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