Can an std::map be moved while iterating through it?

Question

When I'm iterating through an std::map, is there a possibility that by for example adding an element to the map in another thread, the objects in it will be removed causing the iteration to be corrupt? (As the iterator will be pointing to a non-existing variable as it's moved)

Answer 1

In theory when you add an element to an std::map, all the iterators in that map should stay valid. But the problem is that the operations are not atomic. If the OS suspends the inserting thread in the middle of the operation and gives control back to the iterating thread, the state of std::map might be invalid.

You need to synchronize access to the map via mutex or something similar. Alternatively you could use concurrency friendly collection from TBB or another similar library. TBB provides concurrent_unordered_map and concurrent_hash_map.

Answer 2

STL containers aren't thread safe. No guarantees at all. So you need to synchronize access to any standard container if they are used by different threads.

Answer 3

Yes--if another thread may be modifying the vector, you'll need to use something like a mutex to assure that only one thread has access to the vector at any given time.

With a map, the effects of a modification are much more limited -- rather than potentially moving the entire contents of a vector, a modification only affects an individual node in the map. Nonetheless, if one thread deletes a node just as another thread is trying to read that node, bad things will happen, so you still need a mutex to assure that only one thread is operating on the map at any given time.