Re: [Boost-users] [mpl] how to create a map using an algorithm

From: Yuval Ronen (ronen_yuval_at_[hidden])
Date: 2006-08-26 11:56:23

David Abrahams wrote:
> Yuval Ronen <ronen_yuval_at_[hidden]> writes:
> 
>> Lets say I have several types t1, t2, t3, and so on. Each of them has a 
>> typedef x within it. I want to be able to get a type y, and find the 
>> type t which holds is_same<y, t::x>. I thought of doing it using 
>> mpl::map< pair<t1::x, t1>, pair<t2::x, t2>, ...>.
>>
>> The problem is how to create the map. I can create it manually, just 
>> like in the previous sentence, but I hope there's a better way. Is there 
>> some transform algorithm that I can pass it an mpl::set<t1, t2, ...> and 
>> a meta-function that yields t::x for any given t, and this algorithm 
>> would produce the desired map?
> 
>   // untested
>   template <class T> struct getx { typedef typename T::x type; };
> 
>   mpl::fold<
>       the_ts
>     , mpl::insert<_1, mpl::pair<getx<_2>,_2> >
>     , mpl::map0<>
>   >::type the_map;

Looks cool, will check it out.
Thanks.