The Interface Principle by Herb Sutter
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.
For in depth discussion read Namespaces and the Interface Principle by Herb Sutter.
EDIT
Actually, if you want to understand C++ go and read everything what Herb Sutter has written 🙂