[Boost-users] [boost] [fusion] fusion::map doesn't define 'category'

Subject: [Boost-users] [boost] [fusion] fusion::map doesn't define 'category'
From: Steve Lorimer (steve.lorimer_at_[hidden])
Date: 2012-07-26 23:48:00

I am attempting to create a fusion map which looks like:

    map< pair<field1, int>, pair<field2, int> >

I am then attempting to find out if *field1 *exists in the map using *has_key,
*but am getting a compile error:

    error: no type named ‘category’ in ‘struct
boost::fusion::result_of::as_map<boost::fusion::vector2<boost::fusion::pair<field1,
int>, boost::fusion::pair<field2, int> > >

Can anyone offer insight on what I'm doing wrong?

I have a simple example below:

#include <iostream>
#include <boost/fusion/container.hpp>
#include <boost/fusion/sequence.hpp>
#include <boost/fusion/mpl.hpp>
#include <boost/fusion/include/has_key.hpp>
#include <boost/mpl/transform.hpp>

namespace fusion = boost::fusion;
namespace mpl    = boost::mpl;

*// given a field, returns a fusion pair of <field, field::type>*
template<class field>
struct make_pair
{
    typedef typename fusion::result_of::make_pair<field, typename
field::type>::type type;
};

*// given a sequence of fields, returns a fusion map which maps field ->
field::type*
template<class fields>
struct make_map
{
    typedef typename mpl::transform<fields, make_pair<mpl::_1>>::type
pair_sequence;
    typedef typename fusion::result_of::as_map<pair_sequence> type;
};

*// sets boolean value to true if field is in fields*
template <class field, class... fields>
struct contains_key
{
    typedef typename boost::fusion::vector<fields...> vector;
    typedef typename make_map<vector>::type map;
    typedef typename fusion::result_of::has_key<map, field>::type result;

    static const bool value = result();
};

struct field1 { typedef int type; };
struct field2 { typedef int type; };

int main()
{
    std::cout << contains_key<field1, field1, field2>::value << std::endl;
    return 0;
}