title: two classes contain objects of each other
body: <p>i hope there's a simple answer to this.
  without getting too complicated i have two classes. an "Animal" and a "Cell"
  Cell contains an Animal as a member.
  Animal has methods that accept Cells in their parameters.</p>
  
  <p>i'm just confused as to how i #include each class the others header?
  i've only ever had to go up a chain of classes and just included the last class header in the next and so on, but this circular situation has me a bit lost.</p>
  
  <p>i hope that makes sense, any help greatly appreciated.</p>