title: NSMutableArray with only a particular type of objects
body: <p>is it possible to specify that a NSMutableArray can only contain a certain type of objects.
  For example, if I want to store only this kind of objects :</p>
  
  <pre><code>@interface MyObject : NSObject {
      UInt8 value; 
  }
  </code></pre>
  
  <p>In order to be able to use the instance variable like this :</p>
  
  <pre><code>- (void)myMethod:(NSMutableArray *)myArray{
      for (id myObject in myArray){
          [self otherMethod:myObject.value];
      }
  }
  </code></pre>
  
  <p>because I'm getting this error :</p>
  
  <pre><code>request for member 'value' in something not a structure or union
  </code></pre>
  
  <p>Thank you for your help</p>