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. ^[1] 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, explicitly 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 (visibility modes) data may lessen the value of encapsulation of separate classes in object-oriented programming and may indicate a problem in the overall architecture design. Generally though, friend functions are a good thing for encapsulation, as you can keep data of a class private from all except those who you explicitly state need it, but this does mean your classes will become tightly coupled.

Use cases

This approach may be used in friendly function 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

1. Holzner, Steven (2001). C++ : Black Book. Scottsdale, Ariz.: Coriolis Group. p. 397. ISBN 1-57610-777-9. When you declare a function a friend of a class, that function has access to the internal data members of that object (that is, its protected, and private data members.)

• The C++ Programming Language by Bjarne Stroustrup

External links

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