Removing last trailing comma from the arguments of a macro-CodePudding

I need to remove the last trailing comma from a list of macro arguments (because they will be eventually expanded into template arguments where the trailing comma is not admitted).

So I need a macro remove_trailing_comma() which called like remove_trailing_comma(arg1, arg2, arg3, ) expands to arg1, arg2, arg3.

I've tried with different combinations of varargs and __VA_OPT__ but I seem to not be able to do it.

For example:

#define discard_trailing_comma(arg, ...) \
    arg __VA_OPT__(,) discard_trailing_comma(__VA_ARGS__)

discard_trailing_comma(1, 2, 3, )

does not work (with g 10) because expands to 1 , discard_trailing_comma(2, 3,), I do not know why (are macro not expanded recursively?)

Is this possible in C 20?

CodePudding user response：

Assuming you have a bounded number of parameters, you could make a delegate function-like macro as a helper to choose the appropriate implementation macro.

#define GET_DTC(_1,_2,_3,_4,NAME,...) NAME
#define DTC4(_1,_2,_3,_4) _1,_2,_3
#define DTC3(_1,_2,_3) _1,_2
#define DTC2(_1,_2) _1
#define DTC1(_1)
#define discard_trailing_comma(...) \
    GET_DTC(__VA_ARGS__, DTC4, DTC3, DTC2, DTC1)(__VA_ARGS__)

int arr[] = {
discard_trailing_comma(1, 2, 3, )
};

CodePudding user response：

gcc has a non-standard extension, ##__VA_ARGS__, which should do the trick. It is documented here (search the page for ##__VA_ARGS__).

Example usage (simplified to the bare minimum to make things easy on my tired eyeballs):

void call_me (int first, int second, int third) { }

#define EXPAND_ME(first, ...)   call_me (first, ##__VA_ARGS__)

int main ()
{
    EXPAND_ME (1, 2, 3);
}

Note that you need at one fixed argument, and this also works in Clang.

Demo

MSVC has a similar extension. If anyone's interested I'll add it here.