#pragma的作用
#pragma
The #pragma command gives the programmer the ability to tell the compiler to do certain things. Since the #pragma command is implementation specific, uses vary from compiler to compiler. One option might be to trace program execution.
//MSDN-描述
Pragma directives specify machine- or operating-specific compiler features. The __pragma keyword, which is specific to the Microsoft compiler, enables you to code pragma directives within macro definitions.
#pragma token-string __pragma(token-string)
Remarks
Each implementation of C and C++ supports some features unique to its host machine or operating system. Some programs, for example, must exercise precise control over the memory areas where data is put 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 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 that contains 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 that it does not recognize, it issues a warning and continues compilation.
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
strict_gs_check
unmanaged
vtordisp1
warning
1. Supported only by the C++ compiler.
Pragmas and Compiler Options
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 sections of the code with pack:
Copy Code
cl /Zp8 ... <file> - packing is 8 // ... #pragma pack(push, 1) - packing is now 1 // ... #pragma pack(pop) - packing is 8 </file>
The __pragma() Keyword
Microsoft specific
The compiler also supports the __pragma keyword, which has the same functionality as the #pragma directive, but can be used inline in a macro definition. The #pragma directive cannot be used in a macro definition because the compiler interprets the number sign character ('#') in the directive to be the stringizing operator (#).
The following code example demonstrates how the __pragma keyword can be used in a macro. This code is excerpted from the mfcdual.h header in the ACDUAL sample in "Compiler COM Support Samples":
Copy Code
#define CATCH_ALL_DUAL \ CATCH(COleException, e) \ { \ _hr = e->m_sc; \ } \ AND_CATCH_ALL(e) \ { \ __pragma(warning(push)) \ __pragma(warning(disable:6246)) /*disable _ctlState prefast warning*/ \ AFX_MANAGE_STATE(pThis->m_pModuleState); \ __pragma(warning(pop)) \ _hr = DualHandleException(_riidSource, e); \ } \ END_CATCH_ALL \ return _hr; \
End Microsoft specific