Question

I am struggling with a small piece of functionality I’m looking for.

I have a class which contains a fusion::map. I would like to use a variadic constructor to initialise the elements in that map.

I expect the easiest way to do this is to construct a fusion::vector from the constructor arguments, and then call for_each on the map, and setting each pair’s value to its corresponding element in the vector.

However, in order to do this I need to calculate the index of the pair, based on its key type. (pair::first_type)

Can anyone help me?

Please see example code 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/fusion/include/algorithm.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 boost::fusion::vector<fields...> vector;
    typedef typename mpl::transform<vector, make_pair<mpl::_1>>::type pair_sequence;
    typedef typename fusion::result_of::as_map<pair_sequence>::type type;
};

// initialise each member of a map with the corresponding element in the vector
template<typename vector>
struct init
{
    init(vector& v) : _v(v) {}

    template <typename pair>
    void operator()(pair const& data) const
    {
       // TODO: use pair::first_type to find the index of this pair in the map, and set 
       // data.second to at_c<index>(_v);
    }
    vector& _v;
};

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

struct my_map
{
    template<typename... args>
    my_map(args... a)
    {
        typedef typename boost::fusion::vector<args...> vector;
        vector arg_vec(a...);

        fusion::for_each(_map, init<vector>(arg_vec));
    }
    typedef typename make_map<field1, field2>::type map;
    map _map;
};

struct print
{
    template <typename pair>
    void operator()(pair const& data) const
    {
        std::cout << data.second << " ";
    }
};


int main()
{
    my_map m(1, 2);
    fusion::for_each(m._map, print()); // should print '1 2'
    return 0;
}

Answer 1

Adding another answer to my own question, this time for the use-case where the number of arguments is less than or equal to the number of elements in the map.

A working solution is 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/fusion/include/algorithm.hpp>
#include <boost/mpl/transform.hpp>
#include <boost/fusion/include/copy.hpp>
#include <boost/fusion/include/begin.hpp>
#include <boost/fusion/include/next.hpp>
#include <boost/fusion/include/key_of.hpp>
#include <boost/fusion/iterator/key_of.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 fusion::result_of::as_map<typename mpl::transform<boost::fusion::vector<fields...>, make_pair<mpl::_1>>::type>::type type;
};
//-------------------------------------------------------------------------------------------

// iterate through a fusion map to find the index of a given key
template<typename iter, typename key_type, typename seek_type>
struct key_index
{
    typedef typename fusion::result_of::next<iter>::type        next_iter;
    typedef typename fusion::result_of::key_of<next_iter>::type next_key;

    enum { value = 1 + key_index<next_iter, next_key, seek_type>::value };
};

template<typename iter, typename seek_type>
struct key_index<iter, seek_type, seek_type>
{
    enum { value = 0 };
};
//-------------------------------------------------------------------------------------------

// copy an element from a vector to a map, if the index in the vector exists
template<typename map, typename vector, int index, bool in_vec>
struct do_copy
{
    template<typename T>
    void operator()(const vector& v, const T& dest)
    {
        const_cast<T&>(dest) = fusion::at_c<index>(v);
    }
};

template<typename map, typename vector, int index>
struct do_copy<map, vector, index, false>
{
    template<typename T>
    void operator()(const vector&, const T&)
    { }
};
//-------------------------------------------------------------------------------------------

// initialise a map with the corresponding elements in a vector, vector may be smaller than the map
template<typename vector, typename map>
struct init
{
    init(const vector& v) : _v(v) {}

    template <typename pair> void operator()(const pair& data) const
    {
        typedef typename fusion::result_of::begin<map>::type          begin_iter;
        typedef typename fusion::result_of::key_of<begin_iter>::type  key_type;

        enum { index  = key_index<begin_iter, key_type, typename pair::first_type>::value };
        enum { in_vec = fusion::result_of::size<vector>::type::value > index };

        do_copy<map, vector, index, in_vec>()(_v, data.second);
    }
private:
    const vector& _v;
};
//-------------------------------------------------------------------------------------------

struct field1 { typedef std::string type; };
struct field2 { typedef int         type; };
struct field3 { typedef double      type; };
struct field4 { typedef std::string type; };
struct field5 { typedef int         type; };
struct field6 { typedef double      type; };

struct my_map
{
    template<typename... args>
    my_map(args... a)
    {
        typedef typename fusion::vector<args...> vector;
        fusion::for_each(_map, init<vector, map>(vector(a...)));
    }
    typedef typename make_map<field1, field2, field3, field4, field5, field6>::type map;
    map _map;
};

struct print
{
    template <typename pair>
    void operator()(pair const& data) const
    {
        std::cout << data.second << " ";
    }
};
//-------------------------------------------------------------------------------------------

int main()
{
    my_map m("hello world", 5, 2.4);
    fusion::for_each(m._map, print());
    std::cout << std::endl;
    return 0;
}