title: unordered_map thread safety
body: <p>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.. </p>
  
  <p>At one time many threads will be writing, and at another many will be reading but not both reading and writing at the same time i.e. either all the threads will be reading or all will be writing. Will that be thread safe and the container  designed for this? And if it will be, will it really be concurrent and improve performance? Do I need to use some locking mechanism?</p>
  
  <p>I read somewhere that the C++ Standard says the behavior will be undefined, but is that all?</p>
  
  <p>UPDATE: I was also thinking about Intel concurrent_hash_map. Will that be a good option?</p>