Re: Conversion policies

Daniel Wallin <[email protected]> writes:

> Given a conversion policy which handles conversions
> xxx -> C++, what should be passed to the converter
> function so that it can deduce the type?
>
> struct my_converter_policy
> {
>     template<class T>
>     T convert(const param_type&, ...<T>);
>                                  ^^^^^^ what goes here?
> };
>
> The simplest approach would be to just pass a
> boost::type<T>, but this means partial ordering
> won't work. 

Why?  Comeau accepts this:

    template <class T> struct type {};

    template <class T>
    int* convert(int, type<T>);

    template <class T>
    long* convert(int, type<T*>);

    int* x = convert(0, type<int>());
    long* y = convert(0, type<char*>());

> Also, IIRC, trying to deduce T from
> type<const T&> results in an ICE on VC6.5.

vc6.5 compiles this just fine:

    template <class T> struct type {};

    template <class T>
    int* convert(int, type<T>);

    int* x = convert(0, type<int const&>());

Of course, if you're concerned about partial ordering, vc6.5/7 doesn't
support it; you have to "help", and the ways I know of helping do
seem to produce ICEs with vc6.5 when combined with type<...>.

Why don't we go with class specialization, anyway?

struct my_converter_policy
{
   template <class T>
   struct apply
   {
       static T execute(const param_type&);
   };
};

> FWIW, in luabind, we handle this by decorating the
> type:
>
>    T           by_value<T>
>    T&          by_reference<T>
>    const T&    by_const_reference<T>
>    T*          by_pointer<T>
>    const T*    by_const_pointer<T>

I guess I don't really understand the motivation, here.  Could you
say a little more about why you want to distinguish those cases?
Is there something wrong with the way Boost.Python does this now
(whatever "this" is ;->)?

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com



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