Friend function: Difference between revisions

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In [[object-oriented programming]], a '''friend function''', that is a "friendsfriend" of a given [[class (computer science)|class]], is a function that is given the same access as methods to private and protected [[data]].<ref>{{cite book|last=Holzner|first=Steven|title=C++ : Black Book|year=2001|publisher=Coriolis Group|___location=Scottsdale, Ariz.|isbn=1-57610-777-9|page=397|quote=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.)}}

<ref>{{cite book|last=Holzner|first=Steven|title=C++ : Black Book|year=2001|publisher=Coriolis Group|___location=Scottsdale, Ariz.|isbn=1-57610-777-9|page=397|quote=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.)}}

</ref>

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==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:

*aA function of global or [[namespace]] scope may be declared as friend of both classes.

*aA member functionmethod of one class may be declared as friend of another one.

<sourcesyntaxhighlight lang=Cpp"cpp">

// 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:

Bar(): a(0) {}

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 Bar::show(Bar& x, Foo& y); // declaration of friendglobal fromfunction other classas friend

Foo(): b(6) {}

friend void Bar::show(Bar& x, Foo& y); // declaration of global friend from other class

friend void Bar::show(Bar& x, Foo& y); // declaration of friend from other class

};

// Definition of a member functionmethod of Bar; this member is a friend of Foo

void Bar::show(Bar& x, Foo& y) {

cout << "Show via function member of Bar" << endl;

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a.show(a,b);

}

</syntaxhighlight>

</source>

==References==

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==External links==

*[http://www.codersource.net/c/ctutorials/ctutorialfriend.aspx C++ friend function tutorial] at CoderSource.net

*[http://www.cplusplus.com/doc/tutorial/inheritance.html C++ friendship and inheritance tutorial] at cplusplus.com

[[Category:Method (computer programming)]]