Re: [boost] [smart_ptr] Moving cast for unique_ptr

Subject: Re: [boost] [smart_ptr] Moving cast for unique_ptr
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2015-10-07 02:16:02

Le 06/10/15 12:19, Karolin Varner a écrit :
> Hi all!
>
> There is the family of boost pointer cast (boost::static_pointer_cast, boost::dynamic_pointer_cast, ...) that allow a developer to write generic code by casting pointers regardless of whether they are some smart pointer (e.g. shared_ptr) or plain pointers.
>
> However, those functions have not been implemented for unique_ptr, because they create a copy of their pointer and unique_pointers can not be copied.
>
> In my case I still needed to cast some unique_ptr and since it's ok to move unique_ptrs I wrote a function that does exactly that.
> The code is below – Comments are very, very welcome. It works for my case, but I am sure it is not as generic and safe as it should be ;) . Specifically, for a library, it should be made to work on any kind of smart/plain pointer. Not just unique_ptr.
>
> Is there any interest to include a boost::static_moving_pointer_cast and the other types of casts in boost?
I believe this will be a useful addition.
>
> /// Dynamic cast a unique_ptr.
> ///
> /// This function shall move the pointer in in_ptr, into
> /// a new unique_pointer of the desire type, dynamic_casting
> /// it in the process.
> /// In given pointer shall contain a NULL pointer afterwards.
> ///
> /// If unsuccessful, function shall throw an error and leave
> /// in_ptr unmodified.
> ///
> /// This behaviour contrasts a bit with the behaviour of the
> /// normal dynamic_cast:
> /// 1. dynamic_cast copies objects, this moves them since
> ///    unique_pointers can not be copied.
> /// 2. dynamic_cast returns a NULL pointer if the pointer to
> ///    cast is of an incomplete type, this throws
> ///    incomplete_cast instead, to ease error handling.
I wouldn't throw in this case to be coherent with dynamic_pointer_cast. 
If you need a better error handling for dynamic_moving_pointer_cast you 
need also for dynamic_pointer_cast.

Just wondering if we can not make dynamic_pointer_cast to work as your 
dynamic_moving_pointer_cast, when the parameter is a rvalue reference

template<class T, class U>

shared_ptr<T> dynamic_pointer_cast(shared_ptr<U> && r) noexcept;

C++ International Standard
The use will be

     dynamic_pointer_cast<U>(ret_up());
     dynamic_pointer_cast<U>(move(up));

Vicente