Hi everyone, I'm new to boost but I did search the group as thoroughly as I could before asking this. Anyway, the documentation gives this as an example: #include <boost/preprocessor/cat.hpp> #include <boost/preprocessor/repetition/repeat_from_to.hpp> #define DECL(z, n, text) BOOST_PP_CAT(text, n) = n; BOOST_PP_REPEAT_FROM_TO(5, 10, DECL, int x) /* expands to: int x5 = 5; int x6 = 6; int x7 = 7; int x8 = 8; int x9 = 9; */ What confuses me is that BOOST_PP_REPEAT_FROM_TO expands to BOOST_PP_CAT and so does DECL yet after expanding DECL to BOOST_PP_CAT the (comeau) preprocessor continues to expand BOOST_PP_CAT to give the desired result. However, in the following code: #define CONCAT_1(a, b) CONCAT_1_D(a, b) #define CONCAT_1_D(a, b) a ## b #define CONCAT_2(a, b) CONCAT_2_D(a, b) #define CONCAT_2_D(a, b) a ## b #define AB(c, x, y) CONCAT_ ## c(x,y) CONCAT_1(A, B(1, p, q)) we find that we get: CONCAT_1(p,q) Only by changing the code to: CONCAT_1(A, B(2, p, q)) // note the 2 rather than 1 do we get our desired result: pq Can someone explain what I'm not understanding here (this isn't in my usual field of work)? Many thanks in anticipation Brad