how to add method to extend built-in class

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Erik Johnson on 7 May 2013
Commented: Jan Siegmund on 23 Apr 2020
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?
Jan Siegmund on 23 Apr 2020

Rudy on 29 Jul 2014
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.