Lua 设置table为只读属性

如果转载，请注明博文来源：http://www.cnblogs.com/vanishfan/p/6909153.html  望各位支持！

项目中部分只读表易被人误改写，故决定在非线上环境里对这些表附加只读属性，方便在出现误改写的时候抛出lua错误，最终版代码如下：

--[[------------------------------------------------------------------------------
-** 设置table只读 出现改写会抛出lua error
-- 用法 local cfg_proxy = read_only(cfg)  retur cfg_proxy
-- 增加了防重置设置read_only的机制
-- lua5.3支持 1）table库支持调用元方法，所以table.remove table.insert 也会抛出错误，
--               2）不用定义__ipairs 5.3 ipairs迭代器支持访问元方法__index，pairs迭代器next不支持故需要元方法__pairs
-- 低版本lua此函数不能完全按照预期工作
*]]
function read_only(inputTable)
    local travelled_tables = {}
    local function __read_only(tbl)
        if not travelled_tables[tbl] then
            local tbl_mt = getmetatable(tbl)
            if not tbl_mt then
                tbl_mt = {}
                setmetatable(tbl, tbl_mt)
            end

            local proxy = tbl_mt.__read_only_proxy
            if not proxy then
                proxy = {}
                tbl_mt.__read_only_proxy = proxy
                local proxy_mt = {
                    __index = tbl,
                    __newindex = function (t, k, v) error("error write to a read-only table with key = " .. tostring(k)) end,
                    __pairs = function (t) return pairs(tbl) end,
                    -- __ipairs = function (t) return ipairs(tbl) end,   5.3版本不需要此方法
                    __len = function (t) return #tbl end,
                    __read_only_proxy = proxy
                }
                setmetatable(proxy, proxy_mt)
            end
            travelled_tables[tbl] = proxy
            for k, v in pairs(tbl) do
                if type(v) == "table" then
                    tbl[k] = __read_only(v)
                end
            end
        end
        return travelled_tables[tbl]
    end
    return __read_only(inputTable)
end

测试代码如下：

local t0 = {k = 1}
local t2 = {
    fdsf = {456}
}
local t1 = {
        a = {456, 89},
        b = {456,ddss = 9, t2 = t2},
        d = 45,
        e = "string",
}
t1.c=t1

local t3 = read_only(t1)

print(t3.d, t3.c.e, t3.c.c.b.t2.fdsf)
function q1() t3.d = 4555 end
function q2() t3.c.d = 90 end
function q3() t3.c.c.b.t2.fdsf =90 end
function q4() table.remove(t3.a) end
function q5() t3.b[ddss] = nil end
function q6() t3[f] = 89 end
function q7() table.insert(t3.a, 999) end

print(pcall(q1))
print(pcall(q2))
print(pcall(q3))
print(pcall(q4))
print(pcall(q5))
print(pcall(q6))
print(pcall(q7))
print(t3.a[1])
for k,v in pairs(t3) do
    print("===pairs t3:",k,v)
end
for k,v in pairs(t3.a) do
    print("===pairs t3.a:",k,v)
end
for k,v in ipairs(t3) do
    print("===ipairs t3:",k,v)
end
for k,v in ipairs(t3.a) do
    print("===ipair t3.a",k,v)
end
print("len t3:",#t3)
print("len t3.a:", #t3.a)

local t4 = read_only(t2)

local t5 = read_only(t0)
local t6 = read_only(t0)

print(t3.b.t2, read_only(t2))
print(t5, t6, t0)

测试环境https://www.lua.org/cgi-bin/demo  lua5.3.4：

45    string    table: 0x20d4ba0
false    input:17: error write to a read-only table with key = d
false    input:17: error write to a read-only table with key = d
false    input:17: error write to a read-only table with key = fdsf
false    input:17: error write to a read-only table with key = 2
false    input:17: error write to a read-only table with key = nil
false    input:17: error write to a read-only table with key = nil
false    input:17: error write to a read-only table with key = 3
456
===pairs t3:    e    string
===pairs t3:    b    table: 0x20ccd60
===pairs t3:    a    table: 0x20d4e70
===pairs t3:    d    45
===pairs t3:    c    table: 0x20ca700
===pairs t3.a:    1    456
===pairs t3.a:    2    89
===ipair t3.a    1    456
===ipair t3.a    2    89
len t3:    0
len t3.a:    2
table: 0x20d4870    table: 0x20d4870
table: 0x20d5690    table: 0x20d5690    table: 0x20d1140

代码思路设计：

1.使用proxy={}空表而不是目标表tbl来设置__newindex是因为__newindex必须在原表里面不存在才会调用，这样就依然可以对已存在的字段进行改写

__newindex: The indexing assignment table[key] = value. Like the index event, this event happens when table is not a table or when key is not present in table. The metamethod is looked up in table.

Like with indexing, the metamethod for this event can be either a function or a table. If it is a function, it is called with table, key, and value as arguments. If it is a table, Lua does an indexing assignment to this table with the same key and value. (This assignment is regular, not raw, and therefore can trigger another metamethod.)

Whenever there is a __newindex metamethod, Lua does not perform the primitive assignment. (If necessary, the metamethod itself can call rawset to do the assignment.)

2.避免出现table的互相引用，加入travelled_tables存储已经设置过proxy的table的映射

3.对于原表tbl的访问使用__index=tbl

4.对于表查长度使用__len= function () return #tbl end

5.对于遍历pairs，查到lua5.3的pairs默认迭代器next不支持访问元表__index，故直接__pairs = function () return pairs(tbl) end，以此来生成对目标表的迭代遍历

6.对于ipairs，查到lua5.3 ipairs函数生成的迭代器默认就支持访问元表__index，故不需要添加__ipairs

8.2 – Changes in the Libraries

• The ipairs iterator now respects metamethods and its __ipairs metamethod has been deprecated.

7.对于table.insert , table.remove不用特殊处理，lua5.3的table lib支持元表操作，故依然会抛错

8.2 – Changes in the Libraries

• The Table library now respects metamethods for setting and getting elements.

8.避免重复创建read_only，每个tbl只创建一个proxy代理，在tbl的metatable里和proxy的metatable里都设置属性__read_only_proxy,可以直接访问获得