Bizarre value class behavior

Question

John 2014-8-31

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编辑： per isakson 2014-9-2

Below is a self-explanatory VALUE class I developed for purposes of illustration.

      classdef ClassA
          properties
              x;
              y;
          end
          methods
             function obj = ClassA()
                obj.x = 0; 
                obj.y = 0;
             end
             function setX(obj, val)
                obj.x = val; 
             end       
             function setY(obj, val)
                obj.y = val; 
             end       
             function obj = set.x(obj, val)
                *obj* .x = val; 
             end
             function obj = set.y(obj, val)
                obj.y = val; 
             end
          end
      end

I instantiated it as follows:

a = ClassA();

Good. That worked. Now to test the setter methods:

a.x = 1;
a.y = 2;

Those worked too. But doing this:

a.setX(3)
a.setY(5)

causes the prompt to not echo the value of the properties as is done when the setter methods are used. Why? And when I enter the variable name in the prompt, the property values are echoed back but they do not change upon using setX and setY. This is puzzling. I followed proper VALUE class syntax for setX and setY. Help please!

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Answer 1

per isakson 2014-8-31

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编辑：per isakson 2014-9-2

在 MATLAB Online 中打开

"I followed proper VALUE class syntax for setX and setY." &nbsp No, that's handle class syntax. With a value class you create a new instance, which you need to return.

With the class definition modified

    >> a = ClassA
    a = 
      ClassA with properties:
        x: 0
        y: 0
    >> a = a.setX( 17)
    a = 
      ClassA with properties:
        x: 17
        y: 0

or

    >> a = setY( a,117)
    a = 
      ClassA with properties:
        x: 17
        y: 117

where

    classdef ClassA
        properties
            x;
            y;
        end
        methods
            function obj = ClassA()
                obj.x = 0;
                obj.y = 0;
            end
            function obj = setX(obj, val)
                obj.x = val;
            end
            function obj = setY(obj, val)
                obj.y = val;
            end
            function obj = set.x(obj, val)
                obj.x = val;
            end
            function obj = set.y(obj, val)
                obj.y = val;
            end
        end
    end

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John 2014-8-31

That is right. Thank you per isakson and Image Analyst.

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Answer 2

Image Analyst 2014-8-31

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They're methods. Not all methods and functions echo stuff to the command window. Some do, for example jet, colormap, etc. Some do not, for example "hold on", "axis off", etc. Note that, while your functions setX() and setY() do something, they don't actually have any return arguments defined . So they won't even put anything into "ans" or return anything whatsoever. Hence nothing will be echoed to the command window, like if they returned an output argument.

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John 2014-8-31

编辑：John 2014-8-31

在 MATLAB Online 中打开

The functions set.x and set.y are just setter methods and that is the official MATLAB syntax for setters, a new MATLAB OOP feature inspired by C# and Java.

Anyway,

I changed setX and setY to:

             function setX(obj, val)
                obj.x = val; 
                disp(obj.x)
             end       
             function setY(obj, val)
                obj.y = val; 
                disp(obj.y)
             end

to display the values of the current object's x and y properties before leaving the function and it does echo the updated value (no surprise there). But when I enter the object name on the command prompt, a , the values of x and y are not updated, which is what puzzles me. It is as if the variable name a that refers to the instance of ClassA I am working with always points to a copy of the constructed variable and is immutable.

I tried restarting MATLAB -something which has cleared up quirks related to changing the source for the class- and repeated my tests but I get the same confusing behavior.

Please help :-)

per isakson 2014-8-31

Read my answer

John 2014-8-31

编辑：John 2014-8-31

Thank you per isakson

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Answer 3

John 2014-9-2

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在 MATLAB Online 中打开

So does this mean that for a value class, if I want to call a function on an instance of that value class which modifies the state of the instance and also returns data from the instance, I would have 2 return values: 1) the modified instance and 2) the data value returned

Example: A value class that implements a bag of items.

%instantiate a bag class and add the number 1 into it
bag = Bag();
bag.add(1);
%Now remove the item from the bag
[bag, returnedItem] = bag.remove();

Notice that the state of 'bag' is modified after the remove action and has to be reassigned to the variable 'bag'. If the remove action were called like this:

returnedItem = bag.remove();

, the variable bag would still refer to the unmodified bag value with item '1' still in it even though '1' is returned.

Am I right?

I think that's a bit cumbersome but manageable.