class1 as property of class2, class1 property initialization?

Question

SoderlingPotro 2022-11-1

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https://ww2.mathworks.cn/matlabcentral/answers/1840668-class1-as-property-of-class2-class1-property-initialization

评论： J G 2024-1-25

I have a class:

classdef class1
    properties
        prop1 = 1;
    end
end 

and I can easily access prop1 with:

test = class1
test.prop1
ans = 1

If I then put class1 as a property into another class e.g.:

classdef mainClass
    properties
        class1
    end
end

I would like to access class1 propr 1 through mainClass. So I try the following but get an error:

>> test.class1(1).prop1
Index exceeds the number of array elements. Index must not exceed 0.

Why is prop1 not initialized to the value 1? Thanks

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Answer 1

Benjamin Kraus 2022-11-1

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In your definition of mainClass you created a property with the name class1, not the type class1.

This is what you want:

classdef mainClass
    properties
        class1 (1,1) class1
    end
end

Just be careful using this approach. If class1 is a handle class, then all instances of mainClass will share the same class. In that case, you are better off with this approach:

classdef mainClass
    properties
        class1
    end
    methods
        function obj = mainClass()
            obj.class1 = class1;
        end
    end
end

You may also want to reconsider having a property name that matches a class name. It works, but it may be confusing.

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J G 2024-1-24

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Hi @Benjamin Kraus,

I was wondering about this, and found your post. This may deserve submission as a new question, but I'll put here for now...

Can you explain the thinking behind this more? I'd like to use property validation to restrict the size and class of a particular property only when the property is set. If no default value is set in the classdef, it means I intend for that that property to be set by someone/something else, and that's where I'd expect validation to occur. It seems counterintuitive to assign a default value based on a validation, when the programmer never specifically coded a default value (since the option exist to do so!).

The current behavior makes my use-case somewhat difficult. Consider the following.

classdef MyClassA
    properties
        Bobj (1,1) MyClassB     %Note I do not define a default value!!!
    end
    
    methods
        
        function obj = MyClassA()
        end
        
        
        function output = doStuffWithB(obj,input)
            if isempty(obj.Bobj) % this won't work! Is there something similar that will?
                error('User must first define Bobj')
            else
                output = obj.Bobj.myBMethod(input); % In my case, a default Bobj won't work, because it needs its own props to be set before use
            end
        end
    end
    
end

I'd like to create an object of MyClassA, and leave it to the user to define the Bobj property, subject to the validation rules. In a later method, I implement a check to see if the Bobj has been set before proceeding with calculations. This check won't help, because Matlab will create a default object Bobj, even though I never explicity told it too!

Is there a way to cleanly achieve this? (Yes I know, I could do this with set methods and additional hidden properties, etc. but I was hoping for something simpler).

Benjamin Kraus 2024-1-25

I'm with @Steven Lord on this: If MyClassA is required to have a specific instance of MyClassB, it seems like the property should be Immutable and set by the constructor. You can remove the validation from the property itself and move the validation into the constructor (perhaps using Function Argument Validation).

By removing the validation from the property itself, you eliminate the need to define a default value, and MATLAB won't attempt to populate it with a scalar instance of your class.
By making the property Immutable the user won't be able to change the value after the constructor is called, so you don't need the validation on the property itself any more anyway.
By making the property Immutable it also follows your stated behavior, which is that MyClassA depends on a specific instance of MyClassB, so it can be set once and never changed.
Using Function Argument Validation in the MyClassA constructor allows you to basically reuse the same property validation you have now, but within an arguments block, to guarantee that the user provided value is always valid.
By requiring the property to be set in the constructor, you can still rely on the value of the property always being valid (with the possible exception of save/load, which might need to be handled separately), so you don't need to have isempty checks scattered throughout your code.

One benefit of being able to specify that a property is (1,1) is so that your own code (within the class) no longer has to check isempty when interacting with the property. Any code (inside or outside the class) that is interacting with the property can rely on the fact that the value of the property is always scalar. If you allow that to be empty, then it partially negates the value of having the validation in the first place.

An alternative to using (1,1) would be to use mustBeScalarOrEmpty, but that would allow the user to set the property value back to empty once it was set to a scalar value, which is why I think Immutable works well in this case.

PS: Yes, this probably would have been more appropriate as a new question, with a link back to this question.

J G 2024-1-25

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I had thought of making it required in the constructor. My hesitation was more related trying to maintain a standard object setup experience across different classes used in the application. Additionally having it in the constuctor, means that that object must be instantiated prior to creation of a MyClassB object, something I would rather not impose.

I played around with the matlab.mixin.Scalar idea, but didn't get it to work (I may not have been doing it correctly, and didn't spend a lot of time).

In the end, I found a native validitation function MustBeScalarOrEmpty that does the trick, and achieves most of what I want (i.e. an empty default value, property size and class enforecement, self-documentation). See below:

classdef MyClassA
    properties
        Bobj (1,:) MyClassB {MustBeScalarOrEmpty}  % Still provides decent self documentation
    end
    
    methods
        
        function obj = MyClassA()
        end
        
        
        function output = doStuffWithB(obj,input)
            if isempty(obj.Bobj) % This now will work, because default will be a 1x0 MyClassB object
                error('User must first define Bobj')
            else
                output = obj.Bobj.myBMethod(input); 
            end
        end
    end
    
end

J G 2024-1-25

@Benjamin Kraus, sorry! It seems that I didn't see your post prior to writing my recent comment! As you can see I ended up independently discovering the second solution you proposed using mustBeScalarOrEmpty.

I'll think more about your preffered solution. I do hear the benefits as you so eloquently spelled them out. As I mentioned, my current hesitation relates more to how the user creates objects for the application, and the fact making it a required constructor argument would impose a limitation on the sequence at which different objects must be created...

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