Friend class

A friend class in C++ can access the private and protected members of the class in which it is declared as a friend.^[1]

Rationale

Friendship may allow a class to be better encapsulated by granting per-class access to parts of its API that would otherwise have to be public.^[2] This increased encapsulation comes at the cost of tighter coupling due to interdependency between the classes.^[3]

Example

class B {
    friend class A; // A is a friend of B

private:
    int i;
};

class A {
public: 
    A(B b) {
        b.i = 0; // legal access due to friendship
    }
};

Properties

Friendships are not symmetric – if class A is a friend of class B, class B is not automatically a friend of class A.
Friendships are not transitive – if class A is a friend of class B, and class B is a friend of class C, class A is not automatically a friend of class C.
Friendships are not inherited – if class Base is a friend of class X, subclass Derived is not automatically a friend of class X; and if class X is a friend of class Base, class X is not automatically a friend of subclass Derived.

References

External links

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