RE: [5.0] trying to auto init a nil field on first ref...

["Milano Carvalho" <milanogc@...>]

Lua is very flexible! You can achieve this behavior easily:

meta = {__index = {top = 0, bottom = 0, left = 0, rigth = 0}}
r = setmetatable({}, meta)

print(r.top)
r.top = r.top + 10
print(r.top)

Milano

> From: Ando Sonenblick <ando@spritec.com>
> Reply-To: Lua list <lua@bazar2.conectiva.com.br>
> To: Lua list <lua@bazar2.conectiva.com.br>
> Subject: [5.0] trying to auto init a nil field on first ref...
> Date: Thu, 11 Dec 2003 20:15:22 -0800
>
> Gang,
>
> I have a table that represents a rectangle.  Normally it'd be set up like:
>
> r = {
>    top = 0
>    left = 0
>    right = 100
>    bottom = 100
> }
>
> so that if I then:
>
> r.top = r.top + 10
> print(r.top)
>
> I get "10"
>
> What I'd like to do however is have the four fields, if nil when 
> referenced, auto default to zero.  I've been trying to get this with 
> __index and __newindex in a metatable, but I can't seem to find a way.
>
> In other words, say I do this:
>
> r = {}
> r.top = r.top + 10
> print(r.top)
>
> I want want lua to successfully realize that r.top is nil and instead of 
> throwing an error, auto init r.top to zero, add 10 to it, and then print 
> the result
>
> Is this possible?  I'm staring to confuse myself now more than gain any 
> clarity....
>
> Thx mucho,
> Ando
>
> -----------------------
> Ando Sonenblick
> SpriTec Software
> www.spritec.com

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