Re: When does __index metamethod get called?

[David Morris-Oliveros <david.morrisoliveros@...>]

Yep, when you call:

tbl[1] = 1

It is calling the __newindex() metamethod. Try this:

> tbl = {}
> setmetatable(tbl, {__newindex = function(t,k,v) print ("test", k, v) 
end})
> tbl[1] = 1

You'll see how it works.

Now, to track both insertions and overwriting of existing entries in the 
table, you have to use a proxy table (or something like it). Example:

tbl = {}
tbl.__proxy = {}
mt = {__index = function(t,k) return t.__proxy[k]; end,
     __newindex = function(t,k,v) print('tracked', k, v); t.__proxy[k] 
= v; end}
setmetatable(tbl, mt)

This works by making all values "stored" in tbl not actually stored 
there, so when there's an access to it (get), it'll invoke the __index 
metamethod, and retrieve it from __proxy. And when there's a setting of 
it, since it doesn't exist in tbl, but in tbl.__proxy, it'll go through 
your __newindex metamethod.


// David Morris-Oliveros
// Camera & Lua Coder
// david@teambondi.com



Chris wrote:
> Example:
>
>    tbl = {}
>    setmetatable(tbl, {__index = function(t,k) print("test") end})
>    tbl[1] = 1
>    tbl[2] = 2
>
> Prints nothing... ?? 
>
> Also, is there any metamethod that you can hook to know when a value 
> is being set?  I know there is __newindex but that only appears to be 
> called for new values being inserted into the table.  I want to hook 
> both cases of new values and also setting existing values.
>
> --
> // Chris