Class Property: Array of Objects

I would like to create a "Dependent Property" inside a "Main Class" that will handle a array of objects of "Sub Class".

Exemple:

classdef main_class
   properties
      number
   end
   properties (Dependent)
       % prop will be a array of sub_class object
       % the size of this array will depend/change on the "number" property
       prop = sub_class();
   end
end

If I set:

main = main_class();
main.number = 3;

I would like to get:

main.prop = [sub_class(1) sub_class(2) sub_class(3)]

To me is important that "prop" be a dependent property to automatic change the number of the elements in the array.

How could I do that? Thanks!

[Adam]

I would strongly recommend reconsidering your wish to have this as a dependent property. I use dependent properties a lot and I like them, but in this case they have some serious difficulties.

Creating objects in a dependent property is highly unlikely to be what you want to do. This means that every time you access the dependent property it will create a new array of objects, you will never be able to access the same objects within the class (though you can, of course, if you store main.prop in a local variable and use it thereafter) because they will keep getting replaced every time you get the property 'prop'.

Even if this does happen to produce the behaviour you want it is not very transparent and I would always do something as fundamental as object creation in a function named e.g. createXXXX to be clear what it is doing, not as a side-effect of accessing a dependent property.

Also it is not clear - is sub_class actually a sub class of main_class? As in does it inherit from it, or do you just call it sub_class because it sits inside it? Objects containing objects of their own subclass feels like an odd design to me that would cause a lot of potential problems. Maybe I'm just not thinking of a usage though...

Answer 1

[Guillaume]

I'm with Adam, I don't think you want a dependent property. dependent does not mean that the property value is recalculated when some other property change, it means that property value is created every time you ask for it. That value is not stored in the class.

In your case, you would be better off creating a setter method for your number property that automatically update the array size of the normal prop property:

classdef main_class
   properties
      number
      prop
   end
   methods
      function this = main_class(number)   %constructor
         if nargin == 0
            this.number = 0;
         else
            this.number = number;
         end
         this = this.create_subarray;
      end
      function this = set.number(this, value)  %set method for number. reset prop at the same time
         this.number = value;
         this = this.create_subarray;
      end
      function this = create_subarray(this)
         this.prop = sub_class.empty;
         for n = 1:this.number
            this.prop(n) = sub_class(n);
         end
      end
   end
end

As Adam and Andrew have commented, hopefully sub_class is not actually a subclass of main class.

[Nycholas Maia]

Yes! You all are right!

In my case, I will not use a "dependent property" because I want to access the values stored in the memory.

I think that I'm understanding...

Just to be clear:

Question A: What are: "n" and "value" variables in the Guilleaume code?

Question B: I want add some "properties" inside the prop variable. Like a "struct" or "object" inside the prop propertie.

Example:

Main = main_class(10);

Main.prop(5).color = 'blue';
Main.prop(5).size = 10;
Main.prop(5).collection = [struct struct struct];

Please, how can I do that?

Is there a way to define the properties inside the prop? Like a class, I could define the name and type of all properties inside prop. Is this possible?

Thank you so much!

[Adam]

I think the 1:n in Guillaume's code is a typo and should be 1:number instead. The other n's are just place-holder variables in an arrayfun expression, for each of the 1:number values in succession.

'prop' will be an array of classes so yes you can set properties on them. If you are wanting to make lots of changes then having them embedded inside another class is not necessarily the best option, but it depends on the situation really.

[Nycholas Maia]

Adam, you said that "prop will be an array of classes"...

So, I need to create a new M file and create a new class?

Could you please write a little example?

[Adam]

Well, my comment was a little inaccurate - it will be an array of class objects to be more accurate.

You told us in the original question though that you already have this class, it is called 'sub_class' so prop will be an array of objects of this type, whatever is in that class.

[Nycholas Maia]

Thank you Adam for your quick response!

I will try this solution and I will give a feedback as soon as possible!

Thank you all!

[Guillaume]

Yes, as Adam pointed out there were a few typos in my example (now corrected). In addition, I'd forgotten that arrayfun cannot create arrays of object, so I've changed the implementation to an explicit loop.

prop can be anything you want, array of class objects (whose class you define in a separate m file, a structure array (in which case you don't need another file), or something else.

Note that I've ignored methods and properties attributes in my example. I assumed that prop would be read-only. If prop is read/write as your

Main.prop(5).color = 'blue';

would require (assumin that sub_class is a value class), then the user can also change the size of the prop array independently of the number property, breaking your class. If you want to avoid that, you'd also need a property setter for prop, e.g:

function this = set.prop(this, value);
   assert(isequal(size(value), size(this.prop)), 'Resizing prop is not allowed');
   this.prop = value;
end

Answer 2

[Andrew Newell]

You need to define a get method (see Access Methods for Dependent Properties). For example, the method block could look like this:

methods
    function value = get.prop(obj)
         value(obj.number) = sub_class(obj.number);
         for ii=1:obj.number-1
             value(ii) = sub_class(ii);
         end
    end
end

Note that, since sub_class would share this method, the dependent variable is endlessly recursive; if you ran

main = main_class();
main.number = 1;

then main.prop = main.prop.prop = ...

[Nycholas Maia]

Hi Andrew, thanks for your response.

But I'm newer in MATLAB OOP, and I did not understand how your code will return a array of objects of "sub_class()" inside the "dependent property" prop.

Could you explain better for a young MATLAB programmer? Obs.: My doubt is not the get/set idea. Im a C++ programmer.

Thank you!

[Andrew Newell]

If sub_class is actually a subclass of main_class, it inherits the properties and methods of main_class. That would include the property number and the method get.prop. So if you type

main.prop.prop

get.prop will create a sub_class object and return it as main.prop. Then, in main.prop, get.prop will create yet another sub_class object and return it as main.prop.prop. I agree with Adam and Guillaume - this is not the way to go!

Answer 3

[Nycholas Maia]

Thank you all....now I got everything working great! Thank you so much!