[boost] [preprocessor] ifdef equivalent inside a macro, is it possible?

Hi all, Would there be a way to do some "ifdef" inside a macro? Could be in the form of a macro taking 3 args, - 1st being the macro to test the existance of, - 2nd being the value to expand in case 1st arg is a defined macro - 3rd being the value to expand in case 1st arg is not a defined macro Exemple: #define DUMMY_MACRO1    foo1 // next is commented out and not defined // #define DUMMY_MACRO2    foo2 PP_IFDEF(DUMMY_MACRO1, DUMMY_MACRO1, bar1) // would expand to foo1 PP_IFDEF(DUMMY_MACRO2, DUMMY_MACRO2, bar2) // would expand to bar2 Seems impossible to me, but boost/preprocessor does so many things that seemed impossible to me. Maybe someone will have some more magic... Thanks. Damien.