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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


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...



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