MATLAB - create reference (handle?) to variable

Question

Suppose I have the following class:

classdef myClass < handle
    properties
        A = 1
    end
    methods
        function obj = myClass(val)
                   


        

Answer 1

Since you are working with a handle class, both q and w in your example refer to the same object in memory; they are themselves a "pointer"/"reference" to the object they represent.

So continuing your example, if you make changes to one, it will be reflected in the other.

>> q = myClass(10);
>> w = q;
>> q.A = 99;
>> disp(w.A)
    99

Also note that you are not creating another instance of the class when you call w = q;. Compare the following examples in terms of memory space:

>> q = myClass(rand(7000));
>> m = memory; disp(m.MemUsedMATLAB)
   792870912
>> w = q;
>> m = memory; disp(m.MemUsedMATLAB)
   792834048

Against:

>> q = myClass(rand(7000));
>> w = myClass(rand(7000));
??? Error using ==> rand
Out of memory. Type HELP MEMORY for your options.

---

EDIT

Playing around with this, I came up with the following hackish solution.

First we create a wrapper function around the class constructor. It creates an object as usual, plus it returns a function handle that acts as a read-only accessor to a closure variable synced with the original object property using a "PostSet" events listener.

The only change to the original class is to add the SetObservable property attribute:

myClass.m

classdef myClass < handle
    properties (SetObservable)
        A
    end
    methods
        function obj = myClass(val)
            obj.A = val;
        end
    end
end

myClassWrapper.m

function [w A] = myClassWrapper(varargin)
    w = myClass(varargin{:});
    A = @getWA;

    %# closure variable
    a = w.A;

    %# add listener to when w.A changes
    addlistener(w, 'A', 'PostSet',@changeCallback);

    function val = getWA()
        %# return the value of the closure variable
        val = a;
    end
    function changeCallback(obj,ev)
        %# update the closure variable
        a = ev.AffectedObject.A;
        %#fprintf('Value Changed to %g\n',a)
    end
end

Now we can use the wrapper as:

>> [w a] = myClassWrapper(10);
>> a()
ans =
    10
>> w.A = 99;
>> a()
ans =
    99