Lua继承的实现

实现继承的原理：利用table中的元表来实现 __index

__index元方法：用于查看表中元素或方法是否存在如果不存在则返回nil 如果存在则返回__idnex表中的结果

--单一继承的实现 利用元表来实现继承
Account = {name = "lili", balance = 10}
function Account:widthdraw(value)
	self.balance = self.balance + value
end

function Account:new(o)
	o = o or {}
	setmetatable(o, self)
	self. __index = self
	return o
end

local rto = Account:new()
rto:widthdraw(1000)
print("father area")
print("balance = " .. rto.balance)
print("name = " .. rto.name)

--子类继承父类
childAccount = Account:new({height = 200})
--子类重写父类方法
function childAccount:widthdraw(value)
	print("I am child ")
	self.balance = self.balance - value
end
local cto = childAccount:new()
cto:widthdraw(1)
print("-----------------")
print("child area")
print("balance = " .. cto.balance)
print("name = " .. cto.name)
print("height = " .. cto.height)

childAccount = Account:new({height = 200}) 产生子类在Account:new()中将__index设为你自己是一个table 当cto访问balance时由于自己不存在此字段会去元表Account{}中查找存在返回结果不存在会提示在访问一个不存在的nil值

第二种实现继承

Father.lua

--类的声明 声明了类名和属性 属性初始值
Father = {x = 20, y = 40}

--设置元表的索引
--当访问Father中不存在字段信息时 需要在元表中通过__index元方法查找 
--这里设置__index为自身
Father. __index = Father 

--构造方法
function Father:new(x, y)
	local self = {}
	setmetatable(self, Father) --将self的元表设为Father
	self.x = x
	self.y = y
	return self
end

function Father:Test()
	print("x = " .. self.x)
	print("y = " .. self.y)
end

function Father:add()
	self.x = self.x + 10000
	self.y = self.y + 2000
end
return Father

child.lua


require "Father"

--声明了child的新属性
child = {z = 1000, name = "hqk"}
--将child的元表设置为Father
setmetatable(child, Father)
child. __index = child

function child:new(x, y, z, name)
	local self = {}
	--调用父类的构造函数构造父类部分
	self = Father:new(x, y) 
	--将对象自身的元表设为child类
	setmetatable(self, child)
	self.z = z
	self.name = name
	return self
end

function child:Test()
	print("x = " .. self.x)
	print("y = " .. self.y)
	print("z = " .. self.z)
	print("name = " .. self.name)
end
function child:Sub()
	self.x = self.x - 100
	self.y = self.y - 1000
	self.z = self.z * 2
end
return child

jtest.lua 测试

require "Father"
require "child"

print("Father area")
print("construct Father object")
local f = Father:new()

print("use self function")
Father:add()
f:Test()

print("-----------------")
print("Child area")
print("construct child object")
f = child:new() --实例化子对象

print("use Father's function")
f:add() --子类调用父类成员方法

print("use self function")
f:Sub()

print("use rewrite function")
f:Test() --子类调用重写的方法

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