how to add method to extend built-in class

Prior to R2008a, it was possible to extend a class and a new method by just creating, somewhere in the path, the file @classname/methodname.m. Starting with R2008a, this still works ONLY if the class is defined using the older formulation; for classes defined using the newer classdef introduced in R2008a, how can I add a new method for a standard class (e.g., ss or sym) (without the complications introduced by subclassing)?

For example, if x and y are symbolic values, the minimum of the two can be written x*heaviside(y-x)+y*heaviside(x-y). I had a pre-R2008a function @sym/min.m that (in its simplest implementation) had:

function out = min(x,y)
out = heaviside(y-x).*x + heaviside(x-y).*y;
% a more complicated version handled matrix inputs,
% the dimension over which to minimize, and so forth

My @sym method directory could live anywhere on my path. Once the sym class was implemented with classdef, this no longer worked (consistent with the documentation at http://www.mathworks.com/help/matlab/matlab_oop/organizing-classes-in-folders.html). (Though, strangely, MATLAB's help command still sees methods in my @sym folder.)

The only solution I can find is to define, earlier on the path, a file min.m that uses an if isa(...) formulation to use my min code if one of the arguments is symbolic, and the built-in min otherwise -- something like:

function [varargout] = min(varargin)
varargout = cell(1,max(1,nargout));
if nargin==2 && (isa(varargin{1},'sym') || isa(varargin{2},'sym'))
    x = varargin{1};
    y = varargin{2};
    varargout{1} = heaviside(y-x).*x + heaviside(x-y).*y;
else
    [varargout{:}] = builtin('min',varargin{:});
end;

This seems like an overly-complicated solution, and it gives constant messages: Warning: Function /Users/username/matlab/min.m has the same name as a MATLAB builtin. We suggest you rename the function to avoid a potential name conflict.

An alternate idea was to move my @sym/min.m file into MATLAB's toolbox directory (which would be matlabroot/toolbox/symbolic/symbolic/@sym/min.m in this case); however, MATLAB seems to ignore this file, and this seems like a bad idea in any case.

Subclassing and creating my own symbolic class is possible, but more complicated than necessary for a simple extension like this, and would require recasting every symbolic variable to my subclass before calling the subclassed min method.

What is the alternate solution?

[Rudy]

Dear Erik,

I am facing the same problem right now. I have to update the code of a tool box I just inherited. Did you find a way to solve this problem?

[Jan Siegmund]

Strangely, matlab still recommends the "@sym" folder:

https://de.mathworks.com/matlabcentral/answers/103129-how-do-i-overload-built-in-matlab-functions

Answer 1

[Rudy]

Since I had no luck trying to find a solution, I had to find one on my own. This is the method I came up with.

The toolbox had three new method for the zpk class. I created a new class, called sdzpk, and declared it to be a subclass of the built in zpk class. Then, wherever any of the new methods were used, I first converted the object to the new class before passing it to the method.

The following code may ilustrate that better:

Class definition file:

    classdef sdzpk < zpk & sdlti

        methods (Access = public)

            function obj = sdzpk(varargin)

                % Constructor method. Long code here to perform data validation
                % and pass information to the zpk constructor

                obj = obj@zpk(args{:});
            end

            % Existing methods
            % This are the old files I inherited. No need to edit them.

           tsys = ctranspose(sys);
           sys = delay2z(sys);
           sysr = minreal(sys,tol);
           F = minreals(F,tol);
           FP = setpoles(F,p);
           F = symmetr(F,type,tol);
           F = z2zeta(F,tol);
        end       
    end

At several locations within the toolbox, the function minreals is called. All those calls were replaced with:

    minreals(sdzpk(obj))

In that way, I make sure the new class is used and the correct method is applied.

I hope this helps somebody.