Set properties from Superclass in subclass

Hello, I have defined a superclass 'food.m' that has certain properties:

classdef food
    properties 
        
        vitamins
        protein
        carbs
    end
end

Now I want to define a subclass that inherits from the above, and also sets the vitamins property (in other words, I will want all instances of this subclass to have the property 'Vitamins' set to "C")

classdef fruit < food
    properties (Access = private)
        vitamins = "C"
    end
end

The following error is raised

%Cannot define property 'vitamins' in class 'fruit' because the property has already been defined in the
%superclass 'food'.

How do I fix this?

Thank you in advance for your help!

Answer 1

[Turlough Hughes]

Set the Access attribute in the superclass:

classdef food
    properties 
        protein
        carbs
    end
    
    properties (Access = private)
        vitamins
    end
end    

otherwise, the default is that vitamins is defined as a public property in the superclass.

You can get a list of default property attributes here.

[elevonm]

Wow that was easy, thank you very much!!

[elevonm]

However now a second problem arises. I actually want all my properties in my superclass to be private (for convenience in other subclasses).

classdef food
    properties (Access = private)
        vitamins 
        protein
        carbs
    end
end

Now, if I only define the vitamins in 'fruit' as such:

classdef fruit < food
    properties
        vitamins = "C"
    end
end

and define an instance of the class

apple = fruit

I only obtain the vitamins property. I want all other properties (protein, carbs) to also appear, automatically set to 0. How do I achieve that?

Thank you

[Turlough Hughes]

Ah, I think what you're looking for is to have the SetAccess=private in the superclass. This means you restrict options to set the properties, but you can still "Get" them and hence see them when the object is instantiated, (i.e, the GetAcess attribute stays public, which is the default).

classdef food
    properties (SetAccess = private)
        vitamins = "C"
        protein  = 0
        carbs    = 0
    end
end

[Turlough Hughes]

and in this way you don't need to define the properties in the subclass.

[elevonm]

ok, but what if I have another subclass 'bread' in which I want 'vitamins' to be 0, but carbs and protein to be defined seperately in the subclass 'bread'? Without interfering with my 'fruit' class...

[Turlough Hughes]

In that case the SetAccess is protected. This means the subclass has access to change the property values while default values get defined in the superclass. In the following example, the user cannot change the properties (they can only view them), and a subclass can redine property values as required (typically in the subclass's constructor). See the following:

food (superclass):

classdef food
    properties (SetAccess = protected)
        vitamins = "C"
        protein = 0
        carbs   = 0
    end
end

bread (subclass):

classdef bread < food
    
    methods 
        function obj = bread
            obj.carbs       = "lots";
            obj.vitamins    = 0;
        end
    end
end

[elevonm]

Thank you very much for your help!

[Turlough Hughes]

Happy to help :)