Friend class and encapsulation

Friend function

A friend function is a non-member function that is given permission, by the class, to access its private and protected members. It is declared inside the class with the keyword friend.

class Box
{
    int w;
public:
    Box(int x) { w = x; }
    friend void show(Box b);   // friend function
};
void show(Box b) { cout << b.w; }

Friend class

A friend class is a class all of whose member functions can access the private and protected members of another class. It is declared in the second class as friend class A;. Friendship is not mutual (if A is a friend of B, B is not automatically a friend of A) and not inherited.

Do friends violate encapsulation?

Not really, though they loosen it. Reasons:

• The class itself decides who is its friend, by writing the declaration inside the class. Outsiders cannot grant themselves access.
• Friends are listed in the class definition, so they are part of the class's public interface and are easy to find.
• Without friends we would have to make data public or add many get/set functions, which weakens encapsulation more.

However, a friend can read and change private data directly, so too many friends reduce data hiding and make the code harder to maintain. Friends should be used sparingly, only where closely related classes or operators need them.