I have a project in which I use these things I call "records" to attach data to edges and vertexes in a boost::graph. They are visitable (acyclic visitor described by Alexandrescu) and I have metafunctions that will take their most derived type and give me a list of "fields" that they contain. I can then write generic processors to do things like build editors for them or perform operations that can effect any field of a particular type or concept. Thus I often find myself using mpl::for_each and writing things like this: struct operator { template < typename MetaField > void operator()(MetaField const&, boost::enable_if< some_test >::type * = 0); template < typename MetaField > void operator()(MetaField const&, boost::disable_if<some_test>::type * = 0); }; I imagine that this is common enough that there may already be a utility to write things like this but I don't see one in the docs. Is it out there? What about something that would make this type of thing easy: struct operator { template < typename MetaField > void operator()(MetaField const&, boost::enable_if< test_a >::type * = 0); template < typename MetaField > void operator()(MetaField const&, boost::enable_if< and_< not_ <test_a>, test_b>>::type * = 0) void operator()(MetaField const&, boost::enable_if<not_<or_<test_a, test_b>>>::type * = 0); }; That's supposed to be an if, elseif, else construct. -- http://crazyeddiecpp.blogspot.com/