Overriding superclass methods and access modifiers in MATLAB

Question

Consider the following simple class hierarchy:

A.m

classdef A < handle
    methods (Access = protected)    %# protected vs. private
        function foo(obj)
            disp('class A')
        end
    end
end

B.m

classdef B < A
    methods (Access = public)
        function foo(obj)
            disp('class B')
        end
    end
end

Class B inherits from class A and is supposed to override the protected foo method as public.

If we try to instantiate the derived class, we get the following error:

>> b=B();
Error using B
Method 'foo' in class 'B' uses different access permissions than its superclass 'A'. 

The weird thing is if foo was defined as private method in the superclass A, the code works just fine when we invoke the overridden method:

>> clear classes
>> b=B(); b.foo()
class B

So is this a limitation/bug in MATLAB OOP implementation, or is there a good reason behind this behavior? (Code was tested on R2012b)

---

As a comparison, in Java the rules state that you cannot reduce visibility of a method in the sub-class, but you can increase it, where:

(weakest) private < package < protected < public (strongest)

Answer 1

This appears to be a limitation of Matlab. I've tried all combinations of attributes. Matlab throws errors whenever the attributes are different, except when the method of A is private, in which case the attributes in B don't matter.

In other words, unless the method in A is private, the attributes of the method in A and B have to be the same. I guess this does make sense to some extent, in that TMW say "If a method is visible to the subclass, attributes have to be the same; if a method is not visible to the subclass, the subclasses can do whatever they like".