Friend function

In object-oriented programming, a friend function that is a "friend" of a given class is allowed access to private and protected data in that class that it would not normally be able to as if the data was public. Normally, a function that is defined outside of a class cannot access such information. Declaring a function a friend of a class allows this, in languages where the concept is supported.

A friend function is declared by the class that is granting access, explicately stating what function from a class is allowed access. 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. Generallly though, friend functions are a good thing for encapsulation, as you can keep data of a class private from all except those who you explicately state need it, but this does mean your classes will become thightly coupled.

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