From: Peter Dimov (pdimov_at_[hidden])
Date: 2000-09-04 05:40:26
> >Having copy_to as a free function makes it easier for somebody to make it
> >work with, say, ref<void> instead of any.
> Well, there's nothing preventing anyone from wrapping it :-)
Wrapping should not be necessary. The point is that to_ptr is sufficient;
copy_to and any_cast can be implemented in terms of to_ptr. Consider:
template<class U> bool copy_to(any const & from, U & to)
if(U * p = from.to_ptr<U>())
to = *p;
template<class T, class U> bool copy_to_aux(T const & from, U & to,
to = from;
template<class T, class U> bool copy_to_aux(T const &, U &, false_type)
template<class T, class U> bool copy_to(T const & from, U & to)
return copy_to_aux(from, to, is_convertible_to<T, U>::eval);
This makes copy_to even more generic.
> I would prefer not to provide a hacked interface with deprecated
> features if possible (and I believe it is), and instead provide an
> interface that is a proper supertype of the full one.
I don't quite understand what are you objecting to.
Proper compilers would eat
and all compilers, including MSVC, would compile the alternative
It is a deprecated feature, granted, but this is the lesser evil.
> >So when you std::sort a vector of ref's, the contents are sorted by type,
> >then by value.
> Err, "subtle". This mixes two ordering concepts.
No, this is a strict weak ordering on the <type, value> pair. How else would
you make std::set<ref> work?
> >However, you can't do anything generic with an 'any', regardless of
> >whether you have its type_info or not, except getting its name() (which
> >useless anyway.)
> I would argue that it's not useless, and in simple command line
> interpreters turns out to be quite useful!
The standard doesn't guarantee the usefulness of type_info::name(). It's
useful if you know what your compiler does, but it's not portable.
> >That's why I asked for an example that clearly demonstrates that the
> >to ask an 'any' for its type_info is a part of the minimal interface. So,
> >what are those things that need runtime binding? :)
> map<const std::type_info *, handler, type_info_before> handler_table;
Why didn't you say so before? :)
Yes, this is the example I came up with myself.
Background: I'm trying to decide whether ref<> needs type_info() or not.
This example does prove the need for any::type_info. However, ref doesn't
need this sort of dispatch because you have access to the virtual functions
of the base type.
-- Peter Dimov Multi Media Ltd.
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