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

From: John C. Femiani (john.femiani_at_[hidden])
Date: 2008-08-14 01:44:03

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<class M, class K, class V>
> struct multimap_insert_existing {
>    typedef typename insert<typename erase_key<M, K>::type, pair<K, 
> typename push_back<typename at<M, K>::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