c++ - What are the basic rules and idioms for operator overloading?

Note: The answers were given in a specific
order, but since many users sort answers according to votes, rather than the
time they were given, here's an index of the
answers in the order in which they make most
sense:

_{(Note:
This is meant to be an entry to href="https://stackoverflow.com/questions/tagged/c++-faq"> C++ FAQ. If you
want to critique the idea of providing an FAQ in this form, then href="https://meta.stackexchange.com/questions/68647/setting-up-a-faq-for-the-c-tag">the
posting on meta that started all this would be the place to do that. Answers
to that question are monitored in the href="http://chat.stackoverflow.com/rooms/10/c-lounge">C++ chatroom, where
the FAQ idea started out in the first place, so your answer is very likely to get read
by those who came up with the idea.)}