Re: [boost] [concept] concept-based overloading (N2081)

Subject: Re: [boost] [concept] concept-based overloading (N2081)
From: Lorenzo Caminiti (lorcaminiti_at_[hidden])
Date: 2010-05-27 20:30:18

On Thu, May 27, 2010 at 4:59 PM, Jeffrey Lee Hellrung, Jr.
<jhellrung_at_[hidden]> wrote:
> On 5/27/2010 12:57 PM, Lorenzo Caminiti wrote:
>>
>> On Thu, May 27, 2010 at 1:48 PM, Jeffrey Lee Hellrung, Jr.
>> <jhellrung_at_[hidden]>  wrote:
>>>
>>> On 5/27/2010 10:38 AM, Lorenzo Caminiti wrote:
>>>>
>>>> N2081 briefly mentions this can be done in C++ using SFINAE, etc. I
>>>> think I could use `enable_if` if I had some sort of
>>>> `is_input_iterator` metafunction... Please just point me to the
>>>> documentation if this has already been addressed there.
>>>>
>>> Do you mean the iterator_category / iterator_traversal metafunctions in
>>> boost/iterator/iterator_traits.hpp ?
>
> [...]
>>>
>>> http://www.boost.org/libs/iterator/doc/iterator_traits.html
>>>
>>> template<  class Iterator>
>>> typename boost::disable_if<
>>>    boost::is_convertible<
>>>        typename boost::iterator_traversal<  Iterator>::type,
>>>        boost::bidirectional_traversal_tag
>>>    >
>>>>
>>>> ::type
>>>
>>> advance(...) { ... } // for IncrementableIterator's
>>>
>>> template<  class Iterator>
>>> typename boost::enable_if<
>>>    boost::is_convertible<
>>>        typename boost::iterator_traversal<  Iterator>::type,
>>>        boost::bidirectional_traversal_tag
>>>    >
>>>>
>>>> ::type
>>>
>>> advance(...) { ... } // for BidirectionalIterator's
>>>
>>> I think the primary way to currently effect "concept-based" overloading
>>> is
>>> via tags or similar metafunction-based methods.  There do exist
>>> introspection techniques, though, such as proto's can_be_called or
>>> is_incrementable in boost/detail/, so it is possible to overload based on
>>> (some) syntactic properties.
>>>
>>> Is that what you're looking for?
>>
>> Essentially, the above code from Jeff got the job done for what I
>> needed. I used `enabled_if` and tagging.
>>
>> Also thanks to everyone else for the very informative comments.
>>
>
> Just so an "inferior" approach isn't mistaken as the "correct" solution,
> Thomas is right, you should use tag-dispatching if possible:
>
> template< class Iterator >
> void advance_dispatch(..., boost::incrementable_traversal_tag) { ... }
> template< class Iterator >
> void advance_dispatch( ..., boost::bidirectional_traversal_tag) { ... }
> template< class Iterator >
> void advance(...)
> { advance_dispatch(..., typename iterator_traversal< Iterator >::type()); }
>
> And now that I've written this, I see this exact example is already
> elaborated on at the boost website:
>
> http://www.boost.org/community/generic_programming.html#tag_dispatching

Great! The code from the Boost links above is _exactly_ what I was looking for.

Thanks a lot.

-- 
Lorenzo