Hi! I'm developing with preprocessor a class generator, that generates the wrapper classes needed by boost.python when you want to derive in python from polymorph c++-base classes. I would need a Macro that expands to "return" if its argument is not void. I tried the following, but that obiously does not work: #define Return(type) \ BOOST_PP_EXPR_IF(BOOST_PP_NOT_EQUAL(type, void), return) I made a workaround by adding an integer value to my Array that defines the methods signature. But that is redundant. Is there a way to achieve the behaviour above?