Re: [Boost-users] Thread Safety of shared_ptr

On Wed, May 4, 2011 at 12:17 AM, Cory Nelson wrote:

On Tue, May 3, 2011 at 12:59 PM, Ovanes Markarian wrote:

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On Tue, May 3, 2011 at 2:18 PM, Peter Dimov wrote:

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Peter, can you please give an example how that can happen, given the

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is atomic. I thought the counter is first incremented than the rest of

shared_counter the

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machinery deals with pointer copying etc. and in case of underlying object destruction, the counter is first decremented and than the object is destroyed. How is that possible, that shared_counter reaches 0, destructor is called and afterwards the underlying pointer will be assigned to the new shared instance? With Kind Regards, Ovanes

He means that while the reference count is thread safe, the pointer is not. Here's an example that will cause problems:

Thread A: ptr1 = ptr2; Thread B: ptr2 = ptr;

Note that ptr2 is being concurrently read and modified.

Oh, I see. But this looks for me as the wrong usage of shared_ptr. ptr2 should not be passed to Thread A by reference, but should be copied into the Thead A's context. Otherwise I can't imagine how this can happen. The same can happen in the single threaded environment if one uses references to shared_ptr instances. Best Regards, Ovanes