { private : int b2 ; } ;
class C : public B1 , public B2
{ public : int f ( ) ; private : int d ; } ;
• If a derived class be derived from multiple base classes, and these base
have a common base class, then, when access members of this common
base class may be open to different interpretations.
10.7 VIRTUAL BASE CLASS
• For produce only one sub-object in deriver class object, the base classes
must be declared virtual inheritance the common base class.
Key word virtual
Exampleclass B { public : int b ;} ;
class B1 : virtualvirtual public B { private : int b1 ; } ;
class B2 : virtualvirtual public B { private : int b2 ; } ;
class C : public B1 , public B2
{ private : float d ; } ;
As:C cc ;cc . bcc . b // OK
class C { d }
class B1 { b1} class B2 {b2}
class B { b }
cc . B1 :: b and cc . B2 :: b is same sub-object of class B
Store of object C using virtual inheritance
b1
b2
d
b BB
B1B1
B2B2CC
class C { d }
class B1 { b1} class B2 {b2}
class B { b }Exampleclass B { public : int b ;} ;
class B1 : virtualvirtual public B { private : int b1 ; } ;
class B2 : virtualvirtual public B { private : int b2 ; } ;
class C : public B1 , public B2
{ private : float d ; } ;
• If a derived class be derived from multiple base classes, and these base
have a common base class, then, when access members of this common
base class may be open to different interpretations.
10.7 VIRTUAL BASE CLASS
• For produce only one sub-object in deriver class object, the base classes
must be declared virtual inheritance the common base class.
• Using virtual inheritance, the constructor of common base class is called
only once.
// Example13-6#include < iostream.h >class A{ public : A ( ) { cout << "class A" << endl ; } } ;class B : virtual public A{ public : B ( ) {cout << "class B" << endl ; } } ;class C : virtual public A{ public : C ( ) {cout << "class C" << endl ; } } ;class D : public B , public C{ public : D ( ) {cout << "class D" << endl ; } } ;void main ( ){ D dd ; }
Output class Aclass Bclass Cclass D
// Example13-7#include < iostream.h >class A{ public : A ( ) { cout << "class A" << endl ; } } ;class B : /*virtual*/ public A{ public : B ( ) {cout << "class B" << endl ; } } ;class C : /*virtual*/ public A{ public : C ( ) {cout << "class C" << endl ; } } ;class D : public B , public C{ public : D ( ) {cout << "class D" << endl ; } } ;void main ( ){ D dd ; } Output class A
class Bclass Aclass Cclass D
• Each object of a class has its own copy of all data members of the class.
In certain cases only one copy of a variable should be share by all objects
of a class. A static class variable is used for these and other reasons.
• The declaration of a static member begins with the keyword static.
10.8 STATIC CLASS MEMBERS 静态类成员
• Static data members have class scope.
• Static data members must be initialized once (and only once) at file
scope.
• A class’s public static data members can be accessed through any object
of that class.
• A class’s private and protected static data members must be accessed
through public member functions of the class or through friends of the
class.
10.8.1 Static data members 静态数据成员
class object { char ch ; static s ; …... };
object A , B , C , D ;
object B
char ch ;
object C
char ch ;
object D
char ch ;
ExampleExample
object A static s ;static s ;
char ch ;
// Example13-8#include<iostream.h>class counter{ static int num1 ;static int num1 ; public : void setnum1 ( int i ) { num1 = i ; } void shownum1 ( ) { cout << num1 << " " ; } static int num2 ;static int num2 ;} ;int counter :: num1 = 0 ;int counter :: num1 = 0 ; // initial 0 can default int counter :: num2 = 5;int counter :: num2 = 5;void main ( ){ counter a , b ; cout<<"private static DM : \n" ; a . shownum1 ( ) ; b . shownum1 ( ) ; a . setnum1 ( 10 ) ; a . shownum1 ( ) ; b . shownum1 ( ) ; // cout<<counter::num1;// cout<<counter::num1; //error, can not access num1 cout<<“\n public static DM :\n "<< counter :: num2counter :: num2 <<endl;}
// Example13-9 count the number of object# include < iostream.h >class counter{ static int num ; public : counter ( ) { cout << ++ num <<'\t' ; } ~counter( ) { cout << num -- <<'\t' ; } } ;int counter :: num = 0 ;void main ( ){ counter a , b , c ; cout<<endl;}
Output
1 2 3
3 2 1
• Static member functions access static member only.
