Default Class Property Value That Contains Class

In MATLAB 2019b, property validation was added to classes. I'm finding this is a good way to both easily validate property values as well as keep it clear in the code about what type each property is. I do run into problems when I have a property in a class that will contain an object of another class, and I want the property to contain an object or be empty. As a toy example, imagine two classes:

classdef Dog
    
    properties
        Name (1,1) string
        FavouriteToy (1,1) DoggieToy = DoggieToy.empty
    end
    
    methods
        function obj = Dog(Name, FavouriteToy)
            arguments
                Name (1,1) string
                FavouriteToy (1,1) DoggieToy = DoggieToy.empty
            end
    
            obj.Name = Name;
            obj.FavouriteToy = FavouriteToy;
        end
    end
end

classdef DoggieToy
    
    properties
        Name (1,1) string
        IsChewy (1,1) logical
    end
    
    methods
        function obj = DoggieToy(Name, IsChewy)
            obj.Name = Name;
            obj.IsChewy = IsChewy;
        end
    end
end

Within "Dog", I want the property "FavouriteToy" to be optional, e.g. the user can leave it empty if they so choose.

Leaving the classes as is triggers an error in Matlab that the "FavouriteToy" default value doesn't match the size constraint of (1,1). Is there any way to specify that a property (or argument to a function) can be either empty or of some size (in this case (1,1) )?

My current workaround is to adjust "Dog" to:

classdef Dog
    
    properties
        Name
        FavouriteToy DoggieToy {Dog.mustBeScalarOrEmpty(FavouriteToy)} = DoggieToy.empty
    end
    
    methods
        function obj = Dog(Name, FavouriteToy)
            arguments
                Name (1,1) string
                FavouriteToy DoggieToy {Dog.mustBeScalarOrEmpty(FavouriteToy)} = DoggieToy.empty
            end
            
            obj.Name = Name;
            obj.FavouriteToy = FavouriteToy;
        end
    end
    
    methods (Static = true)
        function mustBeScalarOrEmpty(var)
            if ~(isscalar(var) || isempty(var))
                error('Not scalar nor empty.');
            end
        end
    end
end
    

This achieves my desired behaviour, but I'm thinking there must be a cleaner way. If not...feature request for 2020a? I feel that this is a pretty common use case for both class property and function argument validation.

Answer 1

[Steven Lord]

If a Dog has a FavouriteToy, must that FavouriteToy have a name? If so, could you represent the case where a Dog has no FavouriteToy by setting that DoggieToy's Name property to empty?

noToy = DoggieToy("", false);

That way the Dog's FavouriteToy property will always be scalar. Checking if a dog has a favorite toy would then require checking if the toy's Name has strlength greater than 0.

[David DeVries]

Thanks Steven!

An excellent suggestion, though it feels like a work around to me. It also seems that we would basically be embedding an "is this object empty" flag within objects, whereas using Matlab's provided empty object functionality would be preferrable and lead to less possible future confusion.