title: Error when defining NSString global constants
body: <blockquote>
    <p><strong>Possible Duplicate:</strong><br>
    <a href="https://stackoverflow.com/questions/8460340/interface-type-cannot-be-statically-allocated">Interface type cannot be statically allocated?</a>  </p>
  </blockquote>
  
  
  
  <p>Here is my code just like others:</p>
  
  <p><strong>TZNotifications.h</strong><br>
  <code>extern NSString *const TZNotificationKeywordAuthentication;</code></p>
  
  <p><strong>TZNotifications.m</strong><br>
  <code>NSString *const TZNotificationKeywordAuthentication = @"NOTIFICATION_KEYWORD_AUTHENTICATION";</code></p>
  
  <p>But I get two errors with the code like this:</p>
  
  <p><img src="https://i.stack.imgur.com/ght2C.png" alt="enter image description here"></p>