C: How to Shield Commas in Macro Arguments?

Question

Is there a general method to shield comments in macro arguments in C? I know that parentheses can be used for this purpose, but that will not work in cases where added parentheses result in syntax errors in the macro output. I've heard that ({ }) works to shield commas in GCC, but I need this code to also work in VC++ (one of the recent versions which does conform to the C standard with regard to commas in macros). I also cannot use variadic macros in my case.

The specific case I'm trying to do is this (lengthof is a macro defined elsewhere). I'm trying to write a single macro for the entire thing because this will be used many times, and having a multi-macro solution would add a large amount of additional testing code.

#define TEST_UNFUNC(func, res_type, res_set, op_type, op_set) \
{ \
    static const res_type res[] = res_set; \
    static const op_type op[] = op_set; \
    int i; \
    for (i = 0; i < MIN(lengthof(res), lengthof(op)); i++) \
        assert(func(op[i]) == res[i]); \
}

If possible I would like a general answer and not merely a workaround specific to this particular macro.

Answer 1

Use parentheses to shield the comma, and then pass them through a special unparen macro, defined in the example below:

#include <stdio.h>

#define really_unparen(...) __VA_ARGS__
#define invoke(expr) expr
#define unparen(args) invoke(really_unparen args)


#define fancy_macro(a) printf("%s %s\n", unparen(a))

int main()
{
    fancy_macro(("Hello", "World"));
}

The trick here is that the invoke macro forces an extra expansion, allowing really_unparen to be called even though it's not followed by parentheses in the source.

Edit: per comment below, this appears to not be necessary in this case. Though I'm sure I've hit a case where I needed it sometime ... and it doesn't hurt.