Re: [Boost-users] [mpl] Constant time mpl::contains<mpl::set>

Subject: Re: [Boost-users] [mpl] Constant time mpl::contains<mpl::set>
From: Joel Falcou (joel.falcou_at_[hidden])
Date: 2010-03-29 02:35:11

strasser_at_[hidden] wrote:
> Hi,
> 
> I'd like to be able to query if a MPL Sequence contains a certain type, 
> aka mpl::contains.
> But mpl::contains is documented to always have linear complexity, even 
> for mpl::set's. How can you differentiate between types of MPL 
> Sequences, so you can use mpl::at for sets and mpl::contains for vectors?
> MPL itself seems to do this by template specialization, e.g. for 
> mpl::at, which is overloaded.
> But template specialization relies on MPL implementation details, is 
> there a way to detect any type that models an associative sequence?
> I can't find something like mpl::is_set nor a tag to dispatch with in 
> the AssociativeSequence concept.
> 
> Thank you,
> 
> Stefan

I, myself, use :

http://www.boost.org/doc/libs/1_41_0/libs/mpl/doc/refmanual/sequence-tag.html

and I check if the returned type is one of the defined tag. HJere for 
mpl::set, you can do is_same<sequence_tag<X>, sequence_tag< set<> > >
so you don't rely on the explicit type of the tag itself.