Question on using ## in #define - 未名空间MITBBS历史存档

国际科技财经博客移民网络热点娱乐民生时事公众号

Redian新闻

>未名空间

>Programming - 葵花宝典

Question on using ## in #define

Question on using ## in #define# Programming - 葵花宝典

m*t2007-01-28 08:01

1 楼

#include
#define REPLACE hello world
#define JOIN(a, b) a##b
int main() {
printf("JOIN(REP, LACE)\n");
return 0;
}
run on gcc, output:
JOIN(REP, LACE)
Why?

m*t2007-01-28 08:01

2 楼

Thanks. That works. But i am kind of curious since I thought preprocessor
should handle the string substitution. And i tried the following which gives
me compilation error.
#include
#define REPLACE hello world
#define JOIN(a, b) a##b
int main() {
printf("%s", JOIN(REP, LACE));
return 0;
}

O*d2007-01-28 08:01

3 楼

C preprocessor will not replace literal string. C string inside double quote
is literal except escaped sequence. In your first case, "JOIN(REP, LACE)\n
" is a literal string "JOIN(REP, LACE)" followed by a new line character. In
your second case, hello world is not a string.