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

I'm only using the friend-keyword to unittest protected functions. Some will say that you shouldn't test protected functionality. I, however, find this very useful tool when adding new functionality.

However, I don't use the keyword in directly in the class declarations, instead I use a nifty template-hack to achive this:

template
class FriendIdentity {
public:
  typedef T me;
};

/**
 * A class to get access to protected stuff in unittests. Don't use
 * directly, use friendMe() instead.
 */
template
class Friender: public ParentClass
{
public:
  Friender() {}
  virtual ~Friender() {}
private:
// MSVC != GCC
#ifdef _MSC_VER
  friend ToFriend;
#else
  friend class FriendIdentity::me;
#endif
};

/**
 * Gives access to protected variables/functions in unittests.
 * Usage: friendMe(this, someprotectedobject).someProtectedMethod();
 */
template
Friender & 
friendMe(Tester * me, ParentClass & instance)
{
    return (Friender &)(instance);
}

This enables me to do the following:

friendMe(this, someClassInstance).someProtectedFunction();

Works on GCC and MSVC atleast.