Lua中设置table为只读属性的方法详解

转载  2017-07-07   作者:vanishfan   我要评论
这篇文章主要给大家介绍了关于Lua中设置table为只读属性的方法,文中通过示例代码介绍的非常详细,对大家具有一定的参考学习价值,需要的朋友们下面来一起看看吧。
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:
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 ===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 总结 以上就是这篇文章的全部内容了,希望本文的内容对大家的学习或者工作能带来一定的帮助,如果有疑问大家可以留言交流,谢谢大家对脚本之家的支持。

最新评论

利发国际手机客户端