When should you use 'friend' in C++?

Question

I have been reading through the C++ FAQ and was curious about the friend declaration. I personally have never used it, however I am interested in exploring the language.

Answer 1

Another common version of Andrew's example, the dreaded code-couplet

parent.addChild(child);
child.setParent(parent);

Instead of worrying if both lines are always done together and in consistent order you could make the methods private and have a friend function to enforce consistency:

class Parent;

class Object {
private:
    void setParent(Parent&);

    friend void addChild(Parent& parent, Object& child);
};

class Parent : public Object {
private:
     void addChild(Object& child);

     friend void addChild(Parent& parent, Object& child);
};

void addChild(Parent& parent, Object& child) {
    if( &parent == &child ){ 
        wetPants(); 
    }
    parent.addChild(child);
    child.setParent(parent);
}

In other words you can keep the public interfaces smaller and enforce invariants that cut across classes and objects in friend functions.