Friend class

In object-oriented programming to allow access to "private" or "protected" data of a class in another class, the latter class is declared as a friend class.

Purpose

A friend class gives another class full access to its private and protected data and methods.

Example

//A friend class can be declared as:
class A
{
private:
   //...(statements)
public:
   //...(statements)
   friend class B;
}

In this example class B has access to private and protected data and member functions of class A.

Declaration

Friend declarations can go in either the public, private, or protected section of a class--it doesn't matter where they appear. In particular, specifying a friend in the section marked protected doesn't prevent the friend from also accessing private fields.

Scope

Property of friendships is that they are not transitive: The friend of a friend is not considered to be a friend unless explicitly specified.

Example

class A {

 friend class B;
 int a;

};

class B {

 friend class C;

};

class C {

 void f(A* p) {
  p->a = 2;
 }

};

The compiler would not allow the statement p->a = 2 because class C is not a friend of class A, although C is a friend of a friend of A.

References

• An Introduction to Object-Oriented Programming in C++: with Applications in Computer Graphics, by Graham M. Seed

See also

• Friend function

External links

• C++ Language Tutorial
• http://publib.boulder.ibm.com/infocenter/comphelp/v8v101/index.jsp?topic=%2Fcom.ibm.xlcpp8a.doc%2Flanguage%2Fref%2Fcplr043.htm
• http://www.cplusplus.com/doc/tutorial/inheritance/

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