Thread-safety of c++ maps

Question

This is about thread safety of std::map. Now, simultaneous reads are thread-safe but writes are not. My question is that if I add unique element to the map everytime, will that be thread-safe?

1. So, for an example, If I have a map like this std:map<int, std::string> myMap and I always add new keys and never modify the existing key-value, will that be thread-safe?

2. More importantly, will that give me any random run-time behavior?

3. Is adding new keys also considered modification? If the keys are always different while adding, shouldn't it be thread-safe as it modifies an independent part of the memory?

Thanks Shiv

Answer 1

1) Of course not

2) Yes, I hope you'll encounter it during testing, not later

3) Yes, it is. The new element is added in a different location, but many pointers are modified during that.

The map is implemented by some sort of tree in most if not all implementations. Inserting a new element in a tree modifies it by rearranging nodes by means of resetting pointers to point to different nodes. So it is not thread safe

Answer 2

no, yes, yes. You need to obtain exclusive lock when modifying container (including insertion of new keys), though while there's no modification going on you can, of course, safely read concurrently.

edit: http://www.sgi.com/tech/stl/thread_safety.html might be of interest for you.