Quote:
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
– 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_refstruct 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;}
>
//================================================== ==
you should provide a template argument here too. This is what the
compiler complains about.
