"John Maddock" <john@johnmaddock.co.uk> writes:
All of which suggests that a compressed-tuple would be a good idea, although ordering the items within the tuple to ensure compression is actually a larger challenge than folks may have realised.
Actually I think this is pretty easy. Here's the sketch: struct empty {}; template <class Key, class Value, class Tail = empty> struct node : Tail { // typical associative lookup tricks using overloading, per // Boost.Parameter library, or, I'm sure, Fusion's map }; template < class Types // MPL sequence , class Keys // types by which we'll do lookup
struct compressed_tuple_base { typedef typename mpl::sort< mpl::zip_view<Types,Keys> , mpl::less<is_empty<mpl::first<_1> >, is_empty<mpl::first<_2> > > >::type sorted_pairs; typedef typename mpl::fold< sorted_pairs , node<mpl::second<_2>, mpl::first<_2>, _1> , empty >::type type; }; template < class Types // MPL sequence , class Keys = mpl::range_c<int, 0, mpl::size<Types>::value>
struct compressed_tuple : compressed_tuple_base<Types,Keys>::type { // ctors // The easy ctor will initialize from a tuple of references, // which we can build using tie() ... // get<N> just does lookup with integral_c<int,N> as the key };
In the mean time if additional template constructors can be added, I see no problem with that.
I think I'll code it with fusion when I get a moment :). -- Dave Abrahams Boost Consulting www.boost-consulting.com