Macro definition of variable parameters

Parameters of the output functions printf () and fprintf () are variable. When debugging a program, you may want to define your own variable output functions,

The Variable Parameter macro is an option.

The macro in c99 can have variable parameters like a function, such

#define LOG(format, ...) fprintf(stdout, format, __VA_ARGS__)

Where,... indicates that the parameter is variable, __va_args _ is replaced by the actual parameter set in preprocessing.

 

GCC also supports the following forms

#define LOG(format, args...) fprintf(stdout, format, args)

The usage is basically the same as the above, but the parameter symbols have changed.

 

Note that variable parameters cannot be omitted in the macro definition. Although you can pass an empty parameter, it is necessary to mention the usage of the "#" Concatenation symbol.

"##" Is used to connect token. In the preceding example, format, _ va_args _, and ARGs are token,

If the token is empty, no connection is made. Therefore, variable parameters (_ va_args _ and ARGs) can be omitted. Modify the variable macro as follows:

#define LOG(format, ...)     fprintf(stdout, format, ##__VA_ARGS__)

#define LOG(format, args...) fprintf(stdout, format, ##args)

 

The preceding Variable Parameter macro definition can not only customize the output format, but also works with # ifdef # else # endif for convenient output management,

For example, the debugging information is output during debugging, but not during official release.

#ifdef DEBUG

#define LOG(format, ...) fprintf(stdout, ">> "format"\n", ##__VA_ARGS__)

#else

#define LOG(format, ...)

#endif

In the debugging environment, log macro is a variable parameter output macro that is output in a custom format;

In the release environment, log macro is an empty macro and does nothing.