Re: [boost] Formal Review Request: TypeErasure

Subject: Re: [boost] Formal Review Request: TypeErasure
From: Fabio Fracassi (f.fracassi_at_[hidden])
Date: 2012-06-08 04:16:58

On 6/7/12 11:14 PM, lcaminiti wrote:
>
> Nevin Liber wrote
>>
>> Take the following base class:
>>
>> struct S
>> {
>>      void f();
>>      void g();
>>      void h();
>> };
>>
>> In the virtual function world, to enforce that interface, one merely has
>> to
>> write:
>>
>> struct AbstractS
>> {
>>      virtual void f() = 0;
>>      virtual void g() = 0;
>>      virtual void h() = 0;
>> };
>>
>> and have others derive from it.
>>
>> If I wanted a type erased version of the equivalent callable interface,
>> what do I have to do?  If it isn't as simple as writing AbstractS above,
>> only a small fraction of C++ developers will ever attempt it, let alone do
>> it on a regular basis.
>>
>> Steven's library does a great job at covering a lot of cases, but adding
>> custom concepts is a chore.
>>
>
> What if this were possible? Would this be an acceptable way to add custom
> concepts?
>
> BOOST_TYPE_ERASURE_CONCEPT(
>      concept (sable) ( class S )
>      (
>          void member_body(S, f) ( void ) ,
>          void member_body(S, g) ( void ) ,
>          void member_body(S, h) ( void )
>      )
> )
>
> --Lorenzo
>

I think this would be a step in the right direction, but not quite good 
enough yet. I have used and written similar macro based IDLs on/for 
several systems, and they were always disliked by most developers, even 
though they worked quite well.

For such a common feature the Syntax has to be as easy as Possible, and 
I think (but am not sure) this can't be done without a language feature. 
I could Imagine a Syntax like either the above AbstractS (with or 
without the virtual) or even more ideally be able to do something like 
"create_type_erased_concept_from<S>;" which should be implementable 
(maybe with a macro instead of a template) if we had some kind of 
compile time reflection.

regards

Fabio