Why does the "isempty" function return "false" when used on a deleted handle object?

I have created an app using App Designer, in which I reference an external class as a property of the app. This class is derived from MATLAB's "handle" class.

Later in my app, I use the "delete" method on a property to remove the assigned class object. I perform this action because I need to execute code that verifies if this specific app property is empty.

However, when I run the app, this verification fails because the "isempty" method, when applied to the deleted property, returns a logical 0 (false). This indicates that the app property is not considered empty even after using the "delete" method on it.

Why does this occur, and how can I resolve this issue?

Answer 1

[MathWorks Support Team]

In MATLAB, when the "delete" method is applied to a handle object, it does not automatically clear the handle itself. Instead, it typically performs cleanup tasks specific to the object, such as closing files or freeing resources, and then marks the object as invalid. However, the "handle" variable still exists in the workspace and is not automatically removed.

Consider the following example:

classdef ExampleClass < handle
    properties
        Data
    end
    
    methods
        function obj = ExampleClass(data)
            if nargin > 0
                obj.Data = data;
            end
        end
    end
end

Now, in the MATLAB Command Window:

>> obj = ExampleClass(10);    % define an object for the "ExampleClass" class
>> delete(obj);
>> obj

obj =

   handle to deleted ExampleClass

>> isempty(obj)               % returns logical 0 as "obj" still contains the handle to deleted ExampleClass

From the example above, it can be observed that the "isempty" method, when used on a deleted handle object, returns "false".

As a workaround, consider using the "isvalid" function. This function is used to determine if handles are valid.

From the above example:

>> isvalid(obj) % returns logical 0 (false) as obj is not a valid handle after delete operation

Please refer to the "isvalid" documentation for more information.