我们可以使用外部“C”在C文件没有#ifdef __cplusplus？ [英] Could we use extern "C" in C file without #ifdef __cplusplus?

问题描述

为需要定义为C函数的函数，为什么不应该指定 externC？如果对C编译器没有影响，那么对编译器在编译C源文件时会产生什么影响？

在下面的头文件中删除 #ifdef __cplusplus 检查？

  externC{
 int MyFunc（）; 
} 
  

对另一个问题的回答说需要 #ifdef ，但我不明白为什么：

关于＃2：将为通过C ++编译器运行的任何编译单元定义__cplusplus。通常，这意味着.cpp文件和该.cpp文件包括的任何文件。相同的.h（或.hh或.hpp或what-have-you）在不同的时间可以解释为C或C ++，如果不同的编译单元包括它们。如果你希望.h文件中的原型引用C符号名，那么当它们被解释为C ++时，它们必须具有 externC，并且它们不应该具有 externC当解释为C - 因此 #ifdef __cplusplus 检查。

解决方案

构造 externC是一个C ++构造，不被C编译器识别。通常，它会发出一个语法错误消息。

一个常见的技巧是定义一个宏，例如EXTERN_C，扩展到不同的东西取决于你是否编译使用C或C ++。例如：

在一个公共头文件中：

  #ifdef __cplusplus 
 #define EXTERN_C externC{
 #define EXTERN_C_END} 
 #else 
 #define EXTERN_C 
 #define EXTERN_C_END 
 #endif $ b $ 
 <$> 
 c $ c> EXTERN_C 
 int MyFunc（void）; 
 EXTERN_C_END 
  
 
Why shouldn’t extern "C" be specified for a function that needs to be defined as a C function? What effect would that have on the compiler when compiling the file as a C source?

If there is no effect on the C compiler, can’t we just define a function in a header file as below by removing the #ifdef __cplusplus check?
extern "C" {
    int MyFunc();
}
An answer to another question says that the #ifdef is needed, but I don’t understand why:

  
Regarding #2: __cplusplus will be defined for any compilation unit that is being run through the C++ compiler. Generally, that means .cpp files and any files being included by that .cpp file. The same .h (or .hh or .hpp or what-have-you) could be interpreted as C or C++ at different times, if different compilation units include them. If you want the prototypes in the .h file to refer to C symbol names, then they must have extern "C" when being interpreted as C++, and they should not have extern "C" when being interpreted as C -- hence the #ifdef __cplusplus checking.

解决方案
The construct extern "C" is a C++ construct and is not recognized by a C compiler. Typically, it will issue a syntax error message.

A common trick is to define a macro, for example EXTERN_C, that would expand to different thing depending on if you compile using C or C++. For example:

In a common header file:
#ifdef __cplusplus
#define EXTERN_C extern "C" {
#define EXTERN_C_END }
#else
#define EXTERN_C
#define EXTERN_C_END
#endif
In other files:
EXTERN_C
int MyFunc(void);
EXTERN_C_END


                        
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