Preprocessor directive #ifndef for C/C++ code

This is just a common way to protect your includes - in this way it prevents the code from being included twice. And the identifier used could be anything, it's just convention to do it the way described.

This is a common construct. The intent is to include the contents of the header file in the translation unit only once, even if the physical header file is included more than once. This can happen, for example, if you include the header directly in your source file, and it's also indirectly included via another header.

Putting the #ifndef wrapper around the contents means the compiler only parses the header's contents once, and avoids redefinition errors.

Some compilers allow "#pragma once" to do the same thing, but the #ifndef construct works everywhere.