Vaclav Vesely wrote:
Fernando Cacciola wrote:
Vaclav Vesely wrote:
I would like to do somethink like this:
class Base {...} class Derived: public Base {...}
optional<Base> x = Derived(); y->call_virtual_functions()
It looks a lot like Boost.Any. Have you looked at it?
boost::any doesn't support derived classes as well. For example:
any x = Derived(); any_cast<Base>(x).virtual_function(); // throws bad_any_cast
And moreover boost::any is too general. I want to store only values of Base and its derived classes.
OK. Still, Boost.Any is a lot closer to that than Boost.Optional The fundamental issue is that Boost.Optional stores copies of the values in its own storage. That's a key feature of optional<>. In your design, as in Boost.Any's, the values are stored in a separate storage and the wrapper is merely a proxy to it. So this is more the job of a form of smart pointer (that manages external resources). IIRC, a few years ago Peter Dimov sketched a polymorphic variant class in line with your idea (unless I got it all wrong). I can't recall where I saw it though. Peter? In any event, I think what you need is a new beast rather an extension to optional<> because of the separate storage needed to hold the polymorphic value. Best Fernando Cacciola SciSoft Buenos Aires, Argentina