Variable arguments inside a macro

Question

I have two functions foo1(a,b) & foo2(a,b,c) and a macro

#define add(a,b) foo(a,b)

I need to re-define macro to accomplish,

1.if a

Answer 1

That usual trick for counting arguments may be adapted for this:

#define ADD_EXPAND(...) \
        ADD_EXPAND_(__VA_ARGS__, EXPAND_ADD_FUNCS())

#define ADD_EXPAND_(...) \
        EXPAND_ADD_SEL_FUNC(__VA_ARGS__)

#define EXPAND_ADD_SEL_FUNC(first_, second_, third_, func, ...) func

#define EXPAND_ADD_FUNCS() foo2, foo, dummy

#define add(...) ADD_EXPAND(__VA_ARGS__)(__VA_ARGS__)

Once you plow through the boiler plate, it basically just involves placing all the arguments in a line, with the function tokens after them, and seeing which function stands out. That's what EXPAND_ADD_SEL_FUNC does.

You can see it live on coliru.

But I'll reiterate what we told you in comments. This is likely to be a sub-par solution to a proper function. I haven't tested it thoroughly, so breakage is easily possible. Use at your own risk.