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Boost Users : |
Subject: [Boost-users] Register variadic template as tags in a boost::mpl::map
From: Allan Nielsen (a_at_[hidden])
Date: 2012-01-07 06:55:04
In the following code, why can't I look up BB and CC in the mpl::map type_map?
I would expect the map to look like this:
map <
pair<AA, A< xx > >,
pair<BB, B< xx > >,
pair<CC, C< xx > > >
But it does not seem like this is the case...
#include <iostream>
#include <boost/type_traits/is_same.hpp>
#include <boost/mpl/map.hpp>
#include <boost/mpl/at.hpp>
#include <boost/mpl/insert.hpp>
struct AA {};
struct BB {};
struct CC {};
template<int OFFSET>
struct A {
typedef AA T;
enum O { offset = OFFSET };
enum S { size = 2 };
};
template<int OFFSET>
struct B {
typedef AA T;
enum O { offset = OFFSET };
enum S { size = 4 };
};
template<int OFFSET>
struct C {
typedef AA T;
enum O { offset = OFFSET };
enum S { size = 4 };
};
template < template <int> class Head,
template <int> class... Tail>
struct X : public Head< X<Tail...>::size * X<Tail...>::offset >,
public X<Tail...>
{
typedef Head< X<Tail...>::size * X<Tail...>::offset > _Head;
enum O { offset = X<Tail...>::size * X<Tail...>::offset };
enum S { size = _Head::size * X<Tail...>::size };
typedef typename boost::mpl::insert < typename X<Tail...>::type_map,
boost::mpl::pair<typename _Head::T,
_Head >
>::type type_map;
template <typename T> int get_offset() {
return boost::mpl::at<type_map, T>::type:: offset;
}
};
// base case
template < template <int> class Head>
struct X<Head> : public Head<1> {
enum O { offset = Head<1>::offset };
enum S { size = Head<1>::size };
typedef boost::mpl::map< boost::mpl::pair< typename Head<1>::T,
Head<1> > > type_map;
};
int main()
{
X< A, B, C > x1;
std::cout << x1.get_offset<CC>() << std::endl;
// generates compile errors
//std::cout << x1.get_offset<BB>() << std::endl;
//std::cout << x1.get_offset<CC>() << std::endl;
return 0;
}
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