title: tbb concurrent hash map find & insert
body: <p>I am currently using tbb's concurrent hash map to perform concurrent insertions into a hash map. Each key is a string and a value is a vector of integers. I would like to achieve the following: during insertions, if the key does not exist, I insert it and add the value to its vector. If it exists, I simply add the value to its vector.</p>
  
  <p>After inspecting the tbb concurrent hash map API, I noticed that both the find and insert functions return booleans only. So how can I return a pointer to the key if it exists? </p>