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

Subject: Re: [proto] Extendin proto::switch_
From: Mathias Gaunard (mathias.gaunard_at_[hidden])
Date: 2011-08-06 06:47:13

On 08/06/2011 07:30 AM, Eric Niebler wrote:
> On 8/5/2011 8:52 AM, Joel falcou wrote:
>> There is few use case where I wish i can have a proto::switch_ like
>> transform being extendable externally but based on something else than
>> the expression tag like the result of an arbitrary meta-function.
>>
>> Is cloning proto::swicth_ and changing the way it dispatch over its
>> internal cases_ enough ?
>
> 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::_>().
>
> Could you open a feature request?

Why not just add an extra class Enable = void template parameter to 
case_, which would allow to use SFINAE for case_ partial specializations?

template<class Tag>
struct case_<Tag, typename boost::enable_if< is_what_I_want<Tag> >::type>
  : ....
{
};