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

From: Andreas Huber (ahd6974-spamgroupstrap_at_[hidden])
Date: 2005-08-03 03:43:28

David Abrahams <dave <at> boost-consulting.com> writes:
[snip]
> > I need to modify that sequence internally. 
> 
> You don't mean that, exactly, since all metadata is immutable.

Right, I mean that I can't use e.g. push_front on some sequences.

> > If any sequence was allowed (including immutable ones), I would need
> > to copy that sequence into an mpl::list<> first. 
> 
> 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).

> Are you sure you couldn't get away with using some views onto the
> original sequence?

I'm not. Views are something I probably need to try when I get the time to 
make compilation time optimizations.

> > I suspect that the copying slows down compilation noticeably (I
> > never had the chance to make measurements, so this may well be FUD),
> > so I figured that requiring a list is the best way to give the user
> > the opportunity to keep compilation as fast as possible. 
> 
> It's not; a vector would be faster.

Ok, that's good to know. 

> Or you could just require any
> front extensible sequence, in which case, maybe deque is what you
> need?

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.

> > Sure, I could make a copy only if the user doesn't pass an
> > mpl::list<> and document that in the interface but nobody has ever
> > made a request in this direction. I guess this is due to the fact
> > that pretty much everybody specifies the InnerInitial states inline:
> >
> > struct Y;
> > struct Z;
> > struct X : sc::simple_state< X, Machine, mpl::list< Y, Z > > {};
> 
> Why mpl::list and not mpl::vector or mpl::deque?

See above. All of them should work.

> > That is, I can't really think of a real-world example where you would 
> > want to use an algorithm to fill that list...
> >
> > Would you allow any mpl sequence anyway?
> 
> I certainly wouldn't require a list.  Whether to accept every possible
> sequence is another question.  I'm sure there are many models of the
> concept you actually need from the user.

Agreed, an appropriate item is now on my to-do list.

Thanks & Regards,

-- 
Andreas Huber
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