title: Objective C: Check if int exists, including zero
body: <p>I'm dealing with the addressbook, and I want to check if their is an ID set for the contact being created. However, the address book ID is an integer, which I'm worried might be zero based. So I want to do something like...</p>
  
  <pre><code>if (_contact.id) { ... }
  </code></pre>
  
  <p>But with this I run the risk of it assuming the contact with the id 0 doesn't exist, even though it does. This is a difficult problem to test since it would involve setting up a new address book.</p>
  
  <p>So my question is, is there a way to check if the contact is actually NULL/Nil without it assuming that 0 means it's null? Like an "is integer" function or something to the nature?</p>