But I can't say the same things for non-const function which returns reference. <...> And also iterators are same as the index operator, not thread safe.

It seems that the only way to use such a container in the thread-safe manner, is to perform all the necessary locking at some higher-level of your design. Besides the problem you mentioned, you've got another issue: usually you wish to get not just "physical" thread-safety (meaning that your container remains in a consistent state), but a "logical" one. Consider the following code: if (myVector_.size()) { Element element = myVector_[0]; } It doens't make sense that size() and operator[]() themselves are "thread-safe", as between these calls the size might change. This means that the user of your container has to lock it anyway. If so, your internal locking appears to be useless overhead.