Friend functions

Friend functions

In principle, private and protected members of a class cannot be accessed from outside the same class in which they are declared. However, this rule does not apply to "friends".

Friends are functions or classes declared with the friend keyword.

A non-member function can access the private and protected members of a class if it is declared a friend of that class. That is done by including a declaration of this external function within the class, and preceding it with the keywordfriend:

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// friend functions #include <iostream> using namespace std; class Rectangle { int width, height; public: Rectangle() {} Rectangle (int x, int y) : width(x), height(y) {} int area() {return width * height;} friend Rectangle duplicate (const Rectangle&); }; Rectangle duplicate (const Rectangle& param) { Rectangle res; res.width = param.width*2; res.height = param.height*2; return res; } int main () { Rectangle foo; Rectangle bar (2,3); foo = duplicate (bar); cout << foo.area() << '\n'; return 0; }

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The duplicate function is a friend of class Rectangle. Therefore, function duplicate is able to access the members widthand height (which are private) of different objects of type Rectangle. Notice though that neither in the declaration ofduplicate nor in its later use in main, function duplicate is considered a member of class Rectangle. It isn't! It simply has access to its private and protected members without being a member.

Typical use cases of friend functions are operations that are conducted between two different classes accessing private or protected members of both.