optional: Extending optional to hold derived values
Hi, I would like to do somethink like this: class Base {...} class Derived: public Base {...} optional<Base> x = Derived(); y->call_virtual_functions() Current design of boost::optional doesn'g support this. But I wonder if it's to possible to extend it. I created simple prototype to verify the technique (see attachment). It works this way: optional<BaseT> - has attribut _value_holder of type value_holder<BaseT>* - _value_holder points to the instance of derived_value_holder<BaseT, DerivedT>, where DerivedT is the type of stored value value_holder<BaseT> - abstract class - can be cloned (deep copy) derived_value_holder<BaseT, DerivedT> - is derived from value_holder<BaseT> - contains DerivedT value (cloned with copy constructor) What do you think about it? Regards, Vaclav begin 666 optional_test.cpp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` ` end
Vaclav Vesely wrote:
Hi,
I would like to do somethink like this:
class Base {...} class Derived: public Base {...}
optional<Base> x = Derived(); y->call_virtual_functions()
Current design of boost::optional doesn'g support this. But I wonder if it's to possible to extend it.
I created simple prototype to verify the technique (see attachment). It works this way:
optional<BaseT> - has attribut _value_holder of type value_holder<BaseT>* - _value_holder points to the instance of derived_value_holder<BaseT, DerivedT>, where DerivedT is the type of stored value
It looks a lot like Boost.Any. Have you looked at it? Fernando Cacciola SciSoft http://fcacciola.50webs.com
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. Regards, Vaclav
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
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.
Perhaps you can use Boost.PtrContainer: typedef boost::ptr_vector< boost::nullable<Base> > OptionalContainer; ? -Thorsten
participants (3)
-
Fernando Cacciola -
Thorsten Ottosen -
Vaclav Vesely