Hi, I was grep'ing for my name in the boost repository in order to update any old license reference and found the following definition in implicit_cast.hpp: // Macro for when you need a constant expression (Gennaro Prota) #define BOOST_IMPLICIT_CAST(dst_type, expr) \ ( sizeof( implicit_cast<dst_type>(expr) ) \ , \ static_cast<dst_type>(expr) \ ) This wasn't the form I submitted though, as the comma operator isn't allowed in a constant expression and I remember pointing out on this list that this was a case where it would be useful :) Maybe the above was simply copied from this post: http://lists.boost.org/MailArchives/boost/msg42102.php where I was saying that it *doesn't* work and that I had to use the check_helper<> template. Genny.