(Lua notes): Lua object-oriented

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table of Contents

Lua object-oriented

Typical Case

Create object

Access attributes

Access member functions

Complete example

Lua inheritance

Complete example

Function rewriting

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Lua object-oriented

• Objects are composed of properties and methods .
  • The most basic structure in Lua is table, so table is needed to describe the properties of objects .
  • The function in lua can be used to represent methods.
  • The classes in Lua can be simulated by table + function .
• As for inheritance, it can be simulated by metatable (not recommended, only the most basic objects are enough for most of the time).
• A table in Lua is not only an object in a sense.
  • Like objects, tables also have state (member variables)
  • It also has the nature of being independent of the value of the object, especially an object with two different values ​​(table) represents two different objects
  • An object can have different values ​​at different times, but it is always an object
  • Similar to objects, the life cycle of a table has nothing to do with what or where it was created.
• Objects have their member functions, and tables also have:

Account = {balance = 0}
function Account.withdraw (v)
    Account.balance = Account.balance - v
end

• This definition creates a new function and saves it in the withdraw field of the Account object, which can be called as follows:

Account.withdraw(100.00)

Typical Case

• The following simple class contains three attributes: area, length and breadth. The printArea method is used to print the calculation results:

-- 元类
Rectangle = {area = 0, length = 0, breadth = 0}

-- 派生类的方法 new
function Rectangle:new (o,length,breadth)
  o = o or {}
  setmetatable(o, self)
  self.__index = self
  self.length = length or 0
  self.breadth = breadth or 0
  self.area = length*breadth;
  return o
end

-- 派生类的方法 printArea
function Rectangle:printArea ()
  print("矩形面积为 ",self.area)
end

Create object

• Creating an object is the process of allocating memory for an instance of a class . Each class has its own memory and shares common data.

r = Rectangle:new(nil,10,20)

Access attributes

• You can use the dot (.) to access the properties of the class:

print(r.length)

Access member functions

• You can use the colon: to access the member functions of the class:

r:printArea()

• Memory is allocated when the object is initialized .

Complete example

• The following demonstrates a complete example of Lua object-oriented:

-- 元类
Shape = {area = 0}

-- 基础类方法 new
function Shape:new (o,side)
  o = o or {}
  setmetatable(o, self)
  self.__index = self
  side = side or 0
  self.area = side*side;
  return o
end

-- 基础类方法 printArea
function Shape:printArea ()
  print("面积为 ",self.area)
end

-- 创建对象
myshape = Shape:new(nil,10)

myshape:printArea()

输出：面积为     100

Lua inheritance

• Inheritance means that one object directly uses the properties and methods of another object . Can be used to extend the properties and methods of the base class.
• The following demonstrates a simple inheritance example:

-- Meta class
Shape = {area = 0}
-- 基础类方法 new
function Shape:new (o,side)
  o = o or {}
  setmetatable(o, self)
  self.__index = self
  side = side or 0
  self.area = side*side;
  return o
end
-- 基础类方法 printArea
function Shape:printArea ()
  print("面积为 ",self.area)
end

• In the following example, the Square object inherits the Shape class:

Square = Shape:new()
-- Derived class method new
function Square:new (o,side)
  o = o or Shape:new(o,side)
  setmetatable(o, self)
  self.__index = self
  return o
end

Complete example

• The following example inherits a simple class to extend the methods of the derived class. The derived class retains the member variables and methods of the inherited class:

-- Meta class
Shape = {area = 0}
-- 基础类方法 new
function Shape:new (o,side)
  o = o or {}
  setmetatable(o, self)
  self.__index = self
  side = side or 0
  self.area = side*side;
  return o
end
-- 基础类方法 printArea
function Shape:printArea ()
  print("面积为 ",self.area)
end

-- 创建对象
myshape = Shape:new(nil,10)
myshape:printArea()

Square = Shape:new()
-- 派生类方法 new
function Square:new (o,side)
  o = o or Shape:new(o,side)
  setmetatable(o, self)
  self.__index = self
  return o
end

-- 派生类方法 printArea
function Square:printArea ()
  print("正方形面积为 ",self.area)
end

-- 创建对象
mysquare = Square:new(nil,10)
mysquare:printArea()

Rectangle = Shape:new()
-- 派生类方法 new
function Rectangle:new (o,length,breadth)
  o = o or Shape:new(o)
  setmetatable(o, self)
  self.__index = self
  self.area = length * breadth
  return o
end

-- 派生类方法 printArea
function Rectangle:printArea ()
  print("矩形面积为 ",self.area)
end

-- 创建对象
myrectangle = Rectangle:new(nil,10,20)
myrectangle:printArea()

输出：
面积为     100
正方形面积为     100
矩形面积为     200

Function rewriting

• In Lua, you can rewrite the functions of the base class and define your own implementation in the derived class :

-- 派生类方法 printArea
function Square:printArea ()
  print("正方形面积 ",self.area)
end