I am not terribly familiar with the rules of the preprocessor. Is it the case that the expression of the BOOST_PP_EXPR_IF macro is expanded regardless of the resulting value of the condition? The reason I ask is because the following code produces a warning with VC7 .NET 2003. #define MY_MACRO(array) \ BOOST_PP_EXPR_IF( \ BOOST_PP_ARRAY_SIZE(array), \ BOOST_PP_TUPLE_TO_SEQ( \ BOOST_PP_ARRAY_SIZE(array), \ BOOST_PP_ARRAY_DATA(array) \ ) \ ) MY_MACRO((0, ())); The warning is: C4003: not enough actual parameters for macro 'BOOST_PP_EXPR_IIF_0' The macro expands correctly. Perhaps there is a better way to do what I am after. I would like to create a comma separated list (no parenthesis as would be the case if I only took the array data) from a possibly empty set. Thanks, John