4.30. Lua object oriented

4.30.1. Object-oriented feature #

• 1)Encapsulation: a feature that can load the information, functions, and responses of an entity into a single object.

• 2)Inheritance: the inheritance method allows it to be extended without changing the original program, so that the original function can be preserved and the new function can be extended. This helps to reduce repeated coding and improve the efficiency of software development.

• 3)Polymorphism: the same operation acts on different objects, which can have different interpretations and produce different execution results. At run time, methods in an implementation derived class can be called through apointer to the base class.

• 4)Abstraction: Abstraction is a way to simplify complex real-world problems, it can find the most appropriate class definition for specific problems, and can explain the problem at the most appropriate inheritance level.

4.30.2. Object-oriented in Lua #

We know that objects are made up of properties and methods. The most basic structure in LUA is table , so you need to use table to describe the properties of the object.

lua in function can be used to represent a method. Then the classesin LUA can be simulated by table + function.

As for inheritance, you can use the metetable simulate it (not recommended, only simulate the most basic objects, most of the implementation is enough).

Table in Lua is not only an object in a sense. Like objects, tables also have states (member variables); they also have the nature of being independent of the values of objects, especially objects with two different values (table) represent two different objects; an object can have differentvalues at different times, but it is always an object; like objects, the life cycle of a table has nothing to do with what is created and where it iscreated. Objects have their member functions, and tables 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 Account object’s withdraw domain, we can call the following:

Account.withdraw(100.00) 

-- metaclass Rectangle = {area = 0, length = 0, breadth = 0} -- Methods for Derived Classes 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 -- Methods for Derived Classes printArea function Rectangle:printArea () print("The rectangular area is ",self.area) end 

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

print(r.length) 

r:printArea() 

-- metaclass Shape = {area = 0} -- Basic class methods 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 -- Basic class methods printArea function Shape:printArea () print("The area is ",self.area) end -- create object myshape = Shape:new(nil,10) myshape:printArea() 

The area is 100 

4.30.3. Lua inheritance #

Inheritance means that one object uses the properties and methods of anotherobject directly. Can be used to extend the properties and methods of the underlying class.

The following demonstrates a simple inheritance example:

-- Meta class Shape = {area = 0} -- Basic class methods 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 -- Basic class methods printArea function Shape:printArea () print("The area is ",self.area) end 

The next example Square object inherits from 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 

-- Meta class Shape = {area = 0} -- Basic class methods 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 -- Basic class methods printArea function Shape:printArea () print("The area is ",self.area) end -- create object myshape = Shape:new(nil,10) myshape:printArea() Square = Shape:new() -- Derived class methods new function Square:new (o,side) o = o or Shape:new(o,side) setmetatable(o, self) self.__index = self return o end -- Derived class methods printArea function Square:printArea () print("The square area is",self.area) end -- create object mysquare = Square:new(nil,10) mysquare:printArea() Rectangle = Shape:new() -- Derived class methods 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 -- Derived class methods printArea function Rectangle:printArea () print("The rectangular area is",self.area) end -- create object myrectangle = Rectangle:new(nil,10,20) myrectangle:printArea() 

Area of 100 A square area of 100 A rectangular area of 200 

4.30.4. Function rewriting #

In Lua we can override the functions of the base class and define our own implementation in the derived class:

-- Derived class methods printArea function Square:printArea () print("Square area ",self.area) end