Re: [boost] [shared_ptr] constructor accepting auto_ptr

From: Howard Hinnant (hinnant_at_[hidden])
Date: 2005-12-07 10:47:15

On Dec 7, 2005, at 10:01 AM, Yuval Ronen wrote:

> Hi.
> There's a shared_ptr constructor that accepts an auto_ptr<Y>&.  
> Shouldn't
> it accept auto_ptr<Y> by value rather by non-const reference? It seems
> to me that the reference does not add anything, but only causes a VC
> warning (level 4):
>
> ---
> warning C4239: nonstandard extension used : 'argument' : conversion  
> from
> 'std::auto_ptr<_Ty>' to 'std::auto_ptr<_Ty> &'
> A reference that is not to 'const' cannot be bound to a non-lvalue
> ---
>
> when calling this constructor with a temporary auto_ptr.
> This warning is a good thing and I don't want to disable it.

This shared_ptr ctor is designed such that if it throws, the auto_ptr  
arg retains ownership of the pointer.  To transfer ownership from an  
rvalue auto_ptr you could:

shared_ptr<Y> sp(get_auto_ptr().release());

The semantics of this latter statement are different than the ctor  
your pointed out only under exceptional conditions.  Now if the  
shared_ptr ctor throws, the pointer is deleted instead of being  
retained by the auto_ptr.  By having these two options, shared_ptr  
allows the client to choose which semantics work best in any given  
situation.

<musing>

A future shared_ptr might:

shared_ptr(auto_ptr<T>&&);  // or move_ptr

This would allow lvalues or rvalues to bind, and retain the same  
semantics as we have today for lvalue auto_ptr's.  This is just off  
the cuff, food for thought...

</musing>

-Howard