test_newindex.lua

local function test3_newindex2()
    print("------- test3_newindex2 -------")
    
    local t = {}
    local _t_store = {}
    
    local mt = {
        __index = function (t, k)
            print("*access to element " .. tostring(k))
            return _t_store[k]
        end,
        __newindex = function(t, k, v)
            print("Successfully set element \"" .. tostring(k) ..
                "\" as " .. tostring(v))
             --t[k] = v
             rawset(t, k, v)
        end
    }
    setmetatable(t, mt)
    t[2] = "hello"
    print(t[2])
    t[2] = "hello2" -- 此时不会再次执行__newindex！因为t已经有2了
    print(t[2])
end

local function test3_newindex1()
    print("------- test3_newindex1 -------")
    test3_newindex2()

    local t = {}
    local _t_store = {}
    
    local mt = {
        __index = function (t, k)
            print("*access to element " .. tostring(k))
            return _t_store[k]
        end,
        __newindex = function(t, k, v)
            print("Successfully set element \"" .. tostring(k) ..
                "\" as " .. tostring(v))
             _t_store[k] = v
        end
    }
    setmetatable(t, mt)
    t[2] = "hello"
    print(t[2])
    t[2] = "hello2" -- 此时会再次执行__newindex，因为t中没有2，_t_store中才有2
    print(t[2])
end