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

From: Andriy Tylychko (mail.ru) (telya_at_[hidden])
Date: 2006-07-21 13:07:42

Hi All

 

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?

 

Best regards,

Andriy Tylychko,

telya_at_[hidden]