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

From: David Abrahams (dave_at_[hidden])
Date: 2005-08-03 09:24:29

Andreas Huber <ahd6974-spamgroupstrap_at_[hidden]> writes:

> David Abrahams <dave <at> boost-consulting.com> writes:
>> 
>> 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]

If you're erasing items at the end of a list<>, that's *very*
expensive!  So, sorry to be blunt, but I maintain your arguments about
efficiency are invalid.  Not to mention that they're not backed up by
measurements.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com