title: runtime - What does this "@@:" mean in class_addMethod?
body: <p>Use <code>class_addMethod</code> Code:</p>
  
  <pre><code>class_addMethod(newClass, @selector(inputAccessoryView), accessoryImp, "@@:");
  </code></pre>
  
  <p>What dose the parameter "@@:" mean in this method?</p>
  
  <p>Doc:</p>
  
  <pre><code>/** 
   * Adds a new method to a class with a given name and implementation.
   * 
   * @param cls The class to which to add a method.
   * @param name A selector that specifies the name of the method being added.
   * @param imp A function which is the implementation of the new method. The function must take at least two arguments—self and _cmd.
   * @param types An array of characters that describe the types of the arguments to the method. 
   * 
   * @return YES if the method was added successfully, otherwise NO 
   *  (for example, the class already contains a method implementation with that name).
   *
   * @note class_addMethod will add an override of a superclass's implementation, 
   *  but will not replace an existing implementation in this class. 
   *  To change an existing implementation, use method_setImplementation.
   */
  OBJC_EXPORT BOOL class_addMethod(Class cls, SEL name, IMP imp, 
                               const char *types) 
  OBJC_AVAILABLE(10.5, 2.0, 9.0, 1.0);
  </code></pre>