Re: Re: Conversion policies

At 16:00 2003-08-08, David Abrahams wrote:
>Daniel Wallin <[email protected]> writes:
>
> > Ok I've given this some thought. The issue is that we need to keep the
> > converter
> > object instances on the stack of the invoker function, to be able to have
> > storage between check() and to_cxx(). This is important for 
> lvalue-conversions
> > which can perform the entire conversion in the checking part, and 
> stores the
> > pointer. It's likely important for other more advanced policies as 
> well, which
> > could need intermediate storage.
>
>Right, OK:
>
>template<class R, class A0, class A1, class Policies>
>ret_type invoke(R(*fn)(A0, A1), const Policies& policies, ArgTuple t)
>{
>     typename Policies::conversions_type converters(policies);
>
>     if !(
>          converters.check(_1, identity<A0>(), t[0])
>        , converters.check(_2, identity<A1>(), t[1])
>     )
>     return kOverloadFailed;
>
>     return converters.from_cxx(
>         fn(
>              converters.to_cxx(_1, identity<A0>(), t[0])
>            , converters.to_cxx(_2, identity<A1>(), t[1])
>         )
>     );
>}

The type of the converter objects is dependent on the types they operate 
on, so
it's slightly more complicated:

typename Policies::template conversions_type<
      mpl::list<A0, A1>
   >::type converters;

Which isn't really more efficient than the first implementation. Less code 
in the invoker
functions though.


---
Daniel Wallin



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