Lua object model

Simple object model implementation in Lua.

Inspired by http://lua-users.org/wiki/SimpleLuaClasses

Features

• classes
• constructors
• destructors
• inheritance
• instanceof
• parent method calls

Requirements

• Lua 5.1 or newer

Module method overview

• class([parentClass]): table
  • create a new class, optionally inheriting methods and properties of another one
• instanceof(object, class): boolean
  • check if the given object is an instance of the given class or has that class as one of its parents
• new(class, ...): object
  • create an instance of the given class and invoke its constructor
  • any additional arguments will be passed to the constructor
  • should not be used directly, invoke the class table instead (refer to the Instantiating a class section)
• super(object, methodName, ...): mixed
  • invokes a parent method
  • this is the default implementation of object:super(method, ...)

Feature documentation

Let's assume you have imported the ObjectModel module like this:

local o = require 'ObjectModel'

Creating a new class

The class function returns a new table that you can populate with your methods and properties.

MyClass = o.class()

Inheritance

If another class is passed to the class() function, all of its methods and properties are inherited.

MyClass = o.class(OtherClass)

Internally, a shallow copy of the parent class is created for performance reasons. Modifications of the parent class at a later point will NOT be propagated to the child classes.

Prototype properties

Prototype properties can be defined in the class table. They are available in all instances of the class and can be accessed statically. Instance properties will shadow prototype properties with the same name.

MyClass.foo = 'bar'

Warning! Do not put tables into prototype properties unless you want the table to be shared across all the instances (see instance properties).

Methods

Lua provides syntactic sugar for this purpose.

function MyClass:doSomething()
    -- do something, self points to the instance
	print(self.someProperty) -- getting properties
	print(self:someMethod()) -- calling other methods
end

Same as prototype properties, methods defined this way are available in all instances of the class and can be called statically.

Constructors

Constructor is a special method that is invoked when a class is being instantiated.

function MyClass:constructor(lorem, ipsum)
    -- self points to the instance
end

Destructors

Destructor is a special method that is invoked when an instance is being garbage-collected by Lua.

function MyClass:destructor()
    -- self points to the instance
    -- no arguments are passed
end

Instantiating a class

To create an instance of a class, simply call the class table as a function.

local obj = MyClass('something')

All arguments passed to this function are forwarded to the constructor.

Instance properties

Properties defined on an object are privately owned by that object. They will shadow prototype properties with the same name.

MyClass.test = 'hello from prototype'

local obj = MyClass()

print(obj.test) -- prints: hello from prototype
obj.test = 'hello from instance'
print(obj.test) -- prints: hello from instance
obj.test = nil
print(obj.test) -- prints: hello from prototype

Instanceof

To check whether an object is instance of a specific class or has that class as one of its parents, use the instanceof() function.

local obj = MyClass()

if o.instanceof(obj, MyClass) then
    print('obj is instance of MyClass')
end

Calling parent methods

If a class has inherited from another and overridden some of its methods, these methods can be invoked using the :super(methodName, ...) method.

BaseClass = o.class()

function BaseClass:constructor(foo)
    self.foo = foo
end

DerivedClass = o.class(BaseClass)

function DerivedClass:constructor(foo, bar)
    self:super('constructor', foo) -- invoke the parent constructor
    self.bar = bar
end

local obj = DerivedClass('hello', 'world')

print(obj.foo, obj.bar) -- prints: hello world

Metamethods

Metamethods can be defined the same way as any other methods and will work as expected.

function MyClass:__tostring()
    return 'stringified!'
end

local obj = MyClass()

print(obj) -- prints: stringified!