Matlab Implicit default values are broken when using property validation functions for properties that store handle objects.

EDIT: I submitted a bug report. I will update this post as soon as I have new information.

Hi,

I have a class that has a property of type containers.Map. If I add property validation functions I can't assign default values in the constructor without having an initial value first (otherwise matlab assigns a default value of type double, which throws an error because it is the wrong type). The initial value however - when explicitly stated - triggers matlab to issue a warning because the map object would be shared across all instances of the class if I don't overwrite it.

Here is a small example:

classdef matlabBroke
    properties
        map (:, 1) {mustBeA(map, 'containers.Map')};
    end
    
    methods
        function obj = matlabBroke()
            obj.map = containers.Map();
        end
    end
end

This code doesn't run. It throws the error message:

Error using implicit default value of property 'map' of class 'matlabBroke'. Value must be one of the following types:

'containers.Map'.

It can be shrunken down even more:

classdef matlabBroke
    properties
        map {mustBeA(map, 'containers.Map')};
    end
end

or even:

classdef matlabBroke
    properties
        map containers.Map;
    end
end

It only runs when I init the property first:

classdef matlabKindaBroke
    properties
        map (:, 1) {mustBeA(map, 'containers.Map')} = containers.Map();
    end
    
    methods
        function obj = matlabKindaBroke()
            obj.map = containers.Map();
        end
    end
end

This however triggers a warning regarding the shared map object across all instances of this class.

A property default value that is a handle will cause all instances to share the same object data. [...]

There are two flaws:

• the error shouldn't trigger in this case
• the default values aren't immediately obvious although there's a clearly defined place where default values should be when using property validation (https://de.mathworks.com/help/matlab/matlab_oop/validate-property-values.html, first image).

Additionally, initializing objects in the constructor makes it way more difficult to spot the default value, especially in complex classes.

A solution I came up with (would have to be implemented by mathworks):

add the keyword 'new' to matlab. The first example

classdef matlabBroke

properties

map (:, 1) {mustBeA(map, 'containers.Map')};

end

methods

function obj = matlabBroke()

obj.map = containers.Map();

end

end

end

could look like this:

classdef matlabBroke

properties

map (:, 1) containers.Map = new containers.Map();

end

end

This should trigger the default value to be regenerated every time a class is instantiated.

This could omit the error and the default value is in the same place as the property definition. Additionaly, the word 'new' highlights that this handle is not shared across all objects of this class.

If anybody has an easier way to do this please let me know!

My solution right now is to ignore the error and add the default value in the constructor.

[Nikolaus Koopmann]

https://undocumentedmatlab.com/articles/handle-object-as-default-class-property-value

Answer 1

[Shivam]

Hi Jakob,

As per my understanding, you are getting a warning while creating a property with validators in the class and want to know if the instance can be created using the 'new' keyword.

According to the documentation, in MATLAB, the keyword 'new' is not used for creating objects of a class. You can create properties in the class with validators and default Values and use the constructor to initialize the property with an empty 'containers.Map'.

You can refer to the following documentations to know more about property syntax and initialization:

1. https://www.mathworks.com/help/releases/R2023a/matlab/matlab_oop/defining-properties.html
2. https://www.mathworks.com/help/matlab/matlab_oop/initialize-property-values.html

I hope it helps.