[boost] #ifndef paradigm

From: Eric Woodruff (Eric.Woodruff_at_[hidden])
Date: 2002-08-27 13:58:45

Often, when one wants to make sure a header is included only once, s/he uses
the paradigm:

#ifndef HEADER_NAME_H__
#define HEADER_NAME_H__

// code here...

#endif // HEADER_NAME_H__ -- this comment is to make it clear what the endif
is for

It is better to remove the duality of the comment at the end, by switching
to this paradigm:

#ifndef HEADER_NAME_H__

// code here...

#define HEADER_NAME_H__
#endif

When maintiaining a file -- renaming it -- with the comment, one has to make
sure the comment is also updated. This alternative removes the comment
because it is superfluous, and the compiler will help you if you forget to
change the #define at the end.