Re: LightUserData and metatables

[Jerome Vuarand <jerome.vuarand@...>]

2009/10/12 liam mail <liam.list@googlemail.com>:
> Whilst integrating C++ with Lua which enables the passing of C++ objects to
> Lua, calling methods, operators and vice versa I have encountered a problem
> with my current implementation. I use the method with is described on the
> wiki entitled Luna and which is also described in many other publications
> such as Game Programming Gems.
> The crux of my problem is that a new user data is only created when an
> object is currently not alive in Lua, when it is the pointer which was
> returned when the lua_newuserdata was created is pushed onto the stack using
> lua_pushlightuserdata and has it's metatable attached. This metatable
> defines the functions to be called for the operators such as __eq and __le;
> yet a lightuserdata is only equal to itself and does not look at the
> metatable to see if the operator is defined for the type, whilst it does
> look at the metatable for methods.
>
> Is there a way whilst still using the operators instead of say a le function
> (obj:le(rhs)) to accomplish this, or is using this type of function for the
> operator or using a table to represent the data the only method. Ie. is
> there a way to trick Lua into looking at the metatable of the lightuserdata
> for the operators without modifying the source of Lua itself?

Your first paragraph is a little confusing. I'm not sure what you did
exactly, and what userdata is full or light.

Light userdata are not meant to represent objects, but rather as a
convenience to pass C values around. If you have a C object that you
want to manipulate from Lua you should embed them in a full userdata.
You cannot push a full userdata. What you can do however is associate
a userdata with its address in some table (e.g. the registry), and
from the address you can push back the full userdata with a table
indexing operation (lua_gettable).