C++ preprocessor

#include <stdio.h> #include <string.h> #include <stdarg.h> #define NUMARGS(...) (sizeof((int[]){__VA_ARGS__})/sizeof(int)) #define SUM(...) (sum(NUMARGS(__VA_ARGS__), __VA_ARGS__)) void sum( int numargs, ... ); int main( int argc, char *argv[ ] ) { SUM( 1 ); SUM( 1, 2 ); SUM( 1, 2, 3 ); SUM( 1, 2, 3, 4 ); return 1; } void sum( int numargs, ... ) { int total = 0; va_list ap; printf( "sum() called with %d params:", numargs ); va_start( ap, numargs ); while ( numargs-- ) { total += va_arg(ap, int); } va_end( ap ); printf( " %d\n", total ); return; }

It is completely valid C99 code. It has one drawback, though - you cannot invoke the macro SUM() without params, but GCC has a solution to it - http://gcc.gnu.org/onlinedocs/cpp/Variadic-Macros.html

So in case of GCC you need to define macros like this and it will work even with empty parameter list

#define NUMARGS(...) (sizeof((int[]){0, ##__VA_ARGS__})/sizeof(int)-1) #define SUM(...) sum(NUMARGS(__VA_ARGS__), ##__VA_ARGS__)

use this GNU extension, Just remember - it only works with GNU compiler.

#define macro(format, arguments...) fprintf(stderr, format, ##arguments)

The ## token in combination with __VA_ARGS__ is a gcc extension that's not part of ISO C99.

转载于:https://www.cnblogs.com/shangdawei/archive/2013/05/28/3102927.html

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