Programming in Lua(Thrid Edition)笔记---16 Object-Oriented Programming

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16 Object-oriented Programming

  • 对象和方法
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    Account = {
    	balance = 0,
    	withdraw = function (self, v)
    		self.balance = self.balance - v
    	end
    }
    function Account:deposit(v)
    	self.balance = self.balance + v
    end
    Account.deposit(Account, 200.00)
    Account:withdraw(100.00)

在用冒号:而不是.创建或使用一个table的函数时，会自动生成或调用一个self指代table自身

  • 类，用__index来类的实例化
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    function Account:new(o)
    	o = o or {} -- create table if user does not provide one
    	setmetatable(o, self)
    	self.__index = self
    	return o
    end
    a = Account:new{balance = 0}
    print(a.balance) --> 0
    a:deposit(100.00)
    print(a.balance) --> 100

初次调用a.balance = a.balance + v，右侧的balance是类的默认值，右侧是对象的新域，之后再调用，右侧的balance为对象的域

  • 继承，子类可以在父类的基础上修改方法，修改后的方法是在子类自身的域

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    SpecialAccount = Account:new()
    s = SpecialAccount:new{limit = 1000.00}
    function SpecialAccount:withdraw(v)
    	if v - self.balance >= self:getLimit() then
    		error"insufficient funds"
    	end
    	self.balance = self.balance - v
    end
    function SpecialAccount:getLimit()
    	return self.limit or 0
    end
    print(s.balance) --> 0
    s:deposit(100.00)
    print(s.balance) --> 100
    s:withdraw(200.00)
    print(s.balance) --> 200

  • 多重继承

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    -- look up for 'k' in list of tables 'plist'
    local function search(k, plist)
    	for i = 1, #plist do
    		local v = plist[i][k] -- try 'i'-th superclass
    		if v then return v end
    	end
    end
    function createClass(...)
    	local c = {} -- new class
    	local parents = {...}
    	-- class will search for each method in the list of its parents
    	setmetatable(c, {__index = function (t,k)
    		return search(k, parents)
    	end})
    	-- prepare 'c' to be the metatable of its instances
    	c.__index = c
    	-- define a new constructor for this new class
    	function c:new(o)
    		o = o or {}
    		setmetatable(o, c)
    		return o
    	end
    	return c -- return new class
    end
    Named = {}
    function Named:getname()
    	return self.name
    end
    function Named:setname(n)
    	self.name = n
    end
    NamedAccount = createClass(Account, Named)
    account = NamedAccount:new{name = "Paul"}
    print(account:getname()) --> Paul
    print(account)

用createClass()创造一个继承自两个父类的子类。为了减少search()在父类中搜索的复杂度，可以采用记忆化搜索，将搜索的结果保存在原table中

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setmetatable(c, {__index = function (t, k)
	local v = search(k, parents)
	t[k] = v -- save for next access
	return v
end})

这样做的缺点：It is difficult to change method definitions after the system is running, because these changes do not propagate down the hierarhy chain.（说实话，我没看懂这句话……）

  • privacy，用局部变量和闭包构造私有成员，使得私有成员从外部无法直接获得，只能在内部获得

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    function newAccount(initialBalance)
    	local self = {
    		balance = initialBalance,
    		LIM = 10000.00,
    	}
    	local extra = function ()
    		if self.balance > self.LIM then
    			return self.balance * 0.10
    		else
    			return 0
    		end
    	end
    	local withdraw = function (v)
    		self.balance = self.balance - v
    	end
    	local deposit = function (v)
    		self.balance = self.balance + v
    	end
    	local getBalance = function ()
    		return self.balance + extra()
    	end
    	return {
    		withdraw = withdraw,
    		deposit = deposit,
    		getBalance = getBalance
    	}
    end
    acc1 = newAccount(100.00)
    acc1.withdraw(110.00)
    print(acc1.getBalance()) --> -10
    acc1.deposit(50.00)
    print(acc1.getBalance()) --> 40

  • single-method object，用闭包创建对象，无需创建一个接口table，根据参数调用不同的方法，没有继承，但是有私有性

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    function newObject(value)
    	return function (action, v)
    		if action == "get" then return value
    		elseif action == "set" then value = v
    		else error("invalid action")
    		end
    	end
    end

    d = newObject(0)
    print(d("get")) --> 0
    d("set", 10)
    print(d("get")) --> 10