Re: [Boost-users] [fusion] Why no multi{map,set} ?

Steven Watanabe wrote:

AMDG

John C. Femiani wrote:

...

I was just wondering why fusion (and mpl) don't include multi-maps or multi-sets. Are they unnecessary at compile time? Are they too trivial? Do they conceptually not work?

The easiest way to implement a multimap is in terms of an ordinary map. In a fusion/mpl map the key is a type and lookup uses overload resolution. This means that there is necessarily a 1-1 correspondence between keys and values. I would expect a multimap to contain pairs, and to provide an iterator range or sequence of like keys via an 'equal_range' function (like unordered_multimap).

I also just noticed there is no 'equal_range' query metafunction, although there is a 'lower_bound' and 'upper_bound'.

The obvious way to implement a multimap is to make the value a container. Because of the function nature of fusion/mpl this would be like

template struct multimap_insert_existing { typedef typename insert::type, pair::type, V>::type> > type; };

<more code> So it is feasible, but the authors just wanted to keep the scope limited? That works for me; I've been toying with that idea anyhow but I wanted to make sure I was not going waste my time. Thanks, --John