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

Subject: Re: [boost] [smart_ptr] Moving cast for unique_ptr
From: Gavin Lambert (gavinl_at_[hidden])
Date: 2015-10-07 03:03:29

On 7/10/2015 19:16, Vicente J. Botet Escriba wrote:
> 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));

It would theoretically be possible if you have access to the internals 
of shared_ptr (otherwise it would be no better than copying).

However I'm dubious about this usage -- dynamic casts naturally admit 
the possibility of failure, which results in discarding the input 
parameter in cases that it's a temporary (hence the changed semantics in 
the proposed dynamic_moving_pointer_cast).

This basically translates into an operation that says "return the 
pointer without touching its refcount if the cast succeeds, otherwise 
decrement the refcount".  In particular the latter case can result in 
almost immediately deleting a value newly-created by the called 
function, or otherwise unique prior to the cast.

The equivalent operation on unique_ptr would simply be "delete input 
pointer if cast fails".

Either of these seem like an odd thing for code to want to do in 
practice.  Usually polymorphic code doesn't transfer ownership at the 
same time.  I'm curious if there would be any good real-world examples 
for this.