Unordered Fusion Map constructors.

[Dean Michael Berris <mikhailberis@…>]

Currenty, when constructing Fusion Maps, the order of type arguments used as template parameters need to be followed in the constructor as well.

Example:

struct tags {
  struct type_1 { };
  struct type_2 { };
};

using namespace boost::fusion ;

typedef map< pair<tags::type_1, int>, pair<tags::type_2, int> > my_map_type ;

{
  // following line will not compile, because of the argument ordering
  my_map_type instance( make_pair<tags::type_2>(2), make_pair<tags::type_1>(1) );

  // following line will compile, because the argument order is the same as
  // the order of elements in the fusion map specialization
  my_map_type instace_works( make_pair<tags::type_1>(1), make_pair<tags::type_2>(2) );

}

The feature request is to support arbitrary ordering of constructor arguments, not necessarily to follow the order of argument types as prescribed in the template arguments to the fusion map.

[Stjepan Rajko]

[Bug Sprint] One way I see of implementing this would be through a view which reorders the elements of a source map to match the order of keys of a target map. This view could then be used for construction and assignment from an existing map:

my_map_type instance (
  reorder_map_to<my_map_type> (
    make_map( make_pair<tags::type_2>(2), make_pair<tags::type_1>(1) ) ) );

instance =
  reorder_map_to<my_map_type> (
    make_map( make_pair<tags::type_2>(2), make_pair<tags::type_1>(1) ) ) );

// or

my_map_type instance ( reorder_map_to<my_map_type> (some_other_map) );

instance = reorder_map_to<my_map_type> (some_other_map);

The map class could use the reordering view automatically in construction from a list of arguments, construction from a Sequence, and assignment (so that the OP's original code snippet compiles), but that would complicate the otherwise simple implementation. Would using the reordering view manually fulfill the need here?