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Help with setmetatable and _G?

Asked by

CodingEvolution 490

9 years ago

Trying to lock a variable (and it's value) in _G, by making a new environment and resetting a changed _G variable back to default, if attempted to change it.

01-- CodingEvolution
02-- Locking variables (and values) in _G
03 
04_G={} -- New _G
05_G.x=1
06_G.y=2
07 
08setfenv(1,setmetatable({},{
09    __index=function(t,k)
10        if k=='_G' then
11            return setmetatable({},{
12                __newindex=function(t,k,v)
13                    if k=='x' then
14                        print'This value is locked; changing back to default.'
15                        rawset(t,k,_G[k]) -- should set back to 1, but doesn't?
16                    else
17                        print'changing global...'
18                        rawset(t,k,v) -- again, should return normal. but returns nothing (nil).
19                    end
20                end
21            })
22        end
23    return getfenv()[k] -- If not _G, return old environment and index.
24    end
25}))
26 
27print(_G.y) --> "nil" shouldn't it be 2?
28 
29_G.x='Changed' --> as expected, it triggers the __newindex function.
30 
31print(_G.x) --> but even then, it still returns nil.
32 
33-- I have to be somewhat close, because it does detect when and if a value in _G changed. It just
34-- Doesn't do anything about it. 
 View all 34 lines...

Shouldn't rawset return the value of _G[k] from the previous environment? If not, why? Thank you for reading.

-- EDIT: You can run this code and test it here: http://repl.it/BCF6

1 answer

Answered by

funyun 958

9 years ago

I suppose you just have to set __index to _G, since the table is empty.

01_G={}
02_G.x=1
03_G.y=2
04 
05setfenv(1,setmetatable({},{
06    __index=function(t,k)
07        if k=='_G' then
08            return setmetatable({},{
09                __newindex=function(t,k,v)
10                    if k=='x' then
11                        print'This value is locked; changing back to default.'
12                        rawset(t,k,_G[k])
13                    else
14                        print'changing global...'
15                        rawset(t,k,v)
16                    end
17                end,
18 
19                __index=_G, --See? Index is _G.
20                __metatable = "This metatable is locked." --Lock up that metatable!
21            })
22        end
23    return getfenv()[k]
24    end
25}))
26 
27print(_G.y)
28 
29_G.x='Changed'
30 
31print(_G.x)
 View all 31 lines...

Thank you very much sir. CodingEvolution 490 — 9y

Help with setmetatable and _G?

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