Re: [boost] Downcast taking account of virtual inheritance

Subject: Re: [boost] Downcast taking account of virtual inheritance
From: Kim Barrett (kab.conundrums_at_[hidden])
Date: 2010-02-14 16:52:56

On Feb 14, 2010, at 2:55 PM, vicente.botet wrote:
> The implementation is based on whether the base type is polymorphic, not whether the base type is_virtual_base_of<B,D>.
> I have replaced 
> 
>            template<class U>
>            static T cast(U * u){
>                    typedef
>                        BOOST_DEDUCED_TYPENAME mpl::eval_if<
>                            BOOST_DEDUCED_TYPENAME mpl::and_<
>                                mpl::not_<is_base_and_derived<
>                                    BOOST_DEDUCED_TYPENAME remove_pointer<T>::type,
>                                    U
>>> ,
>                                mpl::not_<is_base_and_derived<
>                                    U,
>                                    BOOST_DEDUCED_TYPENAME remove_pointer<T>::type
>>> 
>> ,
>                            mpl::identity<cross>,
>                            mpl::identity<linear>
>> ::type typex;
>                    return typex::cast(u);
>            }
> by
>            template<class U>
>            static T cast(U * u){
>                    typedef 
>                        BOOST_DEDUCED_TYPENAME mpl::eval_if<
>                            is_virtual_base_of<U, BOOST_DEDUCED_TYPENAME remove_pointer<T>::type>,
>                            mpl::identity<cross>,
>                            mpl::identity<linear>
>> ::type typex;
>                    return typex::cast(u);
>            }
> 
> 
> And it work also. But now dynamic_cast is used only if U is virtual_base_of T.

I don't think this change is correct, as it will result in using static_cast in an actual cross-cast situation, i.e. when there is no base/derived relationship between U and T, but they have a common derived type of which the value being cast is an instance.

However, it seems to me that there is a bug in the existing smart_cast implementation, in that it may incorrectly attempt to use static_cast in the case where U is a virtual_base_of T.

I think the correct fix is to leave the existing derivation test in place, but augment it with the is_virtual_base_of test, i.e. mpl::or_ the existing test for cross-cast (T and U have no base/derived relationship) with the is_virtual_base_of test.