How to handle the error "The 'A' class definition uses an instance of itself directly or indirectly as the default value for the 'b' property. This is not allowed."?

I got an error:

The 'A' class definition uses an instance of itself directly or indirectly as the default value for the 'b' property. This is not allowed.

Below is my code:

classdef A
    properties
        b = B; 
    end
end

classdef B
    properties
        a = A;
    end
end

What is wrong in the code? How should I resolve this error?

Answer 1

[MathWorks Support Team]

Cause of the error:

This error is due to recursive property definition. It occurs occurs when a MATLAB class sets a property’s default value to an instance of itself, either directly or indirectly (for example, through another class). This creates an infinite loop when MATLAB tries to initialize the property, so the error is thrown to prevent it. 

In the code example, creating an "A" object needs to create a "B" object as the default value of the property "b". However, when initializing "B"'s property 'a', it requires to create an "A" object as the default value. This leads to infinite recursion.

How to fix:

Use [] (empty array) as the default value for properties that reference other objects. If you need to assign an object to the property, do so explicitly in the class constructor instead of as a property default value.

For example, you can modify the class "A" and "B" as follows: 

classdef A
    properties
        b = []; % Empty by default
    end
    methods
        function obj = A(bObj)
            if nargin > 0
                obj.b = bObj;
            end
        end
    end
end

classdef B
    properties
        a = []; % Empty by default
    end
    methods
        function obj = B(aObj)
            if nargin > 0
                obj.a = aObj;
            end
        end
    end
end

Then, when you run 

b = B();      % Create B object
a = A(b);     % Create A object with b as property
b.a = a;      % Set B's a property to the A object

it should run without any error. 

General recommendation:

1. Review the class you are using as the default value, as well as its superclasses, to check if any of them reference the original class (directly or indirectly) as a property default. 
2. Use "clear classes" after editing a class file to clear the cached classes.