Re: [boost] [functional] [overloaded_function] polymorphic functions?

Subject: Re: [boost] [functional] [overloaded_function] polymorphic functions?
From: Gordon Woodhull (gordon_at_[hidden])
Date: 2012-03-14 16:50:45

Hi Lorenzo,

On Feb 18, 2012, at 6:24 AM, John Bytheway wrote:
> On 18/02/12 03:16, Lorenzo Caminiti wrote:
>> I spent some time trying to make overloaded_function support
>> polymorphic functions but I had little success :(

Thank you for attempting it!  This is something that came up as a possible better solution in the Conversion review in August.  (For which my report is grievously late but not forgotten.)

>> 
>> 1) I was able to do this (see code below):
>> 
>>    overloaded_function<
>>          void (int)
>>        , functor<poly, void (generic)>
>>> o(f, p);
>>    o(321);
>>    o(3.21);
>>    o("cba");
>> 
>> But I don't like it because I explicitly have to pass the functor type
>> poly. I wanted instead to only use generic to tag the template
>> parameters in the signature:
>> 
>>    overloaded_function<
>>          void (int)
>>        , void (generic)
>>> o(f, p);
>>    o(321);
>>    o(3.21);
>>    o("cba");
>> 
>> I'd consider this acceptable but I wasn't able to program this because
>> I loose p's type (poly) in base so base cannot hold a reference to the
>> functor p... Is there a way around this? (Some type-erasure magic
>> maybe??)
> 
> If the type of o did not involve poly, then you could pass a reference
> to o to another translation unit where the definition of poly was not
> visible.  In that other translation unit you could call it with an
> arbitrary type, which would have to instantiate poly::operator() without
> sight of its definition.  Therefore, I conclude that this is clearly
> impossible.

I had neglected the fact that overloaded_function does type erasure (duh).  I guess this would only be possible if the polymorphic result_of (or equivalent) is passed to overloaded_function.  Probably not worth it.

Sorry if I sent you on a wild goose chase!  I really appreciate the effort, and knowing that this technique could be used as long as type erasure is not required (as it isn't for Conversion).

Cheers,
Gordon