Re: [boost] More flexible any_cast for boost::any

Subject: Re: [boost] More flexible any_cast for boost::any
From: DeadMG (wolfeinstein_at_[hidden])
Date: 2013-11-27 04:32:44

I'd agree with implementing it separately, at least for now. Hopefully in
the future, the Standard will provide facilities for it. It shouldn't be
too difficult to put through Committee since it's already been demonstrated
that the feature must be implemented by all implementations today, even if
the existing characteristics of accessing it are very unfriendly.

But what I will note is that for some implementations, you can do a lot
better than this. For example, on Clang, you can use __has_include
toconditionally fall back to the Itanium ABI function
directly.<https://svn.boost.org/trac/boost/ticket/6773> This
wouldn't require exceptions and would reduce the cost from throw/catch to
dynamic_cast, which is quite a bit more reasonable.

In any case, I merely wanted to make the maintainers of Boost.Any aware
that there is a generic implementation of such a polymorphic_any_cast, even
if it's not ideal. Then you can specialize for different implementations
and such.

On 27 November 2013 08:25, Antony Polukhin <antoshkka_at_[hidden]> wrote:

> 2013/11/26 DeadMG <wolfeinstein_at_[hidden]>
>
> > Hello,
> >
> > I've been looking at ABIs and such (various unpleasant reasons). But I
> > recently created the following code:
> >
> > #include <iostream>
> >
> > struct Base {
> >     virtual void check() = 0;
> >     template<typename T> T* IsDerivedFrom() {
> >         try {
> >             check();
> >         } catch(T* p) {
> >             return p;
> >         } catch(...) {
> >             return nullptr;
> >         }
> >         return nullptr;
> >     }
> > };
> >
> > template<typename T> struct Derived : public Base {
> >     Derived(T obj) : t(obj) {}
> >     T t;
> >     void check() override final {
> >         throw &t;
> >     }
> > };
> >
> > struct A { virtual ~A() {} virtual void print() = 0;};
> > struct B : public A { int x; B(int y) : x(y) {} void print() override
> final
> > { std::cout << x; } };
> >
> > int main() {
> >     Base* b = new Derived<B>(B(5));
> >     A* p = b->IsDerivedFrom<A>();
> >     p->print();
> > }
> >
> > Whilst this is hardly the cleanest piece of code I've ever written, it
> also
> > provides a base for implementing a more generic any_cast<ValueType>,
> which
> > could succeed if ValueType is a base class of the stored type. Amazingly,
> > the Standard requires the code to work but does not provide a cleaner
> means
> > of implementation.
> >
>
> This is a very interesting approach. But it has two drawbacks, because of
> exceptions usage:
> * it is slow
> * it won't work with exceptions disabled
>
> This can be implemented as a separate `polymorphic_any_cast` cast that is
> available only with exceptions on. In that way it won't affect performance
> of the existing any_casts and will improve usability.
>
> I'm not *the official* maintainer of the Boost.Any library, I'm just the
> one who made all the latest changes with rvalues and C++11 support. But if
> there'll be no objections and the official maintainer won't appear I can
> add this functionality to Boost.Any.
>
> --
> Best regards,
> Antony Polukhin
>
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