Lua中设置table为只读属性的方法详解

技术  /  管理员 发布于 6年前   343

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=t1local t3 = read_only(t1)print(t3.d, t3.c.e, t3.c.c.b.t2.fdsf)function q1() t3.d = 4555 endfunction q2() t3.c.d = 90 endfunction q3() t3.c.c.b.t2.fdsf =90 endfunction q4() table.remove(t3.a) endfunction q5() t3.b[ddss] = nil endfunction q6() t3[f] = 89 endfunction q7() table.insert(t3.a, 999) endprint(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)endfor k,v in pairs(t3.a) do print("===pairs t3.a:",k,v)endfor k,v in ipairs(t3) do print("===ipairs t3:",k,v)endfor k,v in ipairs(t3.a) do print("===ipair t3.a",k,v)endprint("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：

string table: 0x20d4ba0false input:17: error write to a read-only table with key = dfalse input:17: error write to a read-only table with key = dfalse input:17: error write to a read-only table with key = fdsffalse input:17: error write to a read-only table with key = 2false input:17: error write to a read-only table with key = nilfalse input:17: error write to a read-only table with key = nilfalse input:17: error write to a read-only table with key = 3===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 89len t3: 0len t3.a: 2table: 0x20d4870 table: 0x20d4870table: 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 C 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 C 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

总结

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