[boost] scoped models (was Re: [fusion] Support for classic boost::tuple)

From: Jeremy Siek (jeremy.siek_at_[hidden])
Date: 2006-10-02 09:45:54

Hi Dave,

On Oct 2, 2006, at 4:40 AM, David Abrahams wrote:
> Sure.  So, leaving aside that I have the syntax and probably a few
> other things wrong:
>
>     template <class T, Sequence<int*> S1, Sequence<T> S2,  
> UnaryFunction<T,int*> F>
>     void transform(S1 const& src, S2& dst, F f)
>
>     typedef std::pair<int*,int*> r;
>
>     template <Sequence<int> S1>
>     std::pair<int*,int*> f(S1 s)
>     {
>         r ret;
>
>         // treat pair<int*,int*> as a  sequence of int*
>         concept_map<r, Sequence<int*> >
>         {
>             ...
>         };
>
>         transform(s, ret, address());
>     }
>
>     int storage[2];
>     r input = { storage, storage+2 };
>
>     // Treat pair<int*,int*> as a sequence of int
>     concept_map<r, Sequence<int> >
>     {
>         ...
>     };
>
>     // OK?
>     r x = f(input);
>
>     // which concept does x model?

It's early in the morning, but I'll do my best...

In this outer scope, the type 'r' models Sequence<int>, so
the call to f type checks.

Inside of function f, 'S1' models Sequence<int> and 'r' models  
Sequence<int*>.
The call to transform will not type check because the requirement
Sequence<int*> S1 is not satisfied.

Cheers,
Jeremy

__________________________________
Jeremy Siek <siek_at_[hidden]>
http://www.cs.colorado.edu/~siek/
Visiting Assistant Professor
Department of Computer Science
University of Colorado at Boulder