< More C++ Idioms

Include Guard Macro

Intent

To allow inclusion of a header file multiple times.

Also Known As

Motivation

Including the same header file more than once in the same compilation unit can lead to the violation of a basic rule of C++: One Definition Rule (ODR). A header may get included multiple times because of direct and indirect inclusion.

Solution and Sample Code

Include Guard macro idiom is an old idiom, which is also applicable in a C program. It uses simple #define to allow the inclusion of a header file multiple times in a compilation unit. The idiom ensures that after preprocessing the guarded content of the header file is only seen once by the compiler. More precisely, the compiler gets to see the guarded content where the header file gets included for the very first time. The following macros are put at the very beginning and at very end of a header file:

#ifndef MYHEADER_H_ // beginning
#define MYHEADER_H_ 
...
#endif // MYHEADER_H_ // end

Some compilers support 

#pragma once

as an efficient alternative to include guards. It does not require to open the header file more than once, unlike traditional include guard macro in some compilers. On many modern compilers like GCC4 or MSC++2008 #pragma once will not give better compile time performance as they recognize header guards.

Known Uses

Virtually all header files in the world!

References

#pragma once in Wikipedia.

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