[boost] optional: Extending optional to hold derived values

From: Vaclav Vesely (vaclav.vesely_at_[hidden])
Date: 2006-10-06 04:20:32

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