title: Generating and reusing unique ID in C++
body: <p>I am working on a requirement where requests should have unique number from -2 to -101 inclusive, i.e., there are unique 100 requests at a time. If there are more than 100 requests at a given time then I should send error. Initially I have no requests. Once I sent requests I will take unique number say -2 , -3 and so on. Here requirement is that I may get command from client don't sent request to server for example -2 so I should delete this request and I should reuse this number for future request.</p>
  
  <p>What is the best way to implement this in C++? </p>
  
  <p>Also, I am not supposed to use Boost.</p>