title: In which cases there is no constructor at all, even a default constructor?
body: <p>In <a href="http://www.agner.org/optimize/" rel="noreferrer">this book</a> I am currently reading I ran across this:</p>
  
  <blockquote>
    <p>A class doesn't need a constructor. A default constructor is not needed if the object doesn't need initialization. </p>
  </blockquote>
  
  <p>Am I correct in inferring from the above that the compiler does not generate a default constructor for the class/structure in some cases? If yes, what are those cases? I will venture and say POD is probably one. Are there any other?</p>
  
  <p><strong>EDIT:</strong> I have changed the title as the original title gave the meaning that I asked when was a default constructor not defined instead of asking when does a class not have a constructor at all.</p>