#define宏的使用

dengjin_cu

请问下面的宏为什么要加上##，是变长参数的原因吗？

1. #define DEBUG(fmt, args...) fprintf(stderr, fmt, ##args)

复制代码

bruceteen

好象是去除多余的逗号的，免得出现fprintf( ……，)而语法错误

hbmhalley

Second, the `##' token paste operator has a special meaning when placed between a comma and a variable argument. If you write

     #define eprintf(format, ...) fprintf (stderr, format, ##__VA_ARGS__)
and the variable argument is left out when the eprintf macro is used, then the comma before the `##' will be deleted. This does not happen if you pass an empty argument, nor does it happen if the token preceding `##' is anything other than a comma.

万一 __VA_ARGS__ 为空，省去之前的逗号
http://gcc.gnu.org/onlinedocs/cpp/Variadic-Macros.html#Variadic-Macros

x5miao

回复 3# hbmhalley


    http://gcc.gnu.org/onlinedocs/cp ... tml#Variadic-Macros

有逗号的是C99的用法，没有逗号的是gcc的扩展对吗？
#define DEBUG(args,...) fprintf(stderr,args, ##__VA_ARGS__)
#define DEBUG(args...) fprintf(stderr, ##args)

hbmhalley

回复 4# x5miao


    没看过C99
    不过既然这么说了 应该没错