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Am 21.05.2014 03:01 schröbte Tim Hill:
On May 20, 2014, at 5:15 PM, Eric Wing <ewmailing@gmail.com> wrote:I am trying to create a simple strong reference (Object A holds a reference to Object B) such that: 1) Object A keeps Object B alive (if all other references are disposed) 2) B outlives A I'm seeing #1, but #2 is not working. I am using userdata and using set/getuservalue in Lua 5.2/5.3w2 and basically putting objectB in objectA's user value table. (My specific implementation has Object A keep an array of objects it keeps strong references to.)You cannot make assumptions about the order of GCing of unreferenced objects. Once