Re: [Boost-users] Thread Safety of shared_ptr

2 May 2011


      On Mon, May 2, 2011 at 4:23 PM, Ovanes Markarian wrote:
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On Mon, May 2, 2011 at 3:01 PM, Frank Mori Hess wrote:
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On Monday, May 02, 2011, Peter Dimov wrote:
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// thread B
  // p2 goes out of scope: undefined, the destructor is considered a
"write access"
This example just says that you can't make a copy of a destroyed
shared_ptr, or one that is in the process of being destroyed. In
principle, it has nothing to do with reference counts. Doing something
with a destroyed object of any type is undefined behavior in C++.
It looks like the example isn't quite right though, as p2 was declared in
thread A so it can't go out of scope in thread B.
It still can happen if p2 is a weak_ptr in thread "A" and the corresponding
shared_ptr  goes out of scope in thread "B".
Best Regards,
Ovanes
Actually, I am not sure with this statement. Destructor is only called if
shared count reaches zero. I don't think that this can happen, that a dtor
is called in one thread and the other thread is still able to increment the
shared count even from the weak_ptr. I remember reading weak_ptr's source
and saw that it is also protected again such a case.

Thanks,
Ovanes