Special indices (was Re: LightUserData and metatables)

[Mark Hamburg <mark@...>]

On Oct 12, 2009, at 9:01 AM, Mark Hamburg wrote:

> Though note that depending on how you do lifetime management, you  
> may actually want to use a weak table mapping light userdata  
> pointers to full userdata proxies.
>
>
> You could even use a metatable on this mapping table to do the proxy  
> construction thereby allowing one to write:
>
> 	pushMappingTable( L );	/* e.g., fetch it from the registry */
> 	lua_pushlightuserdata( L, p );
> 	lua_gettable( L, -2 );	/* Fetch or construct the proxy. */
> 	lua_remove( L, -2 );	/* Remove the proxy table. */
>
> If you can store the proxy table as the environment to your C code  
> or as an upvalue, then it becomes even simpler. For example:
>
> 	lua_pushlightuserdata( L, p );
> 	lua_gettable( L, LUA_ENVIRONINDEX );

Which incidentally makes me long a bit for a few more special indices  
as much for code convenience as performance. For example, when  
building full userdata objects that use their environment tables to  
link to other Lua objects, a LUA_SELF_ENVIRONINDEX would save one the  
trouble of fetching the environment onto the stack and tracking its  
position. That's obviously not a huge burden, but it makes short  
method implementations a bit harder to follow. In the example here, a  
special index for a global weak table would help.

That said, I don't have a firm proposal on any of these and its a  
slippery slope. I'll just note that LUA_ENVIRONINDEX wasn't  
technically needed given lua_getfenv but it does make using the  
environment table a lot more natural.

Mark