As we known, there is a preprocess metaprogramming library in boost, but it is not flexible enough. we are unable to define an custom preprocess loop(something like #for or #while ?), and we are also unable to use recursion. I have an idea to do that, C/C++ allows recursion of #include, so we can put the codes we want to evaluate repeatedly to a head, and include itself depends on a preprocess condition. But if we wrote code below, we would meet a error of macro redefinition. /**** foo.hpp ****/ #if FOO < 10 // do something with I # define FOO FOO + 1 # include "foo.hpp" #endif /**** foo.hpp ****/ /**** bar.hpp ****/ #define FOO 0 #include "foo.hpp" /**** bar.hpp ****/ The problem is that an macro must not depends on itself. Thus, to change a preprocess variant, I use a bit of #if to transfer value of a macro to some temporary macro. see my code below: /**** begin_set_value.hpp ****/ #if (BOOST_VPP_VALUE & (1 << 0)) == 0 #define BOOST_VPP_TEMPORARY_BIT_0 0 #else #define BOOST_VPP_TEMPORARY_BIT_0 1 #endif #if (BOOST_VPP_VALUE & (1 << 1)) == 0 #define BOOST_VPP_TEMPORARY_BIT_1 0 #else #define BOOST_VPP_TEMPORARY_BIT_1 1 #endif /* ... */ #if (BOOST_VPP_VALUE & (1 << 30)) == 0 #define BOOST_VPP_TEMPORARY_BIT_30 0 #else #define BOOST_VPP_TEMPORARY_BIT_30 1 #endif #if (BOOST_VPP_VALUE & (1 << 31)) == 0 #define BOOST_VPP_TEMPORARY_BIT_31 0 #else #define BOOST_VPP_TEMPORARY_BIT_31 1 #endif #undef BOOST_VPP_VALUE #define BOOST_VPP_VALUE \ (BOOST_VPP_TEMPORARY_BIT_0 << 0 ) \ + (BOOST_VPP_TEMPORARY_BIT_1 << 1 ) \ + (BOOST_VPP_TEMPORARY_BIT_2 << 2 ) \ + (BOOST_VPP_TEMPORARY_BIT_3 << 3 ) \ /* ... */ \ + (BOOST_VPP_TEMPORARY_BIT_29 << 29 ) \ + (BOOST_VPP_TEMPORARY_BIT_30 << 30 ) \ + (BOOST_VPP_TEMPORARY_BIT_31 << 31 ) \ /*BOOST_VPP_VALUE*/ /**** begin_set_value.hpp ****/ Now, the BOOST_VPP_VALUE is depends on some of BOOST_VPP_TEMPORARY_BIT_n, and can be used to change the former macro: #define FOO 0 #define BOOST_VPP_VALUE FOO #include "begin_set_value.hpp" #undef FOO #define FOO (BOOST_VPP_VALUE + 1) But, FOO depends on BOOST_VPP_TEMPORARY_BIT_ns, and can not be change again, the solution is that A must a variant depends on some other BIT_n definition instead of directly depends on BOOST_VPP_VALUE. Actually, I implements an variant stack to allocate and release these variant. I think the concept of variable preprocess is useful, if Boost.Typeof or Boost.Bind use that, these code they need would be less than copying same codes. I have upload a sample on vault, file name is variable_preprocess.zip. http://boost-consulting.com/vault/index.php?&direction=0&order=&directory=Preprocessor%20Metaprogramming http://boost-consulting.com/vault/index.php?action=downloadfile&filename=variable_preprocess.zip&directory=Preprocessor%20Metaprogramming& All code is validated by wave, thank you for your attention.