MATLAB: Does the property of the class object not getting changed via a method

changeclassfunctiongettingMATLABmethodmodifiednotoopproperty

I have a class, with properties and methods. I am using a method in the class to change a particular property, but the property is not getting changed. Why is this?

See the example here :

classdef tempClass 
      properties
          Value = 1;
      end
      methods
          function changeValue(obj)
              obj.Value = 2;
          end
      end
end

Now, I execute :

>> a = tempClass;
>> a.changeValue;
>> a.Value
ans =
 1

Why is this happening?

Best Answer

The way this class is defined, it is a value class. For value classes, functions must return the modified value.

If you do not want to return the modified object in the function, then you can make this class a handle class as follows :

classdef tempClass < handle 
   %Now the class inherits 'handle', so it is a handle class
   properties
       Value = 1;
   end
     methods
         function changeValue(obj)
                   obj.Value = 2;
         end
     end
  end

Now the class and the function work are expected.

Related Solutions

MATLAB: How to modify private properties from inside a class

The default MATLAB class is a 'value' class. If you call a method that changes a property of a 'value' class object, the operation will return a new object with the modification. However the original object will remain unchanged unless reassigned. To avoid this, inherit from the 'handle' class instead. For more information about the difference between 'value' and handle classes, see the following documentation link:

https://www.mathworks.com/help/matlab/matlab_oop/comparing-handle-and-value-classes.html

The following code will give an example of using a public method to change a private property:

classdef CTester < handle
properties (Access = private) 
expectedResult = []; 
end 
methods 
    function setExpectedResult(obj,In) 
        obj.expectedResult = In; 
    end 
    function y = getExpectedResult(obj) 
        y = obj.expectedResult; 
    end 
    end 
end

Now try running to check that it works:

>> tester = CTester;
>> tester.setExpectedResult(2.87);
>> tester.getExpectedResult

MATLAB: How to change the class definition without changing the behavior of old objects (e.g. those saved in a MAT-file)

There are a number of ways that you can achieve this behavior of maintaining a single class with different behavior for different instances.

One simple approach is to use the strategy pattern:

https://en.wikipedia.org/wiki/Strategy_pattern

With this design, you can have multiple class definitions with different behaviors but the same interface, which you can define with an interface class or an interface superclass (which helps avoid code duplication).

https://www.mathworks.com/help/matlab/matlab_oop/defining-interfaces.html

e.g.

% MyClass.m
classdef MyClass < handle
    properties (Access = private)
        instance
    end
    methods
        function obj = MyClass(instance)
            if nargin < 1
                instance = MyClassV1();
            end
            obj.instance = instance;
        end
        function v = getValue(obj)
            v = obj.instance.getValue();
        end
        function v = add(obj,a,b)
            v = obj.instance.add(a,b);
        end
    end
end
------------------------------------------------
% MyClassV1.m
classdef MyClassV1 < handle
    properties
        value (1,1) double
    end
    methods
        function obj = MyClassV1()
            obj.value = 0;
        end
        function v = getValue(obj)
            v = obj.value;
        end
        function v = add(obj,a,b)
            v = a + b;
            obj.value = v;
        end
    end
end
------------------------------------------------
% MyClassV2.m
classdef MyClassV2 < handle
    properties
        value (1,1) double
    end
    methods
        function obj = MyClassV2()
            obj.value = 100;
        end
        function v = getValue(obj)
            v = obj.value;
        end
        function v = add(obj,a,b)
            v = a - b;
            obj.value = v;
        end
    end
end
------------------------------------------------
>> obj = MyClass;
>> obj.getValue()
ans =
       0
>> obj.add(1,2)
ans =
       3
>> save tmp obj
>> % change line 9 of MyClass.m to "instance = MyClassV2();"
>> clear obj
>> load tmp
>> obj.getValue()
ans =
       3
>> obj.add(1,2)
ans =
       3
>> newObj = MyClass;
>> newObj.getValue()
ans =
     100
>> newObj.add(1,2)
ans =
      -1

Best Answer

Related Solutions

MATLAB: How to modify private properties from inside a class

MATLAB: How to change the class definition without changing the behavior of old objects (e.g. those saved in a MAT-file)

Related Question