Re: [boost] optional: Extending optional to hold derived values

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.
> 
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