Use `#include` guards for all header files

Reason

To avoid files being #included several times.

In order to avoid include guard collisions, do not just name the guard after the filename. Be sure to also include a key and good differentiator, such as the name of library or component the header file is part of.

Example

// file foobar.h:
#ifndef LIBRARY_FOOBAR_H
#define LIBRARY_FOOBAR_H
// ... declarations ...
#endif // LIBRARY_FOOBAR_H

Enforcement

Flag .h files without #include guards.

Note

Some implementations offer vendor extensions like #pragma once as alternative to include guards. It is not standard and it is not portable. It injects the hosting machine's filesystem semantics into your program, in addition to locking you down to a vendor. Our recommendation is to write in ISO C++: See rule P.2.