Problem with implementing abstract property in Matlab

Question

In Matlab (2017b) I'm trying to implement a superclass with a yet undefined abstract property.

classdef Class_Test1
    properties (Abstract=true)
        obj
    end
end

Subclasses should then implement this property and restrict it to a certain class, e.g.:

classdef Class_Test2 < Class_Test1
    properties
        obj@double = 123;
    end
end

a = Class_Test2; now throws an error:

The property 'obj' restriction defined in class 'Class_Test2' must match
the property definition in base class 'Class_Test1'.

Is there no way to specify the class / type of obj in the implementation of the abstract class or am I missing something?

In the end I would like obj to be abstract in Class_Test1 to implement several subclasses, which will hold objects of different classes in obj.

Any help appreciated ...

Cris Luengo · Accepted Answer · 2019-08-12 04:05:35Z

MATLAB classes are weird... or in any case very different from what you expect from a class if you've learnt about classes in any other programming language.

One aspect of MATLAB is that it's an interpreted language. Stuff is evaluated when executed, not when compiled. So code doesn't always need to make sense.

So, we can solve your issue like this:

classdef (Abstract) foo
   methods
      function c = getclass(in)
         c = class(in.obj);
      end
   end
end

The function foo.getclass seems to not make sense, since foo doesn't have a property obj. But because foo is abstract, we're never going to have an object of class foo on which we'll call the getclass method. Whenever getclass is called, it'll be a derived class. So, if we want to use this getclass method, we need to make sure that the derived class has a property obj:

classdef foo_uint8 < foo
    properties
        obj@uint8 = uint8(1);
    end
end

It is now possible to call foo_uint8.getclass:

>> a = foo_uint8;
>> a.getclass
ans =
    'uint8'

shoelz · Accepted Answer · 2019-08-09 09:20:40Z

0

OK, so after looking into this further it seems that this is a deliberate limitation based on the design of Matlab. I'm not too familiar with OOP in other languages but would be interested to learn if this is an general OOP fundamental or rather Matlab specific.

As a workaround I have come up with the following solution:

classdef (Abstract) Class_Test1 < handle
    properties
        Obj
    end
    properties (Abstract = true, Constant)
        ObjClass@char
    end
    methods 
        function set.Obj(obj_in,in)
            if isa(in, obj.ObjClass)
                obj_in.Obj = in;
            end
        end
    end
end

... and the implementation ...

classdef Class_Test2 < Class_Test1

    properties (Constant)
        ObjClass@char = 'double';  % Define the allowed class here ...
    end

end

I would still be interested to learn about other possible solutions.

answered Aug 9, 2019 at 9:20

shoelz

554 bronze badges

5 Comments

Cris Luengo Over a year ago

In C++ you can't do this either. You can have virtual methods, where derived classes change the behavior, but the types must all be identical. But MATLAB uses "duck typing" like Python, so you might not need to declare obj in the superclass at all. Just make sure each derived class has one obj property.

shoelz Over a year ago

I need to declare Obj in the superclass because I also declare methods, which operate on Obj without relying on its type. Thus, Obj is defined as abstract but without a type in the superclass.

Andrew Janke Over a year ago

If you do it this way, I would have the set.Obj throw an error when it gets an invalid input, instead of silently ignoring it (which could lead to hard-to-diagnose bugs): if ~isa(in, obj.ObjClass); error('invalid input type for Obj: %s', class(in); end. That would make the behavior more like the property type restriction, too.

Andrew Janke Over a year ago

Re: "I need to declare Obj in the superclass because I also declare methods, which operate on Obj without relying on its type." This isn't actually true in Matlab! In a superclass, you can refer to properties that are defined only in subclasses; as long as they're public, it'll still work. (As long as you don't need to instantiate and use the superclass directly, instead of using subclasses that define those properties.)

shoelz Over a year ago

... I have dropped extended error checking for the sake of brevity. As for using of properties in superclass methods, which are actually not defined in the superclass: It would never have occured to me that this would be possible ... very interesting.

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