Friend functions and Friend classes

Data hiding is the fundamental concept of object-oriented programming. It means a class private or protected data can access within that class or that class members only.

A class private or protected data cannot be accessed in outside the class or another class. To access the private data in outside class or another class, C++ provides the feature called friend functions and friend classes.

Friend Functions

• In general, class private or protected members are not accessed in outside of the class or another class.
• To access the private members of a class in outside the class, C++ provides the feature called “Friend functions‟.
• A friend function is not a member function of a class, that is, it is a non-member function of the class. But it can access the private members of a class.
• A friend function is declared inside the public specifier of the class and is defined (implemented) outside the class as an ordinary function.
• A friend function is identified by the keyword "friend" followed by function declaration.
• A friend function is declared within a class.

Syntax for a friend function is

class class_name

{

Public:

friend return_type fun_name(args_list);

………

};

return_type fun_name(args_list)

{

statements;

}

}

//Demonstration on friend function

#include<iostream.h>

#include<conio.h>

class sample

{

private:

int n;

public:

sample()

{

n=0;

}

sample(int k) {

n=k;

 }

void show(char *msg)

{

cout<<"\n"<<msg<<n;

}

friend int add(sample, sample);

};

int add(sample x, sample y)

{

return x.n+y.n;

}

void main()

{

clrscr();

sample s1(10),s2(40);

s1.show("s1 : ");

s2.show("s2 : ");

cout<<"\n s1+s2 = "<<add(s1,s2); getch();

}

Friend Class

·       In general, class private or protected members cannot be accessed by the members of another class. To access those members, C++ provides the “friend classes‟.

·         A friend class can access the private members of another class.

·         To declare a friend class, use the keyword “friend‟.

Syntax:-       

friend class classname;

·         A friend class is declared in another class for which it is to be friend.

Ex.               

class A

{

friend class B;

………

};

class B

{

………

.…….

};

·         Similar to friend functions, a class can be friend to any number of classes.

·         When you declare a class as friend, then all its members also became the friend of the other class.A friendship must always be explicitly specified (defined).A friendship is not transitive.

Ex. //Adding two objects of a class using friend function

#include<iostream.h>

 #include<conio.h> 

class first

{

private: int n; public:

first()

{

n=0;                  

}

first(int k)

{                       

n=k;                  

}

void show(char *msg)

{

cout<<"\n"<<msg<<n;

}

friend class second;

};

class second

{

public:

int add(first x, first y)

{

return x.n+y.n;

}

};

void main()

{

clrscr();

first f1(10),f2(40);

f1.show("f1 : ");

f2.show("f2 : ");

second s;

cout<<"\nf1+f2 = "<<s.add(f1,f2); 

getch();

}