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.
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