title: Is boost::asio::thread_pool thread safe when posting tasks on multiple threads?
body: <p>I'm submitting more than one tasks to boost::asio::thread_pool concurrently. But official document does not talk about thread safe of boost::asio::thread_pool.</p>
  
  <p>the boost lib version is 1.69. And my code like below:</p>
  
  <pre><code>/*define a thread pool*/    
  boost::asio::thread_pool pool(4);
  
  //on thread 1
  boost::asio::post(pool, my_task_1);
  ...
  //on thread 2
  boost::asio::post(pool, my_task_2);
  </code></pre>
  
  <p>so I want to know is the code able to work on multiple threads</p>