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

27 Nov 2013

      2013/11/26 DeadMG 
...
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