Re: garbage collector question

[Rici Lake <lua@...>]

On 22-Aug-05, at 10:58 AM, Alberto Arri wrote:

>  I'm developing a C++ program which uses lua as scripting language. I 
> need lua to interact with C++ classes, so i'm using full user data of 
> the size of a pointer to store the address of the object. Now 
> sometimes i have to pass objects created in my c code to lua 
> functions; as far as i have understood, the gc will from this moment 
> keep track of these objects and try to free them when they aren't in 
> use anymore by lua. But they are still in use by my program, which 
> will free them on his own when it's the time.

Keeping a Lua userdata containing a pointer to an object is different 
from allocating the object directly in a Lua userdata. Lua will 
automatically free the storage it allocates; in the former case, that 
means it will free the pointer; in the latter case, it means it will 
free the object.

Before doing that, Lua will also run the __gc metamethod, if it finds 
one in the userdata's metatable. Unless you've set up a __gc metamethod 
which calls free() on the pointer in the userdata, the original object 
pointed to will be untouched. That's probably what you wanted.

However, there are a couple of things to think about:

1) What happens if you delete an object which the Lua script still has 
a reference to?

2) Do you want to cache userdata for object pointers, or are you happy 
with creating a new one each time? (You may need to cache if object 
identity is important, or if you're likely to create a lot of 
references to the same C++ object).