Re: Weird __newindex behaviour

["Liam Devine" <liamdevine@...>]

On 22/09/15 16:41, Scott Morgan wrote:
> 
> Seeing the following behaviour on various version of Lua:
> 
> Lua 5.2.2  Copyright (C) 1994-2013 Lua.org, PUC-Rio
> ~ mt = { __newindex = function(t,k,v)
> ~~ print("Setting :", k, "to", v)
> ~~ rawset(t,k,v)
> ~~ end }
> ~ tab = setmetatable({},mt)
> ~ return tab
> table: 0x671df0
> ~ return tab.test
> nil
> ~ tab.test = 123
> Setting :	test	to	123
> ~ tab.test = nil
> ~ tab.test = nil
> Setting :	test	to	nil
> ~ tab.test = nil
> Setting :	test	to	nil
> ~ tab.test = 321
> Setting :	test	to	321
> ~ return tab.test
> 321
> ~ tab.test = nil
> ~ return tab.test
> nil
> ~ tab.test = nil
> Setting :	test	to	nil
> 
> Why isn't the first `tab.test = nil` event being handled by the
> metatable? As mentioned, I'm seeing this across Lua versions, 5.1, 5.2
> (as above) and 5.3 (and different OS's/compilers)
> 
> It's weird because, surely, if this is a bug, it has already been seen.
> It's such a common pattern. I must be doing something wrong here?
> 
> Scott
> 

Nothing weird or even a bug present here, it is your understanding of
__newindex which is incorrect.

""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. "

At the first call to setting the key's associated value to nil, the key
already exists and has an associated value of 123.

-- 
Liam

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