Re: [boost] [fusion::vector] Forwarding packed arguments by reference

From: Marco Costalba (mcostalba_at_[hidden])
Date: 2007-09-18 08:34:30

On 9/18/07, Marco Costalba <mcostalba_at_[hidden]> wrote:
>
> template<typename Values, typename Args>
> Base* doObject(KeyRef nm, Values& v) const
> {
>         Range r;
>         if (!find_range<Args>(nm, r))
>                 return NULL; /* no function with the same number of arguments */
>
>         const SignatureClass<Base, Args>* p = find<Args>(r);

This is the core of all the stuff.

At runtime the passed in argument types  'Args' are checked against
the factory wrappers stored in a map to see if one of them support the
same type of arguments.

template<typename Args>
const SignatureClass<Base, Args>* find(const Range& r) const
{
        for (const_iterator it = r.first; it != r.second; ++it)
        {
                const SignatureClass<Base, Args>* p = check_type<Args>((*it).second);
                if (p)
                        return p;
        }
        return NULL;
}

And finally:

template<typename Args>
const SignatureClass<Base, Args>* check_type(const StorableClass* p) const
{
        // check c'tor arguments type at runtime
        return dynamic_cast<const SignatureClass<Base, Args>*>(p);
}

All this dynamic plomorphism stuff is, again, necessary (at least I
haven't found nothing better) _only_ because supported classes are not
known at factory instantation time. If it was known I could use a
fusion map to store everything and the trick is done.

But a fusion map cannot be changed at runtime! If you add new classes
to your map you have a _new_ map type. So you cannot foreseen a map
variable as a member data.

That's the problem.

class factory {
                ....
a_fusion_map_type  myFactoriesMap;

template<typename F>
void add_factory(const F& f)
{
    myFactoriesMap << f; // not possible because 'a_fusion_map_type' changes
}

}

Thanks
Marco