Lua metamethods not being called
I'm kinda new to Lua (not really done much with it yet) and I'm trying to wrap my mind around metatables. I have had them working before but now (after a number of months) I have encountered something really weird.
What should this script print when run?
__mt = {}
__mt.__index = function(table, key)
print("In __index")
return 99
end
test = {}
test.x = 5
setmetatable(test, __mt)
print(test.x)
Personally, I would expect it to print "In __index" (from the metamethod) followed by 99. However, whenever I run it I get 5. Nothing I do can get the index metamethod to run. It just acts like I'm using rawget()
instead.
Curiously, adding
print(getmetatable(test).__index(tes开发者_StackOverflowt, "x"))
will do the right thing. The metatable is there, __index()
is correct, it just isn't being called.
Is this a bug or am I just doing something stupid? I can't tell.
The metamethod (in old terminology also called fallback) called __index
is only called if the key x
does not exist in the table, when you access t.x
. Try print(t.y)
instead!
Added: Yes, using a proxy table.
function doubletable(T)
local store = T or {}
local mt = {}
mt.__index = function (t, k) return store[k] and 2*store[k] end
mt.__newindex = store
return setmetatable({}, mt)
end
t = doubletable({a=1, b=3})
t.c = 7
print(t.a, t.b, t.c)
-- output: 2 6 14
精彩评论