Question

I want to create a subclass, ess say, to the built-in ss class. I’d like to be able to convert an existing ss object to an ess object and at the same time add the missing properties, e.g. w, by something like this

sys=ss(a,b,c,d);
esys=ess(sys,w);

but I can’t figure out how to setup the constructor correctly. What is the best way to achieve this? My code currently looks like this

classdef ess < ss
    properties
        w
    end
    methods
        function obj = ess(varargin)
            if nargin>0 && isa(varargin{1},'StateSpaceModel')
                super_args{1} = sys;
            else
                super_args = varargin;
            end
            obj = obj@ss(super_args{:});
        end
    end 
end

But this does not work as I get the following error:

 >> ess(ss(a,b,c,d))
 ??? When constructing an instance of class 'ess', the constructor must preserve
 the class of the returned object.

Of course I could copy all the object properties by hand but it seems to me that there should be some better way.

Answer 1

Here is an example of what I had in mind:

classdef ss < handle
    properties
        a
        b
    end

    methods
        function obj = ss(varargin)
            args = {0 0};     %# default values
            if nargin > 0, args = varargin; end
            obj.a = args{1};
            obj.b = args{2};
        end
    end
end

and:

classdef ess < ss
    properties
        c
    end

    methods
        function obj = ess(c, varargin)
            args = {};
            if nargin>1 && isa(varargin{1}, 'ss')
                args = getProps(varargin{1});
            end
            obj@ss(args{:});    %# call base-class constructor
            obj.c = c;
        end     
    end
end

%# private function that extracts object properties
function props = getProps(ssObj)
    props{1} = ssObj.a;
    props{2} = ssObj.b;
end

Lets test those classes:

x = ss(1,2);
xx = ess(3,x)

I get:

xx = 
  ess handle

  Properties:
    c: 3
    a: 1
    b: 2
  Methods, Events, Superclasses