Hi, I'm using Boost.Preprocessor to create specializations of a class I have named InputSource. in the iteration loop, I have the following: # define i BOOST_PP_ITERATION() # define INPUT_repeat(z, n, user) user template<typename t_return, BOOST_PP_ENUM_PARAMS(i, typename t_param)> class InputSource< t_return, BOOST_PP_ENUM_PARAMS(i, t_param) BOOST_PP_COMMA_IF( BOOST_PP_AND(BOOST_PP_NOT_EQUAL(i, 0), BOOST_PP_NOT_EQUAL(i, MAX_PARAMETERS)) ) BOOST_PP_ENUM(BOOST_PP_SUB(MAX_PARAMETERS, i), INPUT_repeat, detail::no_param) > The part that isn't working is the BOOST_PP_COMMA_IF line. When I look at the preprocessor output generated by MSVC9, I get this: template<typename t_return, typename t_param0> class InputSource< t_return, t_param0 BOOST_PP_IIF_BOOST_PP_BOOL_BOOST_PP_BITAND_BOOST_PP_BOOL_BOOST_PP_NOT_EQUAL(1, 0)BOOST_PP_BOOL_BOOST_PP_NOT_EQUAL(1, 5)(BOOST_PP_COMMA, BOOST_PP_EMPTY)() detail::no_param , detail::no_param , detail::no_param , detail::no_param > My iteration limits has been defined as follows: # ifndef MAX_PARAMETERS # define MAX_PARAMETERS 5 # endif # define BOOST_PP_ITERATION_LIMITS (1, MAX_PARAMETERS) I would have expected the preprocessor to show a comma or a blank line, but instead it looks like some macros were not expanded. Any solutions to this problem?