title: set<class> insert problem
body: <p>why is the following programm not working?
  How can i fix it?</p>
  
  <pre><code>#include &lt;iostream&gt;
  #include &lt;set&gt;
  
  using namespace std;
  
  class K {
  
  private:
      long a;
  };
  
  int main ()
  {   
      K a;
      set&lt;K&gt; b;
  
      b.insert(a);
  
      return 0;
  }
  </code></pre>