title: c++ is unordered_map<int, Enum> thread safe with one thread write and a second thread read, no new element insert?
body: <p>I have the following senario,</p>
  <pre><code>class enum class Status {
      Unknown, Pause, Normal, Resume
  }
  
  std::unordered_map&lt;int, Status&gt; status_map;
  </code></pre>
  <p>If at beginning I know all the key in status_map and initialze,</p>
  <pre><code>for(auto key:key_list) {
     status_map[key]=Status::Unknown;
  }
  </code></pre>
  <p>Then later, <code>thread1</code> will change known key's status, and <code>thread2</code> will keep reading the <code>status_map</code> to make some decision, will this whole operation thread-safe?</p>