Lua Object Oriented Programming

Lua itself is not an Object-Oriented language. But it’s metatable system can provide even more powerful functionality. It’s really easy to misuse the metatable system. Follow the example would make life easier.

-- Define a "Base" table.
local Base = {}

-- Provide an extend() function to let other class inherit from Base.
function Base:extend(o)
    o = o or {}
    setmetatable(o, self)
    self.__index = self
    return o
end

-- Create a new instance of Base.
function Base:new(o)
    o = self:extend(o)
    Base.init(o)
    return o
end

-- The init() function of Base. Note, it is also executed when an instance of a derived class is created.
-- The tricky is in Derived:new() function.
function Base:init()
    print("Base:init()")
end

-- The render() function of Base.
function Base:render()
    print("Base:render()")
end

-- Define a Derived table.
local Derived = Base:extend()

-- Create a new instance of Derived class. Note: usually this function is not necessary, since Base:new() provides
-- similar functionality. But we would like both Base:init() and Derived:init() to be executed.
function Derived:new(o)
    -- Note: this line is triple important, we should prefer self / Derived instead of Base to ensure override
    -- functions can take effect.
    -- Changing it to Base:new(o) to have a try.
    o = Base.new(self, o)
    Derived.init(o)
    return o
end

-- This function should override Base:init(). But both should be actively called in new().
function Derived:init()
    print("Derived:init()")
end

-- This function should override Base:render().
function Derived:render()
    print("Derived:render()")
end

Base:new():render()
-- The above statement expects to output
-- Base:init()
-- Base:render()

Derived:new():render()
-- The above statement expects to output
-- Base:init()
-- Derived:init()
-- Derived:render()