Re: getmetatable-esque metamethod [Forked from:] metatables for strings?

[Dirk Laurie <dirk.laurie@...>]

2015-05-03 23:39 GMT+02:00 Nagaev Boris <bnagaev@gmail.com>:
> On Sun, May 3, 2015 at 8:38 PM, Dirk Laurie <dirk.laurie@gmail.com> wrote:
>> 2015-05-03 21:24 GMT+02:00 Andrew Starks <andrew.starks@trms.com>:
>>
>>> I could not bare to triple post, and this idea is different enough
>>> that I'll take the opportunity to fork...
>>>
>>> sand boxing the string's metamethods is something that would be nice
>>> to be able to do, but it is hard to imagine that feature existing
>>> without something being treated as "special".
>>>
>>> I don't know if this mechanism would be general and useful enough for
>>> consideration, but I did think of a way that I believe could be used
>>> to solve this problem.
>>
>> It is trivial to sandbox a string's methods.
>>
>> ---
>> $ lua
>> Lua 5.3.0  Copyright (C) 1994-2015 Lua.org, PUC-Rio
>>> getmetatable"".__metatable = "Sandboxed!"
>>> string = nil
>>> str = "The quick brown fox"
>>> for k in str:gmatch"%S+" do print(k) end
>> The
>> quick
>> brown
>> fox
>
> This is exactly what I want to prevent.

So do this:

local orig_gsub = string.gsub
my_safe_gsub= function(str,patt,repl)
   if type(repl)~="string" then error"This gsub allows only string
replacement" end
   return orig_gsub(str,patt,repl)
end

safe_string_library = { find=string.find, sub=string.sub, gsub=safe_gsub,
   match = string.match, gmatch = string.gmatch }  -- or whatever

debug.setmetatable("",{__index = safe_string_library, __metatable="Sandboxed!"})
string = safe_string_library