Re: Overriding assignment to do userdata copy instead of reference

[Geoff Richards <Ni1phahs@...>]

On Wed, Sep 24, 2008 at 11:28:11AM -0700, Matthew Armstrong wrote:
> ...
> 
> There are methods to get/set _function_ environments, but no
> corresponding function for userdata.  The lua reference manual says
> there's a C API function for userdata environments, but I don't see
> it.  Can you point me in the right direction?

You just use the same functions: lua_getfenv, lua_setfenv.  These work
with either a function or userdata in the stack position you point to.
In either case the environment has to be a table, or it can be nil for a
userdata (the default).

I've used this with a DateTime module I've written (not released yet,
I'm fine tuning the API).  There are C structs called DateTime and
TimeZone, both of which are stored in userdatas.  A DateTime points to a
TimeZone object.  I have a C pointer in the DateTime structure, because
that's convenient and efficient for accessing the C time zone data, but
DateTime userdatas also have an environment that points to the time zone
so that I can retrieve the userdata object if someone calls the
'get_time_zone' method or whatever, and that also ensures that Lua won't
collect the time zone while it's being used.

You have to wrap a userdata in a table to keep it in an environment
(e.g., just have an array with one element).  And of course you'll need
'__gc' metamethods for the userdatas to free up the structures (or C++
objects, makes no difference).

-- 

--- Geoff Richards -------------><-------------- http://ungwe.org/ ---
"I tried to fling my shadow at the moon,
 The while my blood leapt with a wordless song."  --  Theodore Roethke