lua-users home
lua-l archive

[Date Prev][Date Next][Thread Prev][Thread Next] [Date Index] [Thread Index]

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).