Register variadic template as tags in a boost::mpl::map

7 Jan 2012

      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 >,
       pair >,
       pair > >

But it does not seem like this is the case...

#include <iostream>
#include 
#include 
#include 
#include 

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::size * X::offset >,
           public X
{
    typedef Head< X::size * X::offset > _Head;

    enum O { offset = X::size * X::offset };
    enum S { size   = _Head::size * X::size };

    typedef typename boost::mpl::insert < typename X::type_map,
                                          boost::mpl::pair
...
::type type_map;
template <typename T> int get_offset() {
        return boost::mpl::at::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;
}

Allan Nielsen

Allan Nielsen

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