unordered_map thread safety

Question

I am changing a single thread program into multi thread using boost:thread library. The program uses unordered_map as a hasp_map for lookups. My question is..

At on

Answer 1

Will that be thread safe and the container designed for this?

No, the standard containers are not thread safe.

Do I need to use some locking mechanism?

Yes, you do. Since you're using boost, boost::mutex would be a good idea; in C++11, there's std::mutex.

I read somewhere that the C++ Standard says the behavior will be undefined, but is that all?

Indeed, the behaviour is undefined. I'm not sure what you mean by "is that all?", since undefined behaviour is the worst possible kind of behaviour, and a program that exhibits it is by definition incorrect. In particular, incorrect thread synchronisation is likely to lead to random crashes and data corruption, often in ways that are very difficult to diagnose, so you would be wise to avoid it at all costs.

UPDATE: I was also thinking about Intel concurrent_hash_map. Will that be a good option?

It sounds good, but I've never used it myself so I can't offer an opinion.