Re: [Boost-users] [mpl] Placeholders substitution and template partial specialization

From: Aleksey Gurtovoy (agurtovoy_at_[hidden])
Date: 2006-07-25 00:24:57

Andriy Tylychko writes:
> Consider following example:
>
> BOOST_MPL_ASSERT((boost::is_same<
>          mpl::apply<mpl::apply<_1, _2>::type, boost::add_pointer<_1>, int>::type
>          , int*>));
>
> It should apply "add_pointer" to "int" and receive "int*", but it
> doesn't work. It returns just "int".
>
> Consider another example:
>
> template <class F, class X>
> struct apply1: 
>
> mpl::lambda<F>::type::template apply<X>
> {};
>
> BOOST_MPL_ASSERT((boost::is_same<
>          mpl::apply<apply1<mpl::_1, mpl::_2>/*nothing here*/, boost::add_pointer<mpl::_1>, int>::type
>          , int*>));
>
>
> Handmade apply1 is used instead of mpl::apply. And this example
> works. What difference? Usage of nested "::type" of apply
> metafunction. As result:
>
> BOOST_MPL_ASSERT((boost::is_same<
>          mpl::apply<apply1<mpl::_1, mpl::_2>::type, boost::add_pointer<mpl::_1>, int>::type
>          , int*>));
>
> doesn't work. Seems like this change affects time of placeholders
> substitution. Can anybody explain this in detail?
>
> So, since apply1 is just simplified version of mpl::apply, why we
> cannot use mpl::apply w/o "::type"? Because it cannot be compiled,
> at least using VC8.  And seems like the main difference is that
> apply1 is primary template but mpl::apply is partial template
> specialization. Is such behavior standard?  Can anybody explain the
> logic?

There is not much logic here: it's simply a (known) bug. Please see 
http://thread.gmane.org/gmane.comp.lib.boost.user/9699
for more details.

-- 
Aleksey Gurtovoy
MetaCommunications Engineering