title: C++ priority_queue size() issue
body: <p>I tried to get the size of an empty priority_queue. Something strange happened. Could anybody explain why this happened? Thanks a lot.</p>
  
  <pre><code>#include &lt;iostream&gt;
  #include &lt;queue&gt;
  using namespace std;
  
  int main()
  {
      priority_queue&lt;int, vector&lt;int&gt;, less&lt;int&gt; &gt; asc_queue;
      cout &lt;&lt; asc_queue.size() &lt;&lt; " " &lt;&lt; asc_queue.size() - 1 &lt;&lt; endl;
  }
  </code></pre>
  
  <p>Output:</p>
  
  <pre><code>0 18446744073709551615
  </code></pre>