When to use friend class in C++ [duplicate]

Question

Possible Duplicate:
When should you use 'friend' in C++?

I was brushing up on my C++ (I'm a Java developer) and I came across the friend class keyword which I had forgotten about for a while. Is this one of those features that's just part of the kitchen sink, or is there a good reason for doing this rather than just a vanilla getter? I understand the difference in that it limits who can access the data, but I can't think of a scenario when this would be necessary.

Note: I've seen a similar question, but specifically I'm asking, is this just an advanced feature that adds no real value except to confuse people looking at you're code until they realize what you're doing?

Answer 1

friend class mean we all know that is acesss the value of variable from other class so it is mainly used for use the values so we no need to return the value of other class to main function then main to needed class member function but it having the problem that is a class is friend for other class then friend class should be in below of that class

Answer 2

The FAQ's section about friends: here

The FQA's section about friends: here

Two different points of view about friend.

Answer 3

Here is one example, of several, I'm sure, where a friend class can be legitimately used without disregarding the reasons for encapsulation.

MyClass inherits from GeneralClass. MyClass has gotten big, so you created HelperClass to encapsulate some of the function of MyClass. However, HelperClass needs access to some protected functions in GeneralClass to properly perform it's function, so you make HelperClass a friend to MyClass.

This is better than exposing the protected functions, because they don't need to be available to everybody, but it helps keep your code organized in an OOP way to keep MyClass from getting too complex. It makes sense, because although HelperClass isn't concretely related to MyClass by inheritance, it does have some sort of logical connection to it, embodied in the code, and in design, as "friend".

Answer 4

A concrete instance would be a class factory, where you want one class to only be created through another factory class, so you make the constructors private, and the factory class a friend of the produced class.

It's kinda' like a 2" 12-point 3/4"-drive socket - not terribly common, but when you need it, you're awfully glad you have it.

Answer 5

I look at the friend construct as one of those features of the language that should be used in rare occasions, but that doesn't make it useless. There are several patterns that call for making friend classes, many of them already on this site in that "Related" bar on the right. ====>