title: How to judge whether a class or an object is singleton or not?
body: <p>I wrote a class method like this:</p>
  
  <pre><code>Class class;
  
  BOOL isSingleton = [class isSingleton];
  </code></pre>
  
  <p>But I have no idea how to implement the class method <code>+ isSingleton</code>
  How to know whether a <em>class or a object is a singleton or not</em>?</p>