parameterized warning silencing macro trouble

Question

The following doesn't compile:

#define SUPPRESS(w) _Pragma("GCC diagnostic ignored " ## w)

SUPPRESS("-Wuseless-cast")

int main() {
    int a = (int)4;
    return a;
}

Here's the error:

error: pasting ""GCC diagnostic ignored "" and ""-Wuseless-cast"" does not give a valid preprocessing token

How can I get it to work?

Answer 1

The thing is that _Pragma wants to have an escaped string-literal like so

_Pragma("GCC diagnostic ignored \"-Wuseless-cast\"")

So the trick is to add another layer of stringyfication between the call of SUPPRESS and the call of _Pragma like below

#define xSUPPRESS(w) _Pragma(#w)
#define SUPPRESS(w) xSUPPRESS(GCC diagnostic ignored w)

SUPPRESS("-Wuseless-cast")

See it here in action.