Friend function

<h2 id="use-cases">Use cases</h2>
<p>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:
</p>
<ul><li>A function of global or <a href="/facts/Namespace/9efXwMtV">namespace</a> scope may be declared as friend of both classes.</li>
<li>A method of one class may be declared as friend of another one.</li></ul>
// C++ implementation of friend functions.
#include <iostream>
using namespace std;
 
class Foo; // Forward declaration of class Foo in order for example to compile.

class Bar {
  private:
      int a = 0;
  public:
      void show(Bar& x, Foo& y);
      friend void show(Bar& x, Foo& y); // declaration of global function as friend
};
 
class Foo {
  private:
      int b = 6;
  public: 
      friend void show(Bar& x, Foo& y); // declaration of global function as friend
      friend void Bar::show(Bar& x, Foo& y); // declaration of friend from other class
};
 
// Definition of a method of Bar; this 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);
}

<ul><li><a href="/facts/The_C%2B%2B_Programming_Language/pg0jdXo5">The C++ Programming Language</a> by <a href="/facts/Bjarne_Stroustrup/Be1Yq6rn">Bjarne Stroustrup</a></li></ul>
<h2 id="external-links">External links</h2>
<ul><li><a href="http://www.codersource.net/c/ctutorials/ctutorialfriend.aspx">C++ friend function tutorial</a> at CoderSource.net</li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>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.) <a href="1-57610-777-9" target="_blank">1-57610-777-9</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
</ol>

Friend function open-in-new

Friend function