title: is it safe to assign a stl container?
body: <p>For example:</p>
  
  <pre><code>set&lt;int&gt; s1;
  set&lt;int&gt; s2;
  s1.insert(1);
  s2.insert(2);
  s1 = s2;
  </code></pre>
  
  <p>Is it safe? If so, where did the old element(and the memory they occurred) come to?</p>