I have a boost::mpl::vector of nullary metafunctions and I need to transform it into a sequence of the metafunction's inner types. As in: typedef boost::mpl::vector<boost::mpl::identity<int> > tvec; typedef boost::mpl::vector<int> tvecinner; So I try boost::mpl::transform: typedef typename boost::mpl::transform<tvec,_1>::type ttrans; BOOST_MPL_ASSERT((boost::mpl::equal<tvecinner,ttrans>)); But that does not work and the assert fails. Evidently my _1 is not working by itself although it seems as if it should because it is a placeholder expression and its inner type is what I want. So I try: template <class T> struct tself : T { }; and now instead: typedef typename boost::mpl::transform<tvec,tself<_1> >::type ttrans; BOOST_MPL_ASSERT((boost::mpl::equal<tvecinner,ttrans>)); works fine and the assert succeeds. But that 'tself' hack seems like it should be really unnecessary. Have I missed something cleaner and easier ?