matlab,oop,inheritance,superclass , MATLAB: Set inherited properties to read-only


MATLAB: Set inherited properties to read-only

Question:

Tag: matlab,oop,inheritance,superclass

Lets say I have a superclass called rectangle:

classdef Rectangle
    properties
        width
        height
        x0 = 0
        y0 = 0
        angle = 0
    end
    methods
        function obj = Rectangle(x0, y0, width, height, angle)
            obj.x0 = x0;
            obj.y0 = y0;
            obj.width = width;
            obj.height = height;
            obj.angle = angle;
        end
    end
end

And I have a subclass called Map, in which I want all properties to be read-only once set:

classdef Map < Rectangle
    properties (SetAccess=private)
        filename
    end
    methods
        function obj = Map(filename)
            % Get info from map using a function that uses geotiffread
            [x0, y0, width, height] = GetInfoFromMap(filename);
            obj = [email protected](x0, y0, width, height, 0);
            obj.filename = filename;
        end
    end
end

How can I make the inherited properties of Rectangle read-only within Map? I want the properties of any stand-alone (non Map) Rectangle object to remain changeable.

And also, how can I ensure that some of those properties can take only certain values? i.e. for my purposes a rectangle can have any angle, but I'd expect a map to always have an angle of 0. But if I try to create a set.angle method for Map to ensure that angle can only be 0, I get an error telling me that "Cannot specify a set function for property 'angle' in class 'BackgroundMap', because that property is not defined by that class."


Answer:

This seems like a case where you should prefer composition over inheritance.

This idea is mentioned in one of the comments (@Dev-iL), where it's described as "not very OOP". I disagree - although inheritance is the right design in many cases, composition is often preferable. (See here and here for some discussion of that). One of the typical reasons why one might prefer composition is that inheritance exposes the subclass to the details of the superclass, in other words it breaks encapsulation - and that's what's happening in your implementation of Rectangle and Map.

I would suggest that instead of Map inheriting from Rectangle, it should have a Rectangle as a private (maybe hidden) property. So you should think of a map not as being a type of rectangle, but as having an associated rectangle that it delegates some of its functionality to.

Once you've done that, you can then give Map some properties width, height etc, and make them GetAccess=public, SetAccess=private, Dependent. Then within Map, add a get method for each, which just gets the property of the underlying Rectangle and passes it through. Note that Map doesn't need an angle property, as it's always zero.

classdef Map < handle
    properties (SetAccess=private)
        filename
    end
    properties (Hidden, GetAccess = private, SetAccess-private)
        rectangleDelegate
    end
    properties (Dependent, GetAccess = public, SetAccess = private)
        width
        height
        x0
        y0
    end
    methods
        function obj = Map(filename)
            % Get info from map using a function that uses geotiffread
            [x0, y0, width, height] = GetInfoFromMap(filename);
            obj.rectangleDelegate = Rectangle(x0, y0, width, height, 0);
            obj.filename = filename;
        end
    end
    methods
        function val = get.width(obj)
            val = obj.rectangleDelegate.width;
        end
        % other gets
    end
end

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