My teacher say's CALL is the most expensive subroutine, and costs the most overhead, and that is why we use a friend function (according to her, it reduces the overhead by reducing the number of CALLs. But books don't mention this. They say a friend function is to be used when we need objects of two different classes, and mention nothing about CALLs. I am a bit confused as to who I believe, the textbook, or the teacher.
Could someone help me, or give me a link to some webpage, where I can actually see how and when friend functions are to be used (not what friend functions are).