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From: Fernando Cacciola (fernando_cacciola_at_[hidden])
Date: 2006-10-10 10:15:23

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.
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.


Fernando Cacciola
Buenos Aires, Argentina

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