Re: [boost] [mpl] as_sequence< X< Y > > instantiates typedefs inX<Y >

From: Andreas Huber (ahd6974-spamgroupstrap_at_[hidden])
Date: 2005-08-03 07:21:35

David Abrahams <dave <at> boost-consulting.com> writes:
> >> Why an mpl::list<>?  What's special about that structurue?  Why not
> >> mpl::vector?
> >
> > The sequence just needs to support certain operations, e.g. push_front, 
> > reverse, etc. So, mpl::vector<> should work too (I never checked).
> 
> If you're reversing the sequence, all your arguments about requiring
> mutability for efficiency reasons are invalid.  You might as well do

I don't think so, I'm only reversing the sequence *after* performing other 
mutating operations on it. Here's an example from the current implementation 
(context_type_list is assembled from multiple InnerInitial lists):

template< class CommonContext, class DestinationState >
struct make_context_list
{
  typedef typename mpl::reverse< typename mpl::push_front<
    typename mpl::erase<
      typename DestinationState::context_type_list,
      typename mpl::find<
        typename DestinationState::context_type_list,
        CommonContext
      >::type,
      typename mpl::end<
        typename DestinationState::context_type_list
      >::type
    >::type,
    DestinationState
  >::type >::type type;
};

[I know that this code needs improvement]

> > Don't know, I can't think of anything that would keep you from using
> > deque or vector. I guess it would be best to document the exact
> > requirements and make measurements so that I can tell the user which
> > of the standard mpl sequences are best.
> 
> Not a bad idea, but I wouldn't go overboard.  In this case the first
> priority should be to lift needless restrictions.

I've actually already added two to-do items. The one dealing with the 
restrictions appears before the measurements.

> The restrictions
> you're using don't even neccessarily lead to better efficiency.

Right, I got that :)

Regards,

-- 
Andreas Huber
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