Applying method to one object in array affects values of another

I have an array of objects, but when I call a method of one object in the array, the values of other elements of the array change as well.

The code in question is as follows. A 2x2 array of the MixerArray class is initialised. When I call the method add on a(1,1), the values of a(1,2) and a(2,1) change as well. The following code gives an output of 0, 7 even though the add method should only change values of a(1,1)

a(2,2) = MixerArray();
disp(a(1,2).Register_Array.currentVal)
a(1,1).add(3,4);
disp(a(1,2).Register_Array.currentVal)

The class in question is as follows

classdef MixerArray < handle
    properties
        Register_Array          % Array to store values
        Length                  % Number of mixers
    end
    
    methods
        function obj = MixerArray(n)
            if nargin ==0
                obj.Register_Array = RegisterArray();
                obj.Length = 1;
            else
                obj.Register_Array = RegisterArray(n);
                obj.Length = n;
            end
        end
        
        % add function
        function add(obj, A, B, i)
            if nargin == 3
                obj.Register_Array.setval(A+B);
            else
                obj.Register_Array.setval(A+B,i);
            end
        end
        
        % Update function
        function update(obj)
            obj.Register_Array.update();
        end
    end
end

This calls upon a second class

classdef RegisterArray < handle
    properties
        lastVal                     % Last value
        currentVal                  % Current value
    end
    
    methods
        % n is the length of the register
        function obj = RegisterArray(n)
            if nargin == 0
                obj.currentVal = 0;
                obj.lastVal = 0;
            else
                obj.currentVal = zeros(1,n);
                obj.lastVal = zeros(1,n);
            end
        end
        
        % Assignment function
        function setval(obj, new_value, i)
            if nargin == 2
                obj.currentVal = new_value;
            else
                obj.currentVal(i) = new_value;
            end
        end
        
        % Update values
        function update(obj)
            obj.lastVal = obj.currentVal;
        end
    end
end

Answer 1

[Matt J]

It is a curious pitfall you've stumbled on. Basically, the solution is to avoid using default empty elements to fill up a MixerArray. Instead, create each element of the array explicitly within the constructor.

a=MixerArray(2,2);

where you would modify the constructor like the following

        function obj = MixerArray(varargin)
            
           if ~nargin, return; end
           
           obj(varargin{:})=MixerArray();
           for i=1:numel(obj)
              obj(i).Register_Array=RegisterArray(); 
           end
           
        end
        

[Matt J]

The reason this is necessary is because your original code

a(2,2) = MixerArray();

forces Matlab to fill a(1,1), a(1,2) and a(2,1) with default empty objects. However, it doesn't create these objects by calling the MixerArray constructor 3 separate times. It calls it only once and copies the resulting object into the 3 missing spots. But because the property Register_Array is a handle object, this copying uses handle copying semantics and therefore for i=1,2,3, every a(i).Register_Array is in fact the same instance.

[Steven Lord]

Matt J is correct and this is documented on this documentation page.

[An Aloysius Wang]

Thanks, this indeed solves the problem!