subset enumeration class of an enumeration class

I have defined an enumeration class:

classdef globalnumbers<uint8
    enumeration
        number0 (0)
        number1 (1)
        number2 (2)
        number3 (3)
    end
end

Can I create a subset of this globalnumber enumeation? for instance

classdef mynumbers<globalnumbers
    enumeration
       mynumberA (globalnumbers.number1)
       mynumberB (globalnumbers.number2)
    end    
end

If not, what would be the alternative?

[Matt J]

Sure, why not?

[Sylvain]

I have this error.

'mynumbers' cannot subclass 'globalnumbers' because 'globalnumbers' is an enumeration class.

I think it is not possible.

Answer 1

[Steven Lord]

You cannot inherit from an enumeration class as stated on the documentation page listing restrictions on enumerations. They are inherently Sealed. Why?

Suppose you had the class WeekDays (copied from a documentation page):

classdef WeekDays
   enumeration
      Monday, Tuesday, Wednesday, Thursday, Friday
   end
end

and you created a subclass Days.

classdef Days < WeekDays
    enumeration
        Saturday, Sunday
    end
end

Days would have enumeration values Monday, Tuesday, etc. as well. But Days would violate the Liskov substitution principle, as you can't necessarily substitute an instance of Days for an instance of WeekDays and have the program remain correct. Conceptually speaking, suppose I had a program that asked "Is the stock market open?" The stock market is open on days listed in WeekDays but not open on all the days listed in Days. So if that function were something like (not tested, just for illustration purposes)

function tf = isStockMarketOpen(queryDay)
listOfDaysItCouldBeOpen = WeekDays;
tf = ismember(queryDay, listOfDaysItCouldBeOpen);
end

the function could behave differently if I substituted Days for WeekDays. With WeekDays it would return false for Saturday, with Days it would return true.

I believe you can have your class use values from an enumeration class in its definition:

classdef mynumbers
    enumeration
       mynumberA (globalnumbers.number1)
       mynumberB (globalnumbers.number2)
    end    
end

But now mynumbers is-not-a globalnumbers. So there's no substitutability expectation and so Liskov doesn't apply.