title: updating the value of a key in a std::map
body: <p>Assume we have a simple structure such as the following</p>
  
  <pre><code>struct T{
    int x;
    int y;
  };
  T t1, t2;
  </code></pre>
  
  <p>Also assume that I have a <code>map&lt;T, int&gt; myMap</code> and that two structures of type <code>T</code> are compared using their <code>x</code> values only. I.e. <code>t1 &lt; t2</code> iff <code>t1.x &lt; t2.x</code>. I am trying to update some of the <code>y</code> values of the keys over myMap. This should not affect how the map is seeing the keys. Is there any way other than removing the old element and inserting a new one?</p>