Re: (very) Simple Template Metaprogramming question

Hi,

I have a simple question regarding templates and meta programming.

I am going to try and work my way through the C++ Template
Metaprogramming, a book by David Abrahams and Aleksey Gurtovoy. I'm
not doing this because I want to be a Meta Programming guru (because a
lot of that stuff looks too crazy for use in the real world).

Rather I want to learn heavyweight templates and this is the only
hardcode template book that has questions and answers (on the web).

Anyway I have got as far as the first exercise. I have done the
exercise but I'm getting a compile error below.

I'm using Visual Studio 2005, but I'm sure its not a compiler problem
=96 its just I don't understand what is going on.

The compile error is on the line (full code below):

  const type_info& info1a = typeid(add_const_ref<int>::type);

where the compiler does not like the ::type in the call to typeid.

The error is :

error C2955: 'add_const_ref_impl<false>::type_' : use of class
template requires template argument list
see declaration of 'add_const_ref_impl<false>::type_'
see reference to class template instantiation 'add_const_ref<T>' being
compiled
        with
        [
            T=int
        ]

I'm obviously don't understand what is going on here. I would expect
that buy the time the compiler has done its meta programming magic
add_const_ref<int>::type would be a real type and I could pass it into
typeID, however that looks wrong. I think I'm misunderstanding
something fundermetal here so I'm posting.

So my question is why does this line fail to compile. Should I be able
to pass add_const_ref<int>::type into typeid()?

Thanks

stdlib

Ps. sorry if this is in the wrong group!

//========================

// This is an answer to they very first exercise in the
// C++ Template Metaprogramming book.
// Its the impl of a function add_const_ref function which add adds
converts a supplied type into
// a const ref to that type.

#include <boost/type_traits/is_reference.hpp>
#include <iostream>

template <bool is_ref> struct add_const_ref_impl
{};

template <>
struct add_const_ref_impl<false>
{
  template<typename T>
  struct type_
  {
    typedef const T& type;
  };

};

template <>
struct add_const_ref_impl<true>
{
  template<typename T>
  struct type_
  {
    typedef T type;
  };

};

//========================

template <class T>
struct add_const_ref
{
  static const bool is_ref = boost::is_reference<T>::value;
  typedef typename add_const_ref_impl<is_ref>::type_::type type;

};

int main()
{
  const type_info& info1a = typeid(add_const_ref<int>::type); //
error C2955 here
  const type_info& info2a = typeid(add_const_ref<int&>::type); //
error C2955 here

  std::cout << info1a.name() << std::endl << info2a.name() <<
std::endl;

  return 0;}

//========================