Having defined an external structure class in this way, we'd have an uncompiled structure with dynamic field access. Uncompiled structures dynamically access their fields by overriding the #doesNotUnderstand: message that the VM sends to an object when it determines that the object does not implement a method for the message being sent. For example, if we send one of our POINTLs the message #x then the VM will create a Message containing the selector and arguments (in this case there are none), and pass that Message to the #doesNotUnderstand: method of ExternalStructure, which will attempt to access the field named by the selector. In this case it will be successful (the template contains an entry for #x), but otherwise the usual run-time error walkback will result.