title: Increment int in an unordered_map?
body: <pre><code>int c = counts.at(v).at(id);
  std::cout &lt;&lt; c;
  counts.at(v).emplace(id,c+1);
  std::cout &lt;&lt; counts.at(v).at(id);
  </code></pre>
  
  <p>counts is an unordered_map of unordered_map. The first cout prints 1, the second prints 1, why it is not incremented ?</p>