Re: [boost] [type_erasure] Review started (July 18-27, 2012)

Subject: Re: [boost] [type_erasure] Review started (July 18-27, 2012)
From: Christophe Henry (christophe.j.henry_at_[hidden])
Date: 2012-07-24 13:39:26

> 2. Is it possible to convert between "any" instances with different 
> concept
> sets or value types? For instance, are these possible?
> any< mpl::vector<
>   addable<>,
>   incrementable<>,
>   decrementable<> >
> > a;
>
> any< mpl::vector<
>   addable<> >
> > b = a;

I find this an interesting question. I use the review to change a private 
but real project and replace all of my OO-style interfaces by TypeErasure. I 
got the following to compile and work (you need copy_constructible):

typedef any< boost::mpl::vector<
          boost::type_erasure::typeid_<>,
          boost::type_erasure::copy_constructible<>,
          boost::type_erasure::addable<>,
          boost::type_erasure::incrementable<>,
          boost::type_erasure::decrementable<> >
                > AnyA;

        typedef any< boost::mpl::vector<
          boost::type_erasure::copy_constructible<>,
          boost::type_erasure::typeid_<>,
          boost::type_erasure::addable<>
          >
        > AnyB;

          AnyA a(10);
         AnyB b = a;

OTOH, this does not compile:
       AnyB b2(10);
      AnyA a2=b2;

Which brings me to the following use case. Supposing I have 3 classes IA <- 
IB <- IC and 1 object of each held as shared_ptr, I can dynamic_pointer_cast 
from one to another. But if I have 3 any's of concepts A,B,C, B containing 
A's concepts + some more, and C containing B's concepts + some more, I can 
"upcast" as shown above from C to B and B to A, but not the other way 
around, right? Even if I have an object fulfilling A,B and C.

Thanks,
Christophe

PS: so far I got only one warning: check_match does not use its parameter 
op.