title: How can I make an unordered set of pairs of integers in C++?
body: <p>The following program does not compile an unordered set of pairs of integers, but it does for integers. Can <code>unordered_set</code> and its member functions be used on user-defined types, and how can I define it? </p>
  
  <pre><code>#include &lt;unordered_set&gt;
  ...
  
  class A{
  ...
  private: 
      std::unordered_set&lt; std::pair&lt;int, int&gt; &gt; u_edge_;
  };
  </code></pre>
  
  <p>Compiler error:</p>
  
  <blockquote>
    <p>error: no matching function for call to 'std::unordered_set >::unordered_set()'</p>
  </blockquote>