Include Guard: #pragma once vs. #ifndef #define #endif

Mason — Sat, 06 Sep 2008 04:08:38 +0000

In the C and C++ programming languages, an include guard, sometimes called a macro guard, is a particular construct used to avoid the problem of double inclusion when dealing with the #include directive. The addition of include guards to a header file is one way to achieve this. pragma once is a non-standard but widely supported preprocessor directive designed to cause the current source file to be included only once in a single compilation. Both approaches specify that the file will be included only once by the compiler when compiling a source code file.

The table below compared the details of pragma once and #ifndef #define #endif approach:

	pragma once	#ifndef #define #endif
Sample code	// header file #pragma once class foo { };	// header file #ifndef FOO_HEADER #define FOO_HEADER class foo { }; #endif // FOO_HEADER
C++ Standard	No But both GCC and Microsoft Visual C++ support it.	Yes
Compiling Performance	Better This can reduce build times as the compiler will not open and read the file after the first #include of the module.	It will still have to open the file multiple times, and discard the guard part when compiler find the macro guard. In a large project this could cause increased compile times. But you can also optimize the compiling performance of #ifdef #define #endif approach by this way.

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Include Guard: #pragma once vs. #ifndef #define #endif