More C++ Idioms/Include Guard Macro
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[edit]
Include Guard Macro
[edit] Intent
To prevent inclusion of a header file multiple times.
[edit] Also Known As
[edit] Motivation
Including the same header file in same compilation unit is a problem because it violates a basic rule of C++: One Definition Rule (ODR). A header may get included multiple times because of direct and indirect inclusion.
[edit] Solution and Sample Code
Include Guard macro idiom is an old idiom, which is also applicable in a C program. It used simple #define to prevent inclusion of a header file multiple times in a compilation unit. 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 compiler 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.
[edit] Known Uses
Virtually all header files in the world!
[edit] Related Idioms
[edit] References
#pragma once in Wikipedia.
