title: Using custom comparator for std::set with custom constructor
body: <p>Let's say I have a class</p>
  
  <pre><code>class Custom
  {
  public:
      Custom (int x, int y) : x (x), y (x) {} 
      bool operator() (int a, int b)
      {
          return a &lt; x &amp;&amp; y &lt; b;
      }
  private:
      int x, y;
  };
  </code></pre>
  
  <p>And I want a set that is </p>
  
  <pre><code>std::set&lt;int, Custom&gt; s 
  </code></pre>
  
  <p>with Custom's custom constructor (i.e. Custom (1, 2))</p>
  
  <p>How can I achieve this?</p>
  
  <p>In addition, if I want to use this in an unordered_map, such as </p>
  
  <pre><code>std::unordered_map&lt;string, std::set&lt;int, Custom&gt;&gt; ht
  </code></pre>
  
  <p>how can I do this?</p>