Friend function

A friend function for a class is used in object-oriented programming to allow access to public, private, or protected data in the class from the outside. Normally, a function that is not a member of a class cannot access such information; neither can an external class. Occasionally, such access will be advantageous for the programmer. Under these circumstances, the function or external class can be declared as a friend of the class using the friend keyword.

A friend function is declared by the class that is granting access. Friend declaration can be placed anywhere in the class declaration. It is not affected by the access control keywords.

A similar concept is that of friend class.

Friends should be used with caution. Too many functions or external classes declared as friends of a class with protected or private data may lessen the value of encapsulation of separate classes in object-oriented programming and may indicate a problem in the overall architecture design.

Use cases

This approach may be used when a function needs to access private data in objects from two different classes. This may be accomplished in two similar ways:

• a function of global or namespace scope may be declared as friend of both classes
• a member function of one class may be declared as friend of another one.

#include <iostream>
using namespace std;
 
class B; // Forward declaration of class B in order for example to compile
class A
{
private:
    int a;
public:
    A() { a = 0; }
    void show(A& x, B& y);
    friend void ::show(A& x, B& y); // declaration of global friend
};
 
class B
{
private:
    int b;
public:
    B() { b = 6; }
    friend void  ::show(A& x, B& y); // declaration of global friend
    friend void A::show(A& x, B& y); // declaration of friend from other class 
};
 
// Definition of a member function of A; this member is a friend of B
void A::show(A& x, B& y)
{
  cout << "Show via function member of A" << endl;
  cout << "A::a = " << x.a << endl;
  cout << "B::b = " << y.b << endl;
}
 
// Friend for A and B, definition of global function
void show(A& x, B& y)
{
  cout << "Show via global function" << endl;
  cout << "A::a = " << x.a << endl;
  cout << "B::b = " << y.b << endl;
}
 
int main()
{
   A a;
   B b;
 
   show(a,b);
   a.show(a,b);
}

References

• The C++ Programming Language by Bjarne Stroustrup

External links

• C++ friend function tutorial at CoderSource.net
• C++ friendship and inheritance tutorial at cplusplus.com