#pragma once

原文： https://en.wikipedia.org/wiki/Pragma_once#cite_note-11

pragma once

In the C and C++ programming languages, #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. Thus, #pragma once serves the same purpose as #include guards, but with several advantages, including: less code, avoidance of name clashes, and sometimes improvement in compilation speed. [1]

Example

File "grandparent.h"

#pragma once

struct foo 
{
    int member;
};

File "parent.h"

#include "grandparent.h"

File "child.c"

#include "grandparent.h"
#include "parent.h"

Advantages

The most common alternative to #pragma once is to use #define to set aninclude guard macro, the name of which is picked by the programmer to be unique to that file. For example,

#ifndef GRANDPARENT_H
#define GRANDPARENT_H
... contents of grandparent.h
#endif /* !GRANDPARENT_H */

This is more complicated, possibly less efficient, and prone to error as there are no mechanisms to prevent a programmer accidentally using the same macro name in more than one file, which would result in only one of the files being included. This problem renders #pragma once to be advantageous. Since the compiler itself is responsible for handling#pragma once, the programmer cannot make errors which cause name clashes.

Using #pragma once instead of include guards will, for some compilers, improve compilation speed since it is a higher-level mechanism; the compiler itself can compare filenames orinodes without having to invoke theC preprocessor to scan the header for#ifndef and #endif. It is important to note that some compilers such asGCC,Clang, and EDG-based compilers include specific optimizations to recognize and optimize the handling of include guards, and thus little or no speedup benefit is obtained from the use of#pragma once.^[2][3][4]

Caveats

Identifying the same file on a file system is not a trivial task.^[5] Symbolic links and hard links may cause the same file to be found under different names. Compilers may use a heuristic that compares file size, modification time and content.^[6] This backfires when the same file is intentionally copied into several parts of a project. Withinclude guard based on file path these copies would be treated differently while#pragma once may arbitrarily treat them as the same file in a compiler-dependent way.

Portability

Compiler	#pragma once
Clang	Supported[7]
Comeau C/C++	Supported[8]
C++Builder XE3	Supported[9]
Digital Mars C++	Supported[10]
GCC	Supported[11] (since 3.4[5])
HP C/aC++	Supported[12] (since at least A.06.12)
IBM XL C/C++	Supported[13] (since 13.1.1)
Intel C++ Compiler	Supported[14]
Microsoft Visual C++	Supported[15]
Pelles C	Supported[16]
ARM DS-5	Supported[17]
IAR C/C++	Supported[18]
Solaris Studio C/C++	Not supported[19][20]

References

1. "Games from Within: Even More Experiments with Includes". Web.archive.org. 2005-01-25. Retrieved2013-08-19.
2. "The C Preprocessor: 1. The C Preprocessor". Gcc.gnu.org. 1996-02-01. Retrieved 2013-08-19.
3. ""Clang" CFE Internals Manual — Clang 3.4 documentation". Clang.llvm.org. Retrieved2013-08-19.
4. "clang: File manipulation routines". Clang.llvm.org. Retrieved2013-08-19.
5. "GCC 3.4 Release Series — Changes, New Features, and Fixes". Gcc.gnu.org. Retrieved 2013-08-19.
6. "should_stack_file() function in GCC source code".
7. "clang: clang: Pragma.cpp Source File". Clang.llvm.org. Retrieved 2013-08-19.
8. "Comeau C++ Pre-Release User Documentation: Pragmas". Comeaucomputing.com. Retrieved 2013-08-19.
9. "#pragma once - RAD Studio XE3". Docwiki.embarcadero.com. 2010-12-02. Retrieved 2013-08-19.
10. "Pragmas". Digital Mars. Retrieved2013-08-19.
11. "Alternatives to Wrapper #ifndef". Gcc.gnu.org. Retrieved2013-08-20.
12. http://h20565.www2.hp.com/hpsc/doc/public/display?docId=emr_na-c02936807.Missing or empty |title= (help)
13. "Supported GCC pragmas". IBM. Retrieved 2015-02-20.
14. "Diagnostic 1782: #pragma once is obsolete. Use #ifndef guard instead.". Intel Developer Zones. Retrieved4 December 2013.
15. "once (C/C++)". Msdn.microsoft.com. Retrieved 2013-08-19.
16. IDE help/documentation
17. "ARM Information Center". ARM. Retrieved 2013-12-17.
18. "IAR C/C++ Development Guide"(PDF).IAR Systems. Retrieved4 December 2013.
19. "Solaris Studio 12.4: C++ User's Guide". Oracle. Retrieved 2015-02-20.
20. "Solaris Studio 12.4: C User's Guide". Oracle. Retrieved 2015-02-20.

Categories:

• C headers
• C++

=====================笔记======================

原来以为 #pragma once 只是windows平台支持，看完这个wiki，发现gcc也开始支持了。

优点是：

0、使用方便

不用再自己去写 #ifndef ... #define ... #endif

1、防止宏定义冲突

因为头文件的宏都是编码者自己定义的，所以有可能会造成宏定义冲突。

而#pragma once是编译器处理这些问题，不会出现冲突。

2、编译速度得到提升

只是对于某些编译器来说的。gcc编译器就几乎没有速度上的提升。

=====================关于c语言中的声明和定义的知识=====================

http://stackoverflow.com/questions/4769599/declaration-or-definition-in-c

对于内置类型的变量：

 Declaration Definition Tentative Definition Initialized int i; (inside a block) Yes Yes No No int i=5; (inside a block) Yes Yes No Yes(to 5) int i; (otherwise) Yes No Yes Yes(to 0) extern int i; Yes No No No All definitions are declarations but not vice-versa

一个编译单元中不能有结构体的重复定义。

不同编译单元之间可以有结构体的重复定义。

测试代码1：

// test.c

struct a
{
	int x;
};

// error: redefinition
//struct a
//{
//	int y;
//};

// ok
int b; // 声明
int b; // 声明

// ok
int c = 1; // 声明+定义
int c; // 声明

// ok
int d; // 声明
int d = 1; // 声明 + 定义

// error: redefinition
//int e = 1; // 声明+定义
//int e = 2; // 声明+定义

// ok
int f; // 声明
int f = 1; // 声明 + 定义



/* error
warning: data definition has no type or storage class
g = 2;
  ^
error: redefinition of ‘g’
note: previous definition of ‘g’ was here
   int g = 1;
*/
int g = 1; // 声明 + 定义
//g = 2;


//warning: data definition has no type or storage class
i = 5;

int sum()
{
	int c;
	//int c; // error : redefinition

	// ok
	int h = 1;
	h = 2;

	// ok
	i++;
}