Is it possible to put a preprocessor conditional inside a C macro?

Question

Is there a way to write a C preprocessor macro that expands to different things depending on what argument it receives?

#define foo() ???

/* 1 */
foo(name)

/* 2 */
foo(_)

Desired result:

/* 1 */
int name;

/* 2 */
/*ignore*/

Yes, I know macros are evil. I'm asking this mostly out of curiosity.

Answer 1

To expand on Gavin Smith's answer, you actually can check conditions within a macro expansion:

#define FOO_name 1
#define FOO__ 0

#define CONC(a,b) a##_##b

#define IF(c, t, e) CONC(IF, c)(t, e)
#define IF_0(t, e) e
#define IF_1(t, e) t

#define FOO(x) IF(CONC(FOO,x), int x;, )

FOO(name) // -> int name;
FOO(_)    // -> /*nothing*/

If you're feeling adventurous you can easily extend IF to allow commas, suppress macro expansion, etc. with helper macros.

As above though this does require that you know all of the desired names in advance.