C/C++ #error #pragma #

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#error token-string(记号序列)
将使预处理器打印包含该记号序列的诊断信息;Error directives produce compiler-time error messages.
The error messages include the argument token-string and are currently not subject to macro expansion. These directives are most useful for detecting programmer inconsistencies and violation of constraints during preprocessing. The following example demonstrates error processing during preprocessing:
--------------------------------------------------------
#if !defined(__cplusplus)
#error C++ compiler required.
#endif
--------------------------------------------------------
When #error directives are encountered, compilation terminates.

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#pragma token-string(记号序列)
将使预处理器执行一个与具体实现有关的操作.无法识别的pragma(编译指示)将被忽略掉.
Each implementation of C and C++ supports some features unique to its host machine or operating system. Some programs, for instance, need to exercise precise control over the memory areas where data is placed or to control the way certain functions receive parameters. The #pragma directives offer a way for each compiler to offer machine- and operating system-specific features while retaining overall compatibility with the C and C++ languages. Pragmas are machine- or operating system-specific by definition, and are usually different for every compiler.
Pragmas can be used in conditional statements, to provide new preprocessor functionality, or to provide implementation-defined information to the compiler. The Microsoft C and C++ compilers recognize the following pragmas:
alloc_text        auto_inline            bss_seg             check_stack         code_seg
comment         component            conform1            const_seg            data_seg
deprecated     fenv_access          float_control       fp_contract           function
hdrstop           include_alias         init_seg1            inline_depth         inline_recursion
intrinsic           make_public          managed            message              omp
once                optimize                pack                   pointers_to_members1      pop_macro      push_macro          region,endregion                            runtime_checks  section            setlocale               unmanaged        vtordisp1                 warning    

PS:Supported only by the C++ compiler.
The token-string is a series of characters that gives a specific compiler instruction and arguments, if any. The number sign (#) must be the first non-white-space character on the line containing the pragma; white-space characters can separate the number sign and the word pragma. Following #pragma, write any text that the translator can parse as preprocessing tokens. The argument to #pragma is subject to macro expansion.
If the compiler finds a pragma it does not recognize, it issues a warning, but compilation continues.
Some pragmas provide the same functionality as compiler options. When a pragma is encountered in source code, it overrides the behavior specified by the compiler option. For example, if you specified /Zp8, you can override this compiler setting for specific portions of the code with pack:
cl /Zp8 ...

- packing is 8
// ...
#pragma pack(push, 1) - packing is now 1
// ...
#pragma pack(pop) - packing is 8

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#
不执行任何操作

[dictionary]pragma:附注,注记,编译指示

例如:
#pragma once
Specifies that the file will be included (opened) only once by the compiler when compiling a source code file.

// header.h
#pragma once
This can reduce build times as the compiler will not open and read the file after the first #include of the module.

#pragma unmanaged
譬如：“C++/CLI”也即是managed programming language for Visual C++ 2005，如果在代码前面注明#pragma unmanaged，也就把被托管的本应该生成CIL的C++中间代码改为交给系统底层以传统的方式执行，而不是用CLR运行。

文章出处：飞诺网(www.firnow.com):http://dev.firnow.com/course/3_program/c++/cppjs/20091027/180314.html