Matlab: When implementing methods declared abstract in the super class, why must access be public?

Question

In Matlab (2017a), subclasses cannot restrict access to inherited methods that are declared abstract in the abstract superclass. Why wouldn't this be allowed? A small example:

super.m

classdef (Abstract) super
    methods (Abstract)
        out = fun(obj,in)
    end
end

sub.m

classdef sub < super
    properties
        prop
    end
    methods (Access='private') %remove the access restriction to run without errors
        function out = fun(obj,in)
            out = obj.prop * in;
        end
    end
end

testInheritance.m

instance = sub;

Execution of testInheritance.m results in the following error message:

Error using sub Method 'fun' in class 'sub' uses different access permissions than its superclass 'super'.

Wolfie Wolfie · Accepted Answer · 2018-09-13T07:19:36

They don't have to be public, but they have to be accessible by the sub and super class, and as the error states they must be the same. So you've got 2 issues:

Your super class's method is public and your sub class's method is private
You cannot just set the super class's method to private, or the sub class has no visibility of it.

You want to set Access = protected, which means only the super class and sub classes have visibility of the function, therefore the have the same access permissions, can specify their own behaviour and the function is hidden from other objects.

Here are the Access options, with definitions from the documentation:

public — Unrestricted access
protected — Access from methods in class or subclasses
private — Access by class methods only (not from subclasses)

So your classes become:

classdef (Abstract) super
    methods (Abstract = true, Access = protected)
        out = fun(obj,in)
    end
end

classdef sub < super
    properties
        prop
    end
    methods (Access = protected) 
        function out = fun(obj,in)
            out = obj.prop * in;
        end
    end
end

Note that the syntax is Access = protected, not Access = 'protected' as you showed.

Matlab: When implementing methods declared abstract in the super class, why must access be public?

2 Answers