the problem about different treatment to __VA_ARGS__ when using VS 2008 and GCC
- by liuliu
I am trying to identify a problem because of an unusual usage of variadic macros. Here is the hypothetic macro:
#define va(c, d, ...) c(d, __VA_ARGS__)
#define var(a, b, ...) va(__VA_ARGS__, a, b)
var(2, 3, printf, “%d %d %d\n”, 1);
For gcc, the preprocessor will output
printf("%d %d %d\n", 1, 2, 3)
but for VS 2008, the output is
printf, “%d %d %d\n”, 1(2, 3);
I suspect the difference is caused by the different treatment to VA_ARGS, for gcc, it will first expand the expression to va(printf, "%d %d %d\n", 1, 2, 3), and treat 1, 2, 3 as the VA_ARGS for macro va. But for VS 2008, it will first treat b as VA_ARGS for macro va, and then do the expansion.
Which one is correct interpretation for C99 variadic macro? or my usage falls into an undefined behavior?
