Class Property assignment and read

if true
  % code
endI am trying to use classes in MATLAB and I’m tripping over a property assignment in a class. I am trying to assign a value to a property in one function and read this property back in another function.

When I go to read the value back the value I assigned it no longer is applied to the property. I am not sure what I am doing wrong? Test class

    classdef Class_Test

    %-------------------------------
    % Properties
    %-------------------------------
    properties (Access = private)
        X
    end

    %-------------------------------
    %   Methods
    %-------------------------------
    methods 
        function set_X(this)
            this.X = 1;
            this.X 
        end

        function read_X(this)
            this.X 
        end
    end
end

Output of Object:

>> TC = Class_Test;

>> TC.set_X
ans =

       1

>> TC.read_X

ans =

       []

What I expected was:

>> TC.read_X

ans =

1

Answer 1

[Geoff Hayes]

Matt - I think the problem is how you have defined your class. From Comparing Handle and Value Classes

MATLAB associates objects of value classes with the variables to which you assign them. When you copy a value object, MATLAB also copies the data contained by the object. The new object is independent of changes to the original object.

Objects of handle classes use a handle to reference objects of the class. A handle is a variable that identifies an instance of a class. When you copy a handle object, MATLAB copies the handle, but not the data stored in the object properties. The copy refers to the same data as the original handle. If you change a property value on the original object, the copied object reflects the same change.

So there are two types of classes: value and handle. By default, your class is of type value which is fine, but it requires more code to get the behaviour that you expect. Try deriving your class from handle as

 classdef Class_Test < handle

and then re-run your example and it should work fine. If you want to get your class to work as a value class, then you would need to do something like the following to your set method

 function [this] = set_X(this)
    this.X = 1;
    this.X 
 end

and then re-assign the output from this assignment as

 TC = TC.set_X;

Much easier/cleaner to derive your class from handle.

[Geoff Hayes]

Matt's answer moved here

thanks that was it.