Is there more than one way to make a class abstract?

In the documentation for abstract classes, the only way I can find to create an abstract class is to define at least one abstract method. Any subclass must then implement the abstract methods. Yet I can create the following class:

classdef nextToNothing < hgsetget

    properties
        x
    end
end

and it works just fine (I can demonstrate almost all the documented behavior of SET, for example, with just this class). Clearly, I don't have to implement any abstract method. Is there another way to make a class abstract?

EDIT: I tried just adding an empty methods block:

methods(Abstract)
end

but that didn't have any effect.

EDIT: You can also define an abstract property, but then the property has to be redefined in subclasses.

Answer 1

[Anshuman]

If you want to make a class abstract without defining abstract methods or properties, you can use an abstract methods block, but it should not be empty. Instead, you can declare an abstract method without providing its implementation:

classdef nextToNothing < hgsetget
    methods(Abstract)
        someMethod(obj)
    end
end

Here, 'nextToNothing' is abstract because it declares an abstract method someMethod, which must be implemented by any subclass. Simply adding an empty 'methods(Abstract)' block does not make the class abstract. You need to declare at least one abstract method or property.

Answer 2

[Steven Lord]

As stated on this documentation page, there are several ways to make a class abstract.

"A class is abstract when it declares:

• The Abstract class attribute
• An abstract method
• An abstract property

If a subclass of an abstract class does not define concrete implementations for all inherited abstract methods or properties, it is also abstract."

In the case of hgsetget use the metaclass operator ? to confirm it is abstract and matlab.metadata.abstractDetails to determine which abstract methods or properties it defines (if any.)

metadata = ?hgsetget;
metadata.Abstract % true means it is abstract

ans = logical
   1

matlab.metadata.abstractDetails('hgsetget')

No abstract methods or properties for class hgsetget or no class hgsetget.

The fact that this says hgsetget has no abstract methods or properties indicates to me that its implementation makes it abstract by setting the Abstract class attribute. Since the original question's subclass of hgsetget has no abstract methods or properties (it didn't inherit any from hgsetget) and doesn't set the Abstract class attribute, it's not abstract.