How shared_ptr copy constructor work?

Question

I was thinking, if the main thread is deleting a shared_ptr, and cause reference count drop to 0, and in the process of deleting the referenced pointer heap memory, and at the same time, another thread is copying a shared_ptr reference (inside the copy constructor already). What would happen then?

I didn't downvote the question, but if I had to guess, it was probably done because your question is a single run on sentence which is barely readable and doesn't provide any example of the problem you are trying to solve. — Claies, Apr 02 '18 at 06:19
Possible duplicate of - https://stackoverflow.com/questions/9127816/stdshared-ptr-thread-safety-explained ? — badola, Apr 02 '18 at 06:26

score 0 · Answer 1 · edited Apr 02 '18 at 06:51

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The situation you describe isn't allowed to happen in standard compliant code. In case it does appear in code, you have undefined behavior.

The std::shared_ptr class isn't thread safe. You aren't allowed to access the same instance from 2 threads without synchronization, as that is a race condition.

So, when the counter is read, if it isn't zero, it is incremented and some pointers are copied over.

edited Apr 02 '18 at 06:51

Remy Lebeau

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answered Apr 02 '18 at 06:20

JVApen

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What do you mean by shared_ptr class is not thread safe? i believe they do have atomic or lock inside the implementation right? – Bryan Fok Apr 02 '18 at 06:29
You can use multiple instances in multiple threads, you can't use the same instance in multiple threads – JVApen Apr 02 '18 at 06:30
what if i do: std::thread([&](){ //reference to the shared_ptr }) – Bryan Fok Apr 02 '18 at 06:31
Use [=]{} instead – JVApen Apr 02 '18 at 06:32
yea, just replying your last statement – Bryan Fok Apr 02 '18 at 06:32
Let me rephrase in the original answer: impossible -> not valid code – JVApen Apr 02 '18 at 06:33
I am wondering what my example would happen if we really doing referencing of shared_ptr – Bryan Fok Apr 02 '18 at 06:34
Ok lets me try out an example – Bryan Fok Apr 02 '18 at 06:34
std::shared_ptr a = std::make_shared(10); std::thread([&](){ std::shared_ptr b(a); }); – Bryan Fok Apr 02 '18 at 06:39
the above code compile. not sure what you mean not valid code. – Bryan Fok Apr 02 '18 at 06:40
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Using a nullptr or deleted instance also compiles – JVApen Apr 02 '18 at 06:42
I dont think you get the point here that you CAN reference the shared_ptr, and from my answer, it would cause the problem potentially (as the answer explained). – Bryan Fok Apr 02 '18 at 06:51
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@Bryan Fok Just because something *compiles* does *not* mean that the code is valid. Syntactically valid, yes. But not necessarily free of Undefined Behaviour, implementation defined behaviour, use of languagee extensions, non-portable constructs, logic errors, race conditions, etc etc. – Jesper Juhl Apr 02 '18 at 07:21
His original answer was simpler, the code was just showing it is valid syntax Jesper, and it is not the question I was asking anyway – Bryan Fok Apr 02 '18 at 10:23

score 0 · Answer 2 · answered Apr 02 '18 at 06:30

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I think this blog did answer my question? I am not sure. So if the shared_ptr is copy to the thread, there are no chance that the problem I mention would happen.

shared_ptr must have at least 2 pointers, so it’s bigger than a raw pointer. It also guarantees thread-safety for all methods as long as each thread has its own copy.

answered Apr 02 '18 at 06:30

Bryan Fok

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I think it is very interesting, and it changed the way I always thought about shared_ptr until I try to implement a simple version of it for fun. I think the blog, and the response from Lothar at the below link did answer this question https://stackoverflow.com/questions/9127816/stdshared-ptr-thread-safety-explained – Bryan Fok Apr 02 '18 at 06:48

How shared_ptr copy constructor work?

2 Answers2