[boost] rebind<> bad; template template good

From: David B. Held (dheld_at_[hidden])
Date: 2002-10-02 23:54:34

If anyone needs to be convinced that template templates are a good and
useful feature of C++, let them take a look at this code, which is an
adaptation of shared_static_cast for policy-based pointers:

    template <
        typename T,
        BOOST_SMART_POINTER_PARAMETERS,
        typename U
    >
    inline smart_ptr<T,
        typename OwnershipPolicy::template rebind<
            typename StoragePolicy::template rebind<T>::other::pointer_type
        >::other,
        typename ConversionPolicy::template rebind<
            typename StoragePolicy::template rebind<T>::other::pointer_type
        >::other,
        typename CheckingPolicy::template rebind<
            typename StoragePolicy::template rebind<T>::other::stored_type
        >::other,
        typename StoragePolicy::template rebind<T>::other
    >
        smart_static_cast(
            smart_ptr<U, BOOST_SMART_POINTER_POLICIES> const& p
        )
    {
        return smart_ptr<T,
            typename OwnershipPolicy::template rebind<
                typename StoragePolicy::template
rebind<T>::other::pointer_type
            >::other,
            typename ConversionPolicy::template rebind<
                typename StoragePolicy::template
rebind<T>::other::pointer_type
            >::other,
            typename CheckingPolicy::template rebind<
                typename StoragePolicy::template
rebind<T>::other::stored_type
            >::other,
            typename StoragePolicy::template rebind<T>::other
        >(
            p, detail::static_cast_tag()
        );
    }

Note also that this code is somewhat simplified by macros I've been
using, and that this technique requires *every policy* to define a rebind<>
template.  Also keep in mind that there are three other casts like this. :(
What a high price to pay for lack of template templates!!

I'm trying to figure out a way to avoid defining a rebind<> template
for every policy.  I'm thinking of something like this:

template <class T>
struct rebindable
{
    template <typename U>
    struct rebind
    { typedef T<U> other; };
};

Then maybe you could do:

template <typename P>
struct policy : public rebindable<policy>
{
    ...
};

If anyone has any clever ideas, or ways I can avoid rebind entirely in
the code above, I'm open to suggestions.

Dave