Proto for Boost mailing list page: Re: [proto] Extendin proto::switch_

Subject: Re: [proto] Extendin proto::switch_
From: Eric Niebler (eric_at_[hidden])
Date: 2011-08-29 11:33:50

On 8/28/2011 6:17 PM, Joel Falcou wrote:
> Le 06/08/2011 08:10, Eric Niebler a écrit :
>> On 8/5/2011 10:55 PM, Joel falcou wrote:
>>> On 06/08/11 07:30, Eric Niebler wrote:
>>>> That wouldn't be enough because proto::matches "knows" about
>>>> proto::switch_. It would be easy enough to extend proto::switch_ to
>>>> take
>>>> an optional mpl metafunction that accepts and expression and returns a
>>>> type to dispatch on. It would default to proto::tag_of<mpl::_>. Or for
>>>> the sake of consistency with the rest of proto, it should probably be a
>>>> transform, in which case it would default to proto::tag_of<proto::_>().
>>>
>>> OK
>>>
>>>> Could you open a feature request?
>>>
>>> Well, we wanted to know the correct road, i have someone to do it, so
>>> let's say we'll provide you with a patch request instead ;)
>>
>> Even better. :-)
>>
> 
> Here is a first try:
> 
> https://github.com/MetaScale/nt2/blob/30251fccec639a3823179fc04100fb3fba0688b2/modules/sdk/include/nt2/sdk/dsl/select.hpp
> 
> 
> Not sure it is perfect but it works eemingly :)

Good. Obviously, this needs to be called switch_ instead of select_.
There needs to be an appropriate default for the Transform parameter,
something like tag_of<_>(). There should also be a specialization of
switch_ when the transform is tag_of<_>() to make it as efficient as the
current switch_ (but it should be backward-compatible without the
specialization -- test this!). And of course docs and tests.

> My main cocnern is can i get to remove this internediate when + result_of ?

No need to replace the internal use of when, but you can access when's
nested impl template directly instead of using result_of's needlessly
complicated machinery. See the implementation of if_.

Thanks!

-- 
Eric Niebler
BoostPro Computing
http://www.boostpro.com