MATLAB: Are properties of unique objects referring to the same object

handle objectMATLAB

I am confused on how to use handle objects as properties in matlab. For example, I defined the following classes:

classdef Page < handle
properties
    word1;
    word2;
end
classdef Book < handle
properties
    page1 = Page;
    page2 = Page;
end

Now I instantiate two books:

iliad = Book;
odyssey = Book;

If I check if iliad and odyssey are the same:

eq(iliad, odyssey)

I get:

ans = logical 0

So far so good. But if I check if page1 of iliad and odyssey are the same:

eq(iliad.page1, odyssey.page1)

I get:

ans = logical 1

This is not good! It means that if I change page1 of odyssey, page1 of iliad will also change. What am I misunderstanding? How do I deal with this issue?

Best Answer

This is expected behavior. See the "Initialize Properties to Unique Values" section on this documentation page. Initialize the page1 and page2 properties of the Book object in the constructor if you want them not to refer to the same handle.

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MATLAB: When I have nested objects, changing the parameter of one object changes the same parameter in others

This is an intended behavior for handle classes, and is documented here:

https://www.mathworks.com/help/matlab/matlab_oop/specifying-properties.html#br18hyr

When a constructor for a handle class is placed in the 'properties' section of a class definition, MATLAB creates one generic object of that class and makes the property a handle to that object. If multiple constructors for the same class appear in the 'properties' section, or if multiple objects are created that have the same constructor in their 'properties', a handle to that same generic object are assigned to those properties. In other words, MATLAB does not create a new object every time the constructor is called in the properties section, instead it creates one object and links every constructor call to it.

This behavior can be eliminated by moving the constructor call from the properties section to the constructor of the object. For example, Class1 in the questions can be changed to:

classdef Class1 < handle
    properties
        a;
    end
    methods
        function obj = Class1()
            obj.a = Class2();
        end
    end
end

MATLAB: Cross-Talk between handle Class Objects

This depends entirely on what your setValue and getValue functions do, but taking a flying guess...

If you declare a property that is a handle object and you actually create this within the property block, both objects will have the same object because property block initialisations are evaluated once per class not per object.

So if the thing that is having its value set is some handle class object stored in mySuperClass that you initialised in the property block then this behaviour would occur as they will share a reference to the same actual object.

Best Answer

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MATLAB: When I have nested objects, changing the parameter of one object changes the same parameter in others

MATLAB: Cross-Talk between handle Class Objects

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