t+1 - Latest Comments in Lua metatable examples

Re: Lua metatable examples

williamrobert30 — Thu, 08 Oct 2009 06:11:51 -0000

Thanks for posting Lua metatable example.I was looking for it..........

Re: Lua metatable examples

Matt Wilson — Mon, 01 Jun 2009 17:57:35 -0000

Wow -- that 3d engine sounds really neat! Thanks for the tip. I will
have to look at that.

Re: Lua metatable examples

zslevi — Mon, 01 Jun 2009 17:02:26 -0000

I don't know whether you heard about it, but there's a nice opensource 3d engine using Lua. (You don't have to write C code at all.) Physics simulation is integrated as well (ODE). It's called Apocalyx. It ships with a handful of demos. (They're quite straightforward, you can learn the usage of the engine just by modifying them.) It's very easy to use, and that is the main reason I'm learning Lua.

Re: Lua metatable examples

zslevi — Mon, 01 Jun 2009 16:40:59 -0000

It produces the following output:

apple green
------
asas
green

Re: Lua metatable examples

zslevi — Mon, 01 Jun 2009 16:39:59 -0000

I have experimented a little bit with metatables, and found it intresting that the values in metatables doesn't show up in foreach.

a={apple="green"}
b={cucc="asas"}
setmetatable(a, {__index = b})
for k,v in pairs(a) do print(k,v) end
print("------")
print(a.cucc)
print(a.apple)

Re: Lua metatable examples

Matt Wilson — Mon, 01 Jun 2009 15:11:34 -0000

Oh -- sorry, I don't know of any other examples.

Re: Lua metatable examples

zslevi — Mon, 01 Jun 2009 14:02:59 -0000

I meant double indirection in Lua.

Re: Lua metatable examples

Matt Wilson — Mon, 01 Jun 2009 10:07:26 -0000

Double indirection shows up all over the place. In this case, a failed lookup on the first table goes to the metatable. That's the first indirection. The second indirection happens when the metatable uses the __index to handle the lookup.

Filesystems that use inodes can use double indirection too. That might be a good place to study it.

Re: Lua metatable examples

zslevi — Sun, 31 May 2009 21:49:49 -0000

Can you name other uses of double indirection, or it is restricted to metatables?

Re: Lua metatable examples

Matt Wilson — Sun, 31 May 2009 16:52:07 -0000

Yeah, I wrote this post after spending many hours playing with
metatables until I finally felt like I understood them. It took me a
long time before the double-indirection seemed logical.

Re: Lua metatable examples

zslevi — Sun, 31 May 2009 13:39:03 -0000

I checked out your link and now it seems more clear:
"
The use of the __index metamethod for inheritance is so common that Lua provides a shortcut. Despite the name, the __index metamethod does not need to be a function: It can be a table, instead. When it is a function, Lua calls it with the table and the absent key as its arguments. When it is a table, Lua redoes the access in that table.
"

Re: Lua metatable examples

zslevi — Sun, 31 May 2009 13:35:20 -0000

Wow, it still seems a bit voodoo. It seems, as if a metatable were a function.