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

Answer 1

I always ( and only ) use friend for unit testing private methods. The only other way I can imagine to do this would be to load up the public interface with a whole lot of testing methods, which is just too messy and so I prefer to hide the test methods in a seperate test class.

Something like this:

class cMyClassTest;

class cMyClass
{
public:
.....

private:
friend cMyClassTest;
int calc();     // tricky algorithm, test carefully

};

class cMyClassTest
{
public:
int test_calc()
{
    cMyClass test;
    ....
    int result = test.calc();

    if( result == 42 )
        return 1;
    return 0;
}
};

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

When you wish that one class (Factory) be responsible for creating instances of another class (Type). You can make the constructor of the Type private and thus make sure that only the Factory can create Type objects. It is useful when you wish to delegate the checks to some other class which could serve as a validator. Just one usage scenario.

P.S. Really missing the "friend" keyword in C#...

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

Friendship is used when you have multiple classes and/or functions that work together to provide the same abstraction or interface. The classic example is implementing some kind of numerical class, and all the non-member operator functions (*, -, +, <<, etc) are given friendship so that they can work on the private data of the numerical class.

Such use cases are somewhat rare, but they do exist, and friend is very useful.