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

When to make it a member function:

• when the function is logically coupled with the class (like your maze connectedness example)
• when the function needs to access private or protected members, it's better to make it a member than a friend.

When to make it a standalone function

• when it's a generic function that can be templatized to naturally work on other classes (look at the <algorithms> header for good example)

Answer 2

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