Re: [boost] [DynamicAny] Feedback

Subject: Re: [boost] [DynamicAny] Feedback
From: Daniel Larimer (dlarimer_at_[hidden])
Date: 2011-03-04 11:15:23

On 3/4/11 11:05 AM, "Stewart, Robert" <Robert.Stewart_at_[hidden]> wrote:

> Daniel Larimer wrote:
>>> Stewart, Robert escribió:
>> 
>>>> Couldn't this functionality be added to Boost.Any itself?
>>>> That is, add another cast that does dynamic_cast so there's
>>>> only one library but clients get a choice about how to
>>>> extract values.
>> 
>> The change is perhaps a bit to fundamental. Here is the
>> "effective difference".
>> 
>> struct any
>> {
>>     struct holder_base{};
>>     template<typename T>
>>     struct holder_impl<T> : holder_base
>>     {
>>         T held;
>>     };
>>     holder_base* data;
>> }
>> 
>> struct dynamic_any
>> {
>>     struct holder_base{};
>>     template<typename T>
>>     struct holder_impl<T> : T, virtual holder_base {};
>>     holder_base* data;
>> };
> 
> Hmmm.  What about just adding the equivalent of dynamic_cast<U *>(&held) and
> dynamic_cast<U &>(held), in a new cast function template (perhaps through the
> aid of friendship and a private member function) called dynamic_any_cast, to
> the existing Boost.Any?
> 
The problem is that you must first cast any::placeholder*  to
any::holder<U>* before you can access holder<U>::held.

Unfortunately, any::holder<Derived> does not inherit from any::holder<Base>
so you cannot do a dynamic cast from any::placeholder to any::holder<Base>
if any::placeholder is actually an instance of any::holder<Derived>.

boost::any can do a static cast because it has verified that the types are
the same and thus does not need to worry about "offsets", if the types are
different, then you would need a virtual template function to calculate the
offset between any two types for the cast.  Unfortunately, virtual template
methods are not an option.

> _____
> Rob Stewart                           robert.stewart_at_[hidden]
> Software Engineer, Core Software      using std::disclaimer;
> Susquehanna International Group, LLP  http://www.sig.com
> 
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