What's difference between #pragma and #ifndef? [duplicate]

Th开发者_如何学Cis question already has answers here: Closed 12 years ago.

Possible Duplicate:

#pragma once vs include guards?

When should I use #pragma once?

When should I use #ifndef HEADER_H_INCLUDED?

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The #ifndef/#define/#endif trick works on any C compiler, and on some of them it speeds up the compilation process. The #pragma trick is non-standards, and only works on few C compilers, and may result in different semantics in those that do not support it.

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The difference is that the latter is C and the former is not. Never use #pragma once; always use #ifndef.

One other thing to note when using the #ifndef method is that any preprocessor symbol beginning with two underscores or an underscore followed by a capital letter is reserved and cannot be used. You should use things like #ifndef MYHEADER_H and not #ifndef _MYHEADER_H.

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The construct

myfoo.h

#ifndef MYFOO_H
#define MYFOO_H

/* header information for myfoo.h */

#endif

belongs in every header-file. The trick is: you can include a header file (accidentally) more than once without thinking abaout double declarations. so this is for the preprocessor.

The #pragma is for the compiler, and a preprocessor should ignore pragmas it does not understand.

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Use #pragma when you are addressing a specific compiler (or set of compatible compilers) to guide its code generation or if you are using a standardized #pragma like FP_CONTRACT or -CX_LIMITED_RANGE- that any standards-compliant compiler is going to support.

Use #ifndef and ilk if you are addressing the standard C (or C++) pre-processor and wish to have your code rendered portable across all standards-compliant compilers.

Use of any #pragma that is not defined in the C (or C++) standard renders your code non-portable. #pragma once is a bit of an exception in that it is one of the most commonly-implemented of the non-standard #pragma constructs. Its implementation, however, is not universal across standards-compliant compilers. #ifndef is.