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

Friends are also useful for callbacks. You could implement callbacks as static methods

class MyFoo
{
private:
    static void callback(void * data, void * clientData);
    void localCallback();
    ...
};

where callback calls localCallback internally, and the clientData has your instance in it. In my opinion,

or...

class MyFoo
{
    friend void callback(void * data, void * callData);
    void localCallback();
}

What this allows is for the friend to be a defined purely in the cpp as a c-style function, and not clutter up the class.

Similarly, a pattern I've seen very often is to put all the really private members of a class into another class, which is declared in the header, defined in the cpp, and friended. This allows the coder to hide a lot of the complexity and internal working of the class from the user of the header.

In the header:

class MyFooPrivate;
class MyFoo
{
    friend class MyFooPrivate;
public:
    MyFoo();
    // Public stuff
private:
    MyFooPrivate _private;
    // Other private members as needed
};

In the cpp,

class MyFooPrivate
{
public:
   MyFoo *owner;
   // Your complexity here
};

MyFoo::MyFoo()
{
    this->_private->owner = this;
}

It becomes easier to hide things that the downstream needn't see this way.