I'm going to have to pose an educated disagreement. According to the classic C++ article, "What's in a class?" by Herb Sutter (http://www.gotw.ca/publications/

I'm going to have to pose an educated disagreement. According to the classic C++ article, "What's in a class?" by Herb Sutter (http://www.gotw.ca/publications/mill02.htm http://www.gotw.ca/publications/mill02.htm) - The Interface Principle suggests - For a class X, all functions, including free functions, that both (a) "mention" X, and (b) are "supplied with" X are logically part of X, because they form part of the interface of X. So yes, f(x) is part of x's interface in C++, and has been considered to be so for a long time.