What is purpose of abstract property in MATLAB? - matlab

Question

We have the abstract attribute of methods and properties in MATLAB R2014b, and I know the purpose of abstract attribute for methods. We can call functions in that method and define it in the superclass of a class. What I'm confused about is the purpose of the abstract attribute for a property in MATLAB. How do we use this?

Answer 1

The purpose of abstract properties (and abstract methods) is to allow the creation of interfaces:
The basic idea of an interface class is to specify the properties and methods that each subclass must implement without defining the actual implementation.
For example, you can define an abstract Car with the definition
classdef (Abstract) Car
properties(Abstract) % Initialization is not allowed
model
manufacturer
end
end
The abstract properties model and manufacturer cannot be initialized (that would be like instantiating an abstract class) and all classes that inherit from Car must specify their value for the subclass to be concrete.
If the properties were not abstract, the subclass would simply inherit them.
Making the properties abstract forms a contract of sorts that says "for you to be a usable (concrete) car you must have a model and manufacturer defined".
Therefore, in the definition
classdef FirstEveryManCar < Car
properties
model = 'T';
manufacturer = 'Ford';
end
end
the property definitions are compulsory for the class to not be made abstract automatically (which you could do if you have long class hierarchies).

Answer 2

There is a significant consequence for setter/getter methods (i.e. set.Property & get.Property).
Due to the way Matlab works, you can only implement setter/getter methods inside the class definition file of the class that declares the property. Therefore if you want to ensure a property is defined in the interface, but require implementation specific setter/getter methods, then declaring the property Abstract in the interface class ensures the subclass redefines the property and enables that class to define its own setter/getter methods.
Example 1 (Does not work)
Superclass
classdef (Abstract) TestClass1 < handle
properties
Prop
end
end
Subclass
classdef TestClass2 < TestClass1
methods
function obj = TestClass2(PropVal)
if nargin>0
obj.Prop = PropVal;
end
end
function set.Prop(obj, val)
if ~isnumeric(val)
error('Not a number!');
end
obj.Prop = val;
end
end
end
Example 2 (The correct way)
Superclass
classdef (Abstract) TestClass1 < handle
properties (Abstract)
Prop
end
end
Subclass
classdef TestClass2 < TestClass1
properties
Prop
end
methods
function obj = TestClass2(PropVal)
if nargin>0
obj.Prop = PropVal;
end
end
function set.Prop(obj, val)
if ~isnumeric(val)
error('Not a number!');
end
obj.Prop = val;
end
end
end

Answer 3

I don't know any example where you really need it, but it is typically used when a abstract superclass uses properties without having a reasonable default. This is a extremely trimmed down example, but imagine welcome implementing a full user interface while welcomeGer fills all required properties to provide it in german language:
%welcome.m
classdef welcome
properties(Abstract)
text
end
methods
function printText(obj)
disp(obj.text)
end
end
end
%welcomeGer.m
classdef welcomeGer<welcome
properties
text='Willkommen in unserem Hotel'
end
end
Alternative you could skip the definition of text at all, but then matlab would not throw an error when you forget to initialize text