利用__index和__newindex实现默认值表、监控表、只读表

setdefault = function(t, v)
    setmetatable(t, {__index = function () return v end})
end

s = {1, 2, 3}
setdefault(s, 0)        -- 默认值设为0
print(s.x)

key = {}
local mt = {__index = function (t) return t[key] end}
function setdefault(t, d)
    t[key] = d
    setmetatable(t, mt)
end

t = {} --需要监控的表
local _t = t
t = {} --代理表需要为空，因为只有当读写不存在的域时，才会触发__index和__newindex

local mt = {
    __index = function (t, k)
    print("*access to element"..tostring(k))
    return _t[k]
    end,
    
    __newindex = function(t, k, v)
    print("*update of element " .. tostring(k) .. " to " .. tostring(v))    
    _t[k] = v
    end
}

setmetatable(t, mt)

t[2] = ‘hello‘
print(t[2])

local index = {}

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

    __newindex = function (t, k , v)
        print("*update of element " .. tostring(k) .. " to " .. tostring(v))
        t[index][k] = v
    end
}

track = function (t)
    local proxy = {}
    proxy[index] = t
    setmetatable(proxy, mt)
    return proxy
end

t = track(t)

function readOnly(t)
    local proxy = {}
    local mt = {
        __index = t,
        __newindex = function (t, k, v)
            error("attemp to uopdaate a read-only table", 2)
        end
    }

    setmetatable(proxy, mt)
    return proxy
end

days = readOnly{"Sunday", "Monday", "Tuesday", "Wednesday","Thursday", "Friday", "Saturday"}
print(days[1])
days[2] = "Noday"