In what scenarios should one declare a member function a friend?

Question

In what kind of scenarios would we declare a member function as a 'friend function' ?..What exact purpose does 'friend function' which defies one of central concept of 'Encapsulation' of OOP serve?

Answer 1

You would use a friend function for the same sort of reasons that you would use a friend class, but on a member function (rather than entire class) basis. Some good explanations are in this thread.

While friend functions and classes do violate encapsulation, they can be useful in some scenarios. For example, you may want to allow a test harness to access class internals to allow you to do whitebox testing. Rather than opening up the entire class to the test harness, you could open up a particular function which accesses the internals required by the test harness. While this still violates encapsulation, it's less risky than opening up the entire class.

Also see this article for some more information about friend classes and functions.

Answer 2

Friend functions and classes do not violate encapsulation when you are trying to build an abstraction or interface that must physically span multiple C++ classes or functions! That is why friend was invented.

Those types of cases don't come up often, but sometimes you are forced to implement an abstraction or interface with disparate classes and functions. The classic example is implementing some type of complex number class. The non-member operator functions are given friendship to the main complex number class.

I also recall doing this when programming with CORBA in C++. CORBA forced me to have separate classes to implement CORBA servants. But for a particular part of our software, I needed to marry these together as one interface. Friendship allowed these two classes to work together to provide a seamless service to one part of our software.

Having the ability to mark a particular member function on another class as a friend to your class may seem even stranger, but it is just a way of tightly controlling the friendship. Instead of allowing the entire other class "in" as your friend, you only allow one of its member functions access. Again, this isn't common, but very useful when you need it.