[boost] Re: [mpl] sequence details

From: David Abrahams (dave_at_[hidden])
Date: 2005-02-28 14:40:41

Tobias Schwinger <tschwinger_at_[hidden]> writes:

> Hi Dave,
>
>
> David Abrahams wrote:
>> Tobias Schwinger <tschwinger_at_[hidden]> writes:
>>>Tobias Schwinger wrote:
>>>
>>>>Hi,
>>>>What is the best way to convert the result of a mutating mpl
>>>>intrinsic metafunction (such as 'mpl::push_back', for example) to
>>>>numbered form ('vectorN<a1,a2,..>') ?
>> I was hoping Aleksey would answer, but he seems to have dropped off
>> the face of the earth (temporarily, I hope).
>> 
>
> Thanks for your attempt to fill the (hopefully temporary) gap.
>
>> I am guessing that Aleksey would say "don't do that!"  He has always
>> maintained that relying on sequence type identity was just wrong.
>>
>
> This, in itself, makes pretty much sense to me.
>
> However, I need to get to the types "all at once" rather than "one at a 
> time" (details below).
>
>>>>I tried using the type-member (which only works on some compilers -
>>>>not with GCC for example) and figure 'copy' could probably do the
>>>>trick (but is a quite complex operation) so there might be a better
>>>>recipe for this.
>>>>In a library I'm currently working on, I use a decorator pattern to
>>>> make 
>>>
>>>[ This refers to: http://tinyurl.com/6c35p ]
>>>
>>>
>>>>a class model of a sequence it holds (i.e. give it a tag type member
>>>>and specialize *_impl with delegate semantics). After applying the
>>>>delegate metafunction I need the sequence in a form I can match with
>>>>partial specialization.
>> I don't know the details of your problem, so I can't say for sure,
>> but
>> you might consider using partial specialization based on the
>> sequence's tag and/or size.
>
> I am working on a library which allos to work with function types [ the 
> link above points to the announcement of the preview version on this 
> list, yesterday ].
>
> The problem lies in the metafunction to synthesize function types from 
> an MPL Seuqence [ ref. class template function_type ]:
>
> The current procedure is:
>
> - check for the sequence tag
> - if it is a vector use its ::type member
> - if it is an mpl sequence, but not a vector use mpl::copy to make it
> (other cases are checked for, that do not matter in this context)

That seems sorta crazy.

> - the _impl-part of the story then uses a partial specialization cascade 
> to extract the elements
> - build the function type from them
> (this works fine for non-typeof-based sequences)
>
> Specializing by size would mean:
>
> - use at_c for every element

Not at all; just use iterators dereference and increment them.
boost/python/detail/caller.hpp uses this pattern.

> - build the function type from them
> (reads nicely here, but has to be done for the whole cascade, 
> (max_arity*kinds_of_function_types_supported) times, which would result 
> in an unreadable and inefficient mass of code)

This is going to be a job for the PP library, no matter what you do.

> Probably the latter is still the better way to go, however, I was hoping 
> (and still do) for a better solution than these two.
>
> Unless you (or someone on the list) knows one, it's probably a wise idea 
> to just wait for Aleksey to reappear before changing things around...

Suit yourself.

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