Binding C++ objects using Lua tables, light userdata vs. __gc?

[Carsten Fuchs <CarstenFuchs@...>]

Dear list,

in the Game Programming Gems 6, chapter 4.2 about binding C/C++ objects to Lua, the chapter presents 
"Binding Host Objects to Lua Tables" as the last binding method.

It essentially says that when a new C/C++ object is to be made available in Lua,
	a) a new Lua table is created,
	b) The "_pointer" field within this table is set to a light userdata item with the pointer to the 
C/C++ object,
	c) The metatable corresponding to the type of this object is set for this table,
	d) an entry into the environment table is added (for uniqueness checking).


Well, what I don't understand is why a *light* userdata item is used here, rather than a full 
userdata, when thinking about the automatic garbage collection of the C/C++ object.

If I understood the text correctly, the metatable from step c) above has a __gc method properly 
assigned, but this metatable is now assigned to a *table* rather than to full userdata as in the 
prior section of the book.

Now, the PiL2 says that __gc only works for userdata, not for tables, and that metatables cannot be 
set for light userdata anyway. Thus - if I understand this correctly - the C/C++ object in this 
binding method is never collected, because the __gc method is never called.

Therefore, I would kindly like to ask you for your comments: Is my understanding and conclusions 
correct? Is the problem properly solved by employing full rather than light userdata (and assigning 
the metatable with the __gc method also to that userdata)?

Thank you very much,
and best regards,
Carsten




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Carsten Fuchs    http://www.Ca3D-Engine.de/c_Carsten.php