title: Is unique_ptr guaranteed to store nullptr after move?
body: <p>Is <code>unique_ptr</code> guaranteed to store <code>nullptr</code> after move?</p>
  
  <pre><code>std::unique_ptr&lt;int&gt; p1{new int{23}};
  std::unique_ptr&lt;int&gt; p2{std::move(p1)};
  assert(!p1); // is this always true?
  </code></pre>