unordered_map thread safety

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STL containers are designed so that you are guaranteed to be able to have:

A. Multiple threads reading at the same time

or

B. One thread writing at the same time

Having multiple threads writing is not one of the above conditions and is not allowed. Multiple threads writing will thus create a data race, which is undefined behavior.

You could use a mutex to fix this. A shared_mutex (combined with shared_locks) would be especially useful as that type of mutex allows multiple concurrent readers.

http://eel.is/c++draft/res.on.data.races#3 is the part of the standard which guarantees the ability to concurrently use const functions on different threads. http://eel.is/c++draft/container.requirements.dataraces specifies some additional non-const operations which are safe on different threads.

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