Member function vs. nonmember function?

Question

What is your rule for which functions that operate on a class should be member functions vs. nonmember functions? For example, I have a class which represents a maze using a mat

Answer 1

Herb Sutter says "we want to make them nonmember nonfriends if reasonably possible", and he's smarter than I am.

http://www.gotw.ca/gotw/084.htm