title: When I can't use templates?
body: <p>Can someone explain to me when I can't use templates for generic functions or for generic classes?  </p>
  
  <p>I understand that templates are needed at compilation-time, hence, there exist cases where we can't use templates. But it's not clear to me , and I am unable to find examples of generic code that cannot be implemented with with templates (but can be implemented with polymorphism)</p>