Alan M. Carroll writes:

Is this as simple as it gets? Ah, finally, I have an MPL variant that works: struct get_key_type { template < typename T > struct apply { typedef

typename T::key_type type; }; };

typedef typename boost::mpl::eval_if_c, typename boost::mpl::identity<PAYLOAD> >::type testing;

You can make it simpler: template<typename T> struct get_key_type { typedef typename T::key_type type; }; typedef typename boost::mpl::eval_if_c< IS_OUTER, get_key_type<PAYLOAD>, boost::mpl::identity<PAYLOAD>

::type testing;

Joaquín M López Muñoz Telefónica, Investigación y Desarrollo