Q: What does ``#pragma once'' mean? I found it in some header files.
A: It is an extension implemented by some preprocessors to help make header files idempotent; that is, to make sure that their contents are processed exactly once even if they are #included multiple times. It is equivalent to the #ifndef trick mentioned in question 10.7, though less portable. Some people claim that #pragma once can be implemented ``more efficiently'' (of course only compilation efficiency is a factor here), but in fact a preprocessor that is serious about compilation efficiency can arrange for the portable #ifndef trick to be handled just as efficiently.