title: set head to NULL ('NULL' : undeclared identifier)
body: <p>I defined a linked list in C++. I am trying to set a NULL value to the variable head (in the constructor of <code>Movie_LinkedList</code>), but I got:</p>
  
  <p><strong>movie.h(40): error C2065: 'NULL' : undeclared identifier</strong></p>
  
  <p>please note <strong><em>that I can't include any library except of <code>iostream</code></em></strong></p>
  
  <p>Any help appreciated!</p>