C++ What’s in a class?

http://www.gotw.ca/publications/mill02.htm Herb Sutter, 1998

With that example in mind, I'll propose the Interface Principle:

The Interface Principle
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.