Re: [Boost-users] [Fusion] Views of Associative Sequences

Subject: Re: [Boost-users] [Fusion] Views of Associative Sequences
From: Joel de Guzman (joel_at_[hidden])
Date: 2009-10-16 19:29:12

Christopher Schmidt wrote:
> My implementation introduces associative iterators. An associative 
> forward sequence shall provide an iterator that allows direct mapping 
> from the iterator (type) to the underlying element (type), key type and 
> the actual value (type) associated with the key.
> I split fusion::find up in fusion::find_key and a new fusion::find. My 
> fusion::find does not care about the associative-ness of the sequence. 
> It searches for a plain element type. That is it returns the first 
> iterator for which is_same<fusion::result_of::value_of<mpl::_1>, Elem> 
> evaluates to true, or an end-iterator. fusion::find_key returns the 
> first iterator for which is_same<fusion::result_of::key_of<mpl::_1>, 
> Key> evaluates to true, or an end-iterator.
> 
> The compile-time evaluation of a 
> fusion::(result_of::)find_key-invocation should scale linear to the 
> number of elements of the sequence being searched on. The 
> fusion::find_key implementation uses fusion::detail::static_seq_find_if, 
> which is 4-unrolled optimized.
> This should be slightly faster compared to the unoptimized 
> implementation of fusion::map<...>/set<...>::(meta_)find_impl(_const), 
> which are plain recursive metafunctions/n-overloaded functions.
> The runtime performance of my port should be better if the compiler 
> supports rvalue references. If not, there should not be a difference in 
> any case at all. For fusion::find, the run-time performance is of course 
> constant.
> The "old" implementation uses a distinct intrusive (meta-)function, 
> analogue to find, to implement at_key. I implemented fusion::at_key 
> based on fusion::find_key.
> 
> For more information on my port, have a look at the posts over at the 
> development mailing list.

That's a well-thought and very nice design, Christopher!

Regards,
-- 
Joel de Guzman
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