Classes and Objects

We've briefly talked about the class implementation used by Pragtical in the introduction. In this article, we'll talk about how to use the implementation efficiently.

note

Some methods here will be written differently than in the source code. This is to clarify how the methods should be called.

Creating classes

Similar to many other languages, all classes extend the Object class. To extend a class, one can use the Object.extend() method.

function Object.extend(base_class: Object): Object end

This method accepts one parameter — the class to copy. This method will create a copy of a Object that can be extended without affecting the original object.

You can now define or override methods and metamethods of this newly created class. To initialize a class, you can override the Object:new() method.

-- load the Object class
local Object = require "core.object"
-- since the source class is usually the one you want to copy,
-- we often simplify it to Object:extend().
-- This is equivalent to Object.extend(Object).
local Animal = Object:extend()
-- define a constructor for Animal
function Animal:new()
  self.type = "animal"
end

tip

Object implements the __tostring() and __call() metamethods. You can override the __tostring() metamethod to return a string that suits your class.

Super class

You can access the super class via Object.super. For instance, to call the constructor of the super class, you can do:

local Object = require "core.object"
local Animal = Object:extend()
function Animal:new()
  self.type = "animal"
end

local Dog = Animal:extend()
function Dog:new()
  -- call the superclass constructor
  -- note the use of . instead of : and explicitly passing self
  Dog.super.new(self)
end

Note that you must call super class methods with . instead of : and pass self as the first parameter. If you call the method with :, it will call the method with self as super, which is the super class and not the instance.

Class relationships

The Object class also provide utility methods for checking relations between classes and object.

To check whether an object is strictly an instance of a class (not an instance of a child class), use Object:is(). To do the inverse, use Object:is_class_of().

function Object:is(class: Object): boolean end
function Object.is_class_of(class: Object, instance: Object): boolean end

Object:is() accepts a parameter — the class to compare it to. If the object is strictly an instance of the class, the method returns true.

Object.is_class_of() accepts a class and compares it against an instance of an object. If the object is a strictly an instance of the class, the method returns true.

Example:

local Animal = Object:extend()
local Dog = Animal:extend()
local Cat = Animal:extend()
local doggo = Dog()

print(doggo:is(Dog)) -- prints true
print(doggo:is(Cat)) -- prints false
print(doggo:is(Animal)) -- also prints false

-- note that since we're going to pass Dog as the first parameter,
-- we might as well as use the colon notation
print(Dog:is_class_of(doggo)) -- prints true
print(Cat:is_class_of(doggo)) -- prints false
print(Animal:is_class_of(doggo)) -- prints false

To check whether an object is an instance of a class (or an instance of child class), use Object:extends(). To do the inverse, use Object:is_extended_by().

function Object:extends(class: Object): boolean end
function Object.is_extended_by: Object, instance: Object): boolean end

The methods accept the same parameters as Object:is() and Object:is_class_of().

local Animal = Object:extend()
local Dog = Animal:extend()
local Cat = Animal:extend()
local doggo = Dog()

print(doggo:extends(Dog)) -- prints true
print(doggo:extends(Cat)) -- prints false
print(doggo:extends(Animal)) -- prints true

-- note that since we're going to pass Dog as the first parameter,
-- we might as well as use the colon notation
print(Dog:is_extended_by(doggo)) -- prints true
print(Cat:is_extended_by(doggo)) -- prints false
print(Animal:is_extended_by(doggo)) -- prints true