Re: [boost] [DynamicAny] Feedback

Subject: Re: [boost] [DynamicAny] Feedback
From: Sebastian Redl (sebastian.redl_at_[hidden])
Date: 2011-03-04 11:36:16

On 04.03.2011, at 17:15, Daniel Larimer wrote:

> 
> 
> 
> 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.

It would be possible to implement dynamic_any_cast for the existing boost::any by using compiler-specific implementation knowledge. For example, on GCC, you could access type_info::__do_upcast to do the casting.

Of course, such an implementation would have to have cases for every compiler, and it might turn out to be unimplementable on some.

Sebastian