Re: [boost] [type_erasure] Downcasting is now possible

Subject: Re: [boost] [type_erasure] Downcasting is now possible
From: Mathias Gaunard (mathias.gaunard_at_[hidden])
Date: 2015-03-10 13:24:33

On 10/03/2015 17:38, Steven Watanabe wrote:
> AMDG
>
> On 03/10/2015 07:33 AM, Mathias Gaunard wrote:
>> On 08/03/2015 05:27, Steven Watanabe wrote:
>>
>>>     The calls to register_binding are necessary for
>>> this to work, unfortunately.  I don't think it's
>>> possible to implement dynamic_any_cast without
>>> some kind of registration, but at least it only
>>> needs to be called once for each concept/type
>>> combination.  If anyone has any better ideas
>>> about how to handle this, I'd love to hear it.
>>
>> If you're doing this, you might as well implement full dynamic duck typing.
>> If the expression is valid, you call it, otherwise you raise an exception.
>>
>
>    Oh, I agree.  However, the cost of such a call
> is significantly greater then a normal virtual
> call.  Thus, I don't want the library to do
> this without being told explicitly that it's the
> desired behavior.  With what I have already, the
> implementation of such a feature is trivial.  The
> real issue is defining the interface for it.

How about something like this:

any a;
duck_visit(f, a);

with 'f' a polymorphic function object constrained with SFINAE.

if f(a) is callable for the dynamic type of a, then call it, otherwise 
raise an exception.