Class property empty after initialization

OS: Windows 10

Hi every one!

I'm implementing matlab classes and observed some strange behaviour I don't have an explanation for and cannot find an answer.

Here is short example of my code:

classdef A < handle % Some superclass a with an abstract property and an already initialized one
    properties(Abstract)
        prop1
    end
    
    properties(Constant)
        prop2 = someValue;
    end
    
    methods
        obj = function A(prop2)
            obj.prop2 = prop2;
        end
    end
end

classdef C % Class C provides an interface which has to be implemented by specific subclasses of class A
    methods
        obj = function C(prop1)
           obj.prop1 = prop1;
        end
    end
end

classdef B < A 
    properties
        prop1 = @C; % initialize prop1 with interface of class C
        prop3
    end
    
    methods
        obj = function B(prop2)
            obj@A(prop2); % pass prop2 arg to superconstructor of class A
            obj.createC(); % create an instance of class C
        end
    
        function createC(obj)
            obj.prop3 = obj.prop1(obj.prop2);
        end
    end
end

When I create an instance of class B, objectB.prop3 is empty, where it should contain an instance of class C.

I checked the creation of the class C object in the debugger. Function createC properly creates the object and it is stored in B.prop3.

So, only when creation of the class B object is done, prop3 is empty.

Thanks in advance,

cheers

[Sebastian Pietsch]

I found the cause of the problem.

obj.prop3 is a cell array and changing values in a cell array within a method call seems to not change the object variable after the call is done.

My workaround now is

function prop3 = createC(obj)
    prop3 = obj.prop1(obj.prop2);
end

and then set the object parameter in the construcor

obj.prop3 = obj.createC(obj);

.

Has anybody an idea why this is and how to make the changes apear on object level?

Answer 1

[Adarsh]

Hi @Sebastian Pietsch,

After running the code, I've realized that some adjustments are necessary. I've observed that the property ("prop3") is being updated correctly and is not empty, which is the desired behaviour. Since all the classes inherit from the "handle class," there's no need to update the object again, as any changes will automatically be reflected in the properties.

Here are a few changes that I have made to the code:

1. “prop2 = someValue;” => removed “someValue” as it is not defined
2. “obj.prop2 = prop2;” => removed this statement from “A” ’s constructor as prop2 is declared constant so it has to be assigned a value directly and cannot be modified or reassigned.
3. “obj = function C(prop1)” => This line has a syntax error, so modified it accordingly to => “function obj = C(prop1)”
4. “obj = function A(prop2)” => This line has a syntax error, so modified it accordingly to => “function obj = A(prop2)”

Here is a working example with the modified code:

Class A Implementation:

classdef A < handle

properties(Abstract)

prop1

end

properties

prop2 = 1;

end

methods

function obj = A(prop2)

obj.prop2 = prop2;

end

end

end

Class B Implementation:

classdef B < A

properties

prop1 = @C;

prop3

end

methods

function obj = B(prop2)

obj@A(prop2);

obj.createC();

end

function createC(obj)

obj.prop3 = obj.prop1(obj.prop2);

end

end

end

Class C Implementation:

classdef C

properties

prop1

end

methods

function obj = C(prop1)

obj.prop1 = prop1;

end

end

end

Here is the code which I have used for testing the working of handle class:

Obj = B(12);

Obj

For more information on handle classes you can refer to the following documentation link:

https://www.mathworks.com/help/releases/R2021a/matlab/handle-classes.html

I hope it helps.