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Asked: 2026-05-28T15:08:51+00:00

I wonder if size functions (size, length or whatever) are thread safe? They usually

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I wonder if size functions (size, length or whatever) are thread safe? They usually just return some private size member, as I understand. I really doubt they do any sort of calculations. They all are marked as const but are they thread-safe? for example std::list::size?

I have a lock-protected function for writing and another for reading (also lock-protected) but I wonder if my count function should also be lock-protected? IMO, it looks like a waste of response time. I don’t think it may break any iterators or fail if some member is being removed from the list at the same time (as same, as possible).

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1 Answer

  1. Editorial Team

    Yes, it needs to be protected by a lock. Let’s say that your implementation’s std::list::size is a 32-bit value but on your platform 32-bit reads are not atomic, they take 2 16-bit reads. In this case, a second thread may interrupt the first that was reading the size after the first read has occurred, update the size variable and then when the second 16-bit read takes place you may get a real messed up value for size.

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