Objects won't keep member variables

Hello. I have a container class that holds an array 'arr' to store instances of the item class. The item class has to member variables called variable1 and variable2.

classdef container
  properties
      arr;
  end

    methods
        function this = container(n)
            this.arr = item.empty(n,0);
            for i = 1:n
                this.arr(i) = item(4);  %set value1 to 4
            end
        end   
    end 
end
%------------------------------------------------------
classdef item

    properties
        value1;
        value2;
    end

    methods
        function this = item(v)
            this.value1 = v;
        end

        function setValue2(this, v)
            this.value2 = v;
        end 
    end 
end
%------------------------------------------------------
clear all;
c = container(2);%container able to hold 2 item objects
c.arr(1).setValue2(9);    %set value2 of first item to 9
c.arr(1).value1        %display value1 and value2 of first item
c.arr(1).value2

Wenn I run the last part I get this result:

ans = 4

ans = []

So value1 is set correctly in the Constructor, whereas value2 is set via a seperate function called setValue2. When I debug I can see that setValue2(9) sets value2 to 9 but when I leave the function value2 becomes an empty double again.

What am I doing wrong? Why doesnt Matlab keep the value? Do I use item.empty() the wrongt way? I didn't find any other option to declare an empty array of type 'item'.

Tanks you for every answer.

Answer 1

[Titus Edelhofer]

Hi,

the way you write the code is correct, if you have "handle-semantics". For value semantics you have to return the changed object (think of the methods as usual matlab functions), so:

function this = setValue2(this, v)
  this.value2 = v;
end

and call like

c.arr(1) = c.arr(1).setValue2(9);

If you don't like this, change your class to be a "handle" class by changing only the definition:

classdef item < handle

Then you c.arr(1) is not the object itself but a reference to the object.

Titus

[Tilman]

Thank you so much. Everything is working perfectly now.

[Joseph]

Thank you!! This was frustrating me for awhile.

[Jixiong Su]

Thanks a lot！