[Boost-users] is_same_impl

From: Gottlob Frege (gottlobfrege_at_[hidden])
Date: 2005-04-05 19:16:16

template <typename T>
::boost::type_traits::yes_type
BOOST_TT_DECL is_same_tester(T*, T*);

::boost::type_traits::no_type
BOOST_TT_DECL is_same_tester(...);

template <typename T, typename U>
struct is_same_impl
{
   static T t;
   static U u;

   BOOST_STATIC_CONSTANT(bool, value =
      (::boost::type_traits::ice_and<
         (sizeof(type_traits::yes_type) ==
sizeof(detail::is_same_tester(&t,&u))),
         (::boost::is_reference<T>::value == ::boost::is_reference<U>::value),
         (sizeof(T) == sizeof(U))
        >::value));
};

Would it be any better to do is_same_tester with just one arg, like this:

template <typename T>
::boost::type_traits::yes_type
BOOST_TT_DECL is_same_tester(T*);

template <typename T>
::boost::type_traits::no_type
BOOST_TT_DECL is_same_tester(...);

and then the call is:.
         (sizeof(type_traits::yes_type) ==
sizeof(detail::is_same_tester<T>(&u))),

(and then not need a 'static T t' at all?)

And does it make any difference to declare the U as a pointer:

static U *u; // and/or leave off the static

and then not pass it in by address:
         (sizeof(type_traits::yes_type) ==
sizeof(detail::is_same_tester<T>(u))),

Or not bother with a U at all, but just cast 0 into a U*:
         (sizeof(type_traits::yes_type) ==
sizeof(detail::is_same_tester<T>(static_cast<U *>(0)))),

Or does none of it really make a difference?

And while I am at it, what would the value of 'same' be:

   struct A { int x; };
   struct B : A;

   static bool same = boost::type_traits::is_same<A, B>::value;

I suspect same == true, as B is castable to an A, and sizeof(B) == sizeof(A).

checking both is_same_tester<T>(u) and is_same_tester<U>(t) might be better?