A structure groups related data together under one variable, with fields you access using a dot, like tire.diameter. A class takes that same idea a step further: it groups data together and bundles in the functions that operate on that data, so both travel together as a single unit.

The MATLAB Help Center has a guide on Object-Oriented Programming.

Defining a Class

A class is defined in a file named exactly after the class, using the classdef keyword. For example, a Rocket class would be saved in a file named Rocket.m, containing:

classdef Rocket
    properties
        Name
        Thrust % N
        Mass   % kg
    end

    methods
        function obj = Rocket(name, thrust, mass)
            obj.Name = name;
            obj.Thrust = thrust;
            obj.Mass = mass;
        end

        function twr = thrustToWeight(obj)
            g = 9.81;
            twr = obj.Thrust / (obj.Mass * g);
        end
    end
end

The properties block lists the data each Rocket carries around - similar to fields in a structure. The methods block contains functions associated with the class. Every method’s first input, obj by convention, is the object it’s being called on.

The method with the same name as the class (Rocket, here) is the constructor - the function that runs when you create a new Rocket, responsible for filling in its properties.

Creating and Using Objects

An object is one specific instance of a class, created by calling the constructor:

my_rocket = Rocket('Test Vehicle', 5000000, 400000);

This calls the constructor with name='Test Vehicle', thrust=5000000, and mass=400000, and stores the resulting object in my_rocket. Properties are accessed with dot notation, just like a structure’s fields:

disp(my_rocket.Name)

Methods are also called with dot notation, but with parentheses to invoke them:

twr = my_rocket.thrustToWeight();

This calls thrustToWeight with obj automatically set to my_rocket, computing a thrust-to-weight ratio of about 1.27 for the values above.

Why Use a Class?

You can create as many Rocket objects as you like from the same class definition, each with its own independent property values:

falcon = Rocket('Booster A', 7600000, 549000);
sls = Rocket('Booster B', 39100000, 2600000);

Both falcon and sls have their own Name, Thrust, and Mass, and both can call thrustToWeight() - the calculation only has to be written once, inside the class, instead of copied into every script that needs it. This is the same motivation behind user functions, extended to keep data and the functions that act on it defined together in one place.

Reading Questions

  1. How is a class similar to a structure? How does it go further?
  2. What must the filename of a class definition be?
  3. What is a constructor, and when does it run?
  4. How do you access a property of an object? How do you call one of its methods?
  5. If you create two Rocket objects, do they share the same Thrust value, or does each have its own?