Could someone please help clarify the meaning of the _z extension? In what situation should one use BOOST_PP_ENUM_z rather than BOOST_PP_ENUM? Usage BOOST_PP_ENUM_ ## z(count, macro, data) Arguments z The next available BOOST_PP_REPEAT dimension. http://www.boost.org/doc/libs/1_33_1/libs/preprocessor/doc/ref/enum_z.html Examples: #include #include #include #define MACRO(z, n, _) \ template< \ BOOST_PP_ENUM_PARAMS_Z(z, BOOST_PP_INC(n), class T) \ > class X ## n { \ /* ... */ \ }; \ /**/ BOOST_PP_REPEAT(2, MACRO, nil) #define MACRO1(z, n, _) \ template< \ BOOST_PP_ENUM_PARAMS(BOOST_PP_INC(n), class T) \ > class Y ## n { \ /* ... */ \ }; \ /**/ BOOST_PP_REPEAT(2, MACRO1, nil) int main (int argc, char * const argv[]) { typedef X0<int> x0; typedef X1 x1; typedef Y0<int> y0; typedef Y1 y1; return 0; }