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;
}
};