Bizarre value class behavior

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John
John on 31 Aug 2014
Edited: per isakson on 2 Sep 2014
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!

Accepted Answer

per isakson
per isakson on 31 Aug 2014
Edited: per isakson on 2 Sep 2014
"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
  1 Comment
John
John on 31 Aug 2014
That is right. Thank you per isakson and Image Analyst.

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More Answers (2)

Image Analyst
Image Analyst on 31 Aug 2014
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.
  5 Comments
John
John on 31 Aug 2014
Thank you per isakson

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John
John on 2 Sep 2014
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.

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