Metatable & MetaMethod

Intro

算术运算相关元方法

local mt = {}
local Set = {}
function Set.new (l)
    local set = {}
    setmetatable(set, mt)
    for _, v in ipairs(l) do
         set[v] = true
    end
    return set
end

function Set.union (a, b)
    local res = {}
    for k in pairs(a) do
        res[k] = true
    end
    for k in pairs(b) do
        res[k] = true
    end
    return res
end

function Set.intersection (a, b)
    local res = {}
    for k in pairs(a) do
        res[k] = b[k]
    end
    return res
end

function Set.tostring (set)
    local l = {}
    for e in pairs(set) do
        l[#l + 1] = e
    end
    return "{".. table.concat(l, ",") .. "}"
end

mt.__add = Set.union
mt.__mul = Set.intersection
mt.__tostring = Set.tostring

s1 = Set.new{10, 20, 30}
s2 = Set.new({30, 1})
print(getmetatable(s1))
s3 = s1 + s2
print(s1)
print(Set.tostring(s3))

--[[
	table: 00000000006ea150
	{20,10,30}
	{20,1,10,30}
]]--

表相关的元方法

prototype = {x = 0, y = 0, width = 100, height = 100}
function new(o) 
	setmetatable(o, mt)
	return o
end

mt.__index = function (_, key) 
	return prototype[key]
end

mt.__newindex = function (t, key, value)
	print("Add new field: " .. key.tostring())
	t[key] = value
end
--[[ 
	另一种写法是直接用表作为__index字段的值
	mt.__index = prototype
]]--
local w = new {x = 10, y = 20}
print(w.width) --> 100, 当w.width不存在时会调用prototype[width]
w.height = 10
print(w.height)

--[[
	100
	Add new field: height
	10
]]--

Practice: 跟踪对表的访问（Proxy）

function track (t)
    local mt = {
        __index = function (_, k)
            print("*access to element " .. tostring(k))
            return t[k]
        end,

        __newindex = function (_, k, v)
            print("*update element " .. tostring(k) .. " with value " .. tostring(v))
            rawset(t, k, v)
        end,

        __pairs = function ()
            return function (_, k)
                local nextkey, nextvalue = next(t, k)
                if (nextkey ~= nil) then
                    print("*traversing element " .. tostring(nextkey))
                end
                return nextkey, nextvalue
            end
        end,

        __len = function ()
            return #t
        end,
    }

    local proxy = {}
    setmetatable(proxy, mt)
    return proxy
end

local t = track({})

t.x = 1
t.y = 10

print(t["x"])

for k, v in pairs(t) do
    print("(" .. tostring(k) .. ", " .. tostring(v) .. ")")
end


--[[
	*update element x with value 1
	*update element y with value 10
	*access to element x
	1
	*traversing element x
	(x, 1)
	*traversing element y
	(y, 10)
]]--

补充：元方法字段表

注意：

• Lua 不支持 __gt（大于）和 __ge（大于等于）元方法。如果需要实现这些操作，可以通过 __lt 和 __le 间接实现。

• 如果定义了 __eq，则必须同时定义 __le 和 __lt，否则会报错。