How to create a subclass to built-in class?

I fail to create a subclass to the built-in class ss (state space system, part of control systems toolbox). The constructor for the class, which involves a call to the superclass constructor, returns an instance. This instance has the method 'new_method' in that the methods() function lists it, but when I call the method on the instance, it is not found by the interpreter Here is a very short version of a code that generates the described problem:

classdef my_subclass < ss
    properties 
        my_prop
    end
    methods
        function obj= my_subclass(the_value)
            % Constructor.
            obj= obj@ss(); % call to superclass constructor not required, 
                           % but makes code clearer
            obj.my_prop= the_value;
        end     
        function obj= new_method(obj, the_value)
            % Simple example for a method that exists according to the
            % methods() function, but cannot be called.
            obj.my_prop= the_value;
        end
    end
end

Instantiation works fine:

>> instance= my_subclass(3)

instance =

    Empty state-space model.

>> instance.my_prop

ans =

       3

>> 

The new method exists:
>> methods(instance)

Methods for class my_subclass:
... new_method % The result of methods is too long to show it here, but it does contain 'new_method'.

Calling 'new_method' results in an error:

>> instance.new_method(4)
Error using InputOutputModel/subsref (line 44)
No property of the class "my_subclass" matches the string "new_method". Use PROPERTIES to get the list of properties for this class.

>>

[Martin Moennigmann]

Note that the following matlab documentation pages are related, but do not provide an answer to my question (at least not to me): Subclasses of Built-In Types with Properties Subclasses of MATLAB Built-In Types

Answer 1

[Philip Borghesani]

You did it correctly in general but the ss class should probably be marked as Sealed, I recommend not subclassing it. Only properties of an ss class and its children can be accessed with dot indexing, to call a method use a direct method call:

% instance.new_method(4) % will fail
new_method(instance,4) % correctly calls new_method