Note: I corrected a few formatting errors on 29 January 2020.

Acknowledgements: These slides are adapted from slide set 11 for the course Programming in Lua created by Fabio Mascarenhas, Federal University of Rio de Janeiro, Brazil. It was presented at Nankai University, P. R. China, in July 2013.

Advisory: The HTML version of this document may require use of a browser that supports the display of MathML. A good choice as of January 2020 is a recent version of Firefox from Mozilla.

Lua Objects

• Lua tables are objects

  • own state (values associated with keys)
  • own identity independent of state
  • own lifecycle independent of who created them

• Lua tables can have own operations (closures associated with keys)

  Account = { balance = 0.0 }
  function Account.withdraw(v) 
     Account.balance = Account.balance - v 
  end 
  
  Account.withdraw(100.00)

Methods

• Account.withdraw almost method as in OOP languages

• Problem is hard-coding of receiver as global name

• Better to pass receiver as explicit parameter

  function Account.withdraw(self,v) 
     self.balance = self.balance - v 
  end
  
  a1 = Account
  a2 = { balance = 0, withdraw = Account.withdraw }
  a1.withdraw(a1,100.00)  -- operates on a1
  a2.withdraw(a2,200.00)  -- operates on a2

Methods and :

• In most OOP languages, method has implicit receiver object called self or this

• Lua method is function that takes receiver as 1st parameter, call parameter whatever want

• Indexing Lua “receiver” object with method name returns its closure, then closure can be applied to arguments

  > obj.method(obj, <other arguments>)

• Lua has colon operator : as syntactic sugar for above – obj any expression, evaluated once, get method, pass as 1st parameter

  > obj:method(<other arguments>) 

Declaring Methods

• Can use colon to declare a method

   function obj:method(<other arguments>)
      <code of method>
   end

• Effect same as following

  function obj.method(self,<other arguments>)
     <code of method>
  end

Square Object

    local square = { x = 10, y = 20, side = 25 } 

    function square:move(dx, dy) 
       self.x = self.x + dx
       self.y = self.y + dy 
    end

    function square:area() 
       return self.side * self.side 
    end
    
    return square
       
    > print(square:area())
    625
    > square:move(10, -5) 
    > print(square.x, square.y) 
    20      15 

Classes

• Methods in square work with any table with x, y, and side

  > square2 = { x = 30, y = 5, side = 10 } 
  > print(square.area(square2)) 
  100 
  > square.move(square2, 10, 10) 
  > print(square2.x, square2.y) 
  40     15 

• Can put methods in Square class, a prototype for objects like square and square2

• Also add method to create new instances

• Have metatable with metamethod __index pointing to Square

Square Class as Module

    local Square = {} 
    Square.__index = Square

    function Square:new(x, y, side) 
       return setmetatable({ x = x, y = y, side = side }, self) 
    end 

    function Square:move(dx, dy) 
       self.x = self.x + dx 
       self.y = self.y + dy 
    end                               > s1 = Square:new(10, 5, 10)
                                      > s2 = Square:new(20, 10, 25) 
    function Square:area()            > print(s1:area(), s2:area()) 
       return self.side * self.side   100     625 
    end                               > s1:move(5, 10) 
                                      > print(s1.x, s1.y) 
    return Square                     15      15 

Default Fields

• Other fields in Square become default values for instances

   local Square = { color = "blue" }

• Reading field gives default value from class

  > s1 = Square:new(10, 5, 10) 
  > print(s1.color) 
  blue

• But setting field just sets field in instance

  > s1.color = "red" 
  > print(s1.color) 
  red 
  > s2 = Square:new(20, 10, 25) 
  > print(s2.color) 
  blue 

Circle Class

    local Circle = {} 
    Circle.__index = Circle

    function Circle:new(x, y, radius) 
       return setmetatable({ x = x, y = y, radius = radius }, self) 
    end 

    function Circle:move(dx, dy) -- same as for Square
       self.x = self.x + dx 
       self.y = self.y + dy 
    end 

    function Circle:area() 
       return math.pi * self.radius * self.radius 
    end 

   return Circle 

Shape Superclass

• Factor common parts into Shape class:

  local Shape = {} 
  Shape.__index = Shape 
  
  function Shape:new(x, y) 
     return setmetatable({ x = x, y = y }, self) 
  end 
  
  function Shape:move(dx, dy) 
     self.x = self.x + dx 
     self.y = self.y + dy 
  end 
  
  return Shape 

• Metatable of instance is class; metatable of class is superclass

Point extends Shape

• Points are simple shapes with just coordinates, area 0

  local Shape = require "shape"
                                          > p = Point:new(10, 20) 
  local Point = setmetatable({}, Shape)   > print(p:area()) 
  Point.__index = Point                   0
                                          > p:move(-5, 10) 
  function Point:area()                   > print(p.x, p.y) 
     return 0                             5       30 
  end 
  
  return Point 

• setmetatable makes new class inherit methods of Shape including constructor

Circle extends Shape

• Override constructor new in class Circle, but call Shape constructor to do part of work

  local Shape = require "shape" 
  local Circle = setmetatable({}, Shape) 
  Circle.__index = Circle
                                            > c = Circle:new(10, 20, 5)
  function Circle:new(x, y, radius)         > c:move(5, -5) 
     local shape = Shape.new(self, x, y)    > print(c.x, c.y) 
     shape.radius = radius                  15      15 
     return shape                           > print(c:area()) 
  end                                       78.539816339745 
  
  function Circle:area() 
     return math.pi * self.radius * self.radius 
  end
  
  return Circle

• Can use explicit reference like Shape.new to call the super method

Other Object Models

• This just one way to implement objects in Lua

• Advantage: simple!

• Disadvantage: putting “class methods” (new) and “instance methods” (move, area) in same namespace

• Other metamethods not inherited

  To connect __tostring with a tostring method that can be overriden, explicitly set Class.__tostring = Class.tostring for each Class

• Can define more sophisticated object models in libraries, make work with the : operator

Quiz

• With our object model, how could we check whether an object is an instance of a class?

• What about checking whether an object is an instance of a class or one of its subclasses?

instanceof Function

Assuming the object/class model from this set of slides:

    function instanceof(obj, class)
       local mt = getmetatable(obj) -- get class prototype
       if mt == class then          -- is instance of class?
          return true
       elseif mt == nil then        -- top has no metatable
          return false 
       else                         -- check superclass
          return instanceof(mt, class) 
       end
    end