title: change element by iterator
body: <p>i have problem when i want to change element of set by using iterator.
  This simple code may explain what i want to do.</p>
  
  <pre><code>  set&lt;int&gt; s;
    s.insert(12);
  
    set&lt;int&gt;::iterator it = s.begin();
    *it = 4; // error C3892: 'it' : you cannot assign to a variable that is const
  </code></pre>
  
  <p>Why i can not change value pointed to by normal iterator, not const_iterator?</p>
  
  <p>In my code iterator was return by set::find(). Maybe is better way to pick specific element from set and change him.</p>