Re: [Boost-users] MPL if_ not following true branch

Cory Nelson escribió:

This code doesn't seem to ever follow the true branch of the if_... it's like the eval_if is clobbering it. Can someone tell me what I'm doing wrong (other than not using a map.. this is a reduction of some other code that can't use a map)?

#include <iostream> #include <typeinfo> #include #include #include #include #include

template<typename T> struct choose_type { struct int_tag {}; struct float_tag {};

template<typename InvalidT> struct invalid_type_specified { BOOST_STATIC_ASSERT(sizeof(InvalidT) == 0); };

typedef typename boost::mpl::if_< boost::is_same, int_tag, typename boost::mpl::eval_if< boost::is_same, boost::mpl::identity, invalid_type_specified<T> >::type

...

::type type; };

The problem with your code is that the else part of your mpl::if_ expression is always evaluated (it has the form mpl::eval_if<...>::type), so when you you instantiate choose_type<int> the mpl::eval_if_ is also instantiated and static asserts (as int is not float). Try this instead: struct choose_type { struct int_tag {}; struct float_tag {}; template<typename InvalidT> struct invalid_type_specified { BOOST_STATIC_ASSERT(sizeof(InvalidT) == 0); }; typedef typename boost::mpl::eval_if< boost::is_same, boost::mpl::identity, typename boost::mpl::eval_if< boost::is_same, boost::mpl::identity, invalid_type_specified<T> > >::type type; }; Joaquín M López Muñoz Telefónica, Investigación y Desarrollo