• Static member functions have no this pointer.
10.8.2 Static member functions 静态成员函数
10.8.2 Static member functions 静态成员函数 Example class X{ int member ; public : static void func ( int i , X * ptr ) ;static void func ( int i , X * ptr ) ; }:
void g ( ){ X obj ; X :: func ( 1, & obj ) ;X :: func ( 1, & obj ) ; // OK obj . func ( 1 , & obj ) ;obj . func ( 1 , & obj ) ; // OK}
10.8.2 Static member functions 静态成员函数 Example class X{ int member ; public : static void func ( int i , X * ptr ) ;static void func ( int i , X * ptr ) ; }:
void X :: func ( int i ; X * ptr )
{ member = i ; // Error ， whose member ？ ptr -> member = i ;ptr -> member = i ; // OK
}
10.8.2 Static member functions 静态成员函数 Example class X{ int member ; public : static void func ( int i ,static void func ( int i , X aX a ) ;) ; }:
void X :: func ( int i ; void X :: func ( int i ; X aX a ) )
{ { cout << member ;cout << member ; // error, can not access non static member
cout << a . member ;cout << a . member ; // OK, pass parameter
}}
// Example13-10 count goods#include<iostream.h>class Goods { public : static int totalWeight ;static int totalWeight ; Goods ( int w) { weight = w; totalWeight += w ; } ; ~ Goods ( ) { totalWeight -= weight ; } ; int Weight ( ) { return weight ; } ; static int TotalWeight ( ) { return totalWeight ; } ; private : int weight ;} ;int Goods :: totalWeight = 0 ;int Goods :: totalWeight = 0 ;void main ( ){ int w ; cin >> w ; Goods * g1 = new Goods ( w ) ; cin >> w ; Goods * g2 = new Goods ( w ) ; cout << Goods :: TotalWeight ( ) << endl ; delete g2 ; cout << Goods :: TotalWeight ( ) << endl ;}
• A friend function of a class is defined outside that class’s scope.
• Friend functions have right access private members of the class.
• A function or an entire class may be declared to be a friend of another
class.
• Friendship is neither symmetric nor transitive.
10.9 FRIEND FUNCTION AND FRIEND CLASS
10.9 FRIEND FUNCTION AND FRIEND CLASS
Example
class X
{ int i ;
friend void func ( X * , int ) ; // func is friend of X
void member_func ( int ) ;
};
…...
void func ( X * xptr , int a )
{ xptr -> i = a ; }; // access private member
void X :: member_func ( int a )
{ i = a ; };
Example
class X
{ int i ;
friend void func ( X * , int ) ; // func is friend of X
void member_func ( int ) ;
};
// Example13-11 Count Distance#include<iostream.h>#include<math.h>class Point { public : Point ( double xi , double yi ) { X = xi ; Y = yi ; } double GetX( ) { return X ; } double GetY( ) { return Y ; } friend double Distance ( Point & a , Point & b ) ;friend double Distance ( Point & a , Point & b ) ; private : double X , Y ;} ;double Distance ( Point & a , Point & b )double Distance ( Point & a , Point & b ) { double dx = a.X - b.X ; // access private data members double dy = a.Y - b.Y ; return sqrt ( dx * dx + dy * dy ) ;}void main ( ) { Point p1 ( 3.0 , 5.0 ) , p2 ( 4.0 , 6.0 ) ; double d = Distance ( p1 , p2 ) ; cout << "This distance is " << d << endl ;}
// Example13-12 Friend Class#include<iostream.h>class A { friend class B ;friend class B ; // class B is friend of class A public: void Display ( ) { cout << x << endl ; }; private : int x ;};class Bclass B{ public: void Set ( int i ) { a . xa . x = i ; } ; // access private DM of a void Display ( ) { a. Display ( ) ; } ; private: A aA a ;} ;
void main ( ){ B b; b . Set ( 100 ) ; b . Display ( ) ; }<< endl ;}