When not to use include guard in header file?

Question

We all know when to use include guard, but when shall we not use it in our project?

Recently, I saw a project with mix compilation (CUDA + GCC), one header file (CUDA file) is deliberately left without include guard. I am just curious about it.

Answer 1

One case in when you do want to include the same file several times with different parameters. In this case the include file would act as a sort of template. An example are the scalers on Dosbox.

Answer 2

There are 2 scenarios off the top of my head:

1. when you want to turn on/off debugging capabilities (as how assert.h works)
2. for 'x-macro' type of functionality where you have the include file perform 2 parts of problem, such as defining an enum then defining an array of stringified names corresponding to the enums

Answer 3

In our projects we never use include guard. We are using include antiguard:

#ifndef _stdafx_h_
#define _stdafx_h_
#else
#error reinclude stdafx.h
#endif

Because if you reincluded same header - you written wrong code or worked with wrong architecture.

Answer 4

Include guards are used so that the include file can be included multiple times in a single compilation unit without resulting in duplicate declarations.

Do not use include guards when the file should be included multiple times in a single compilation unit and this does not result in duplicate declarations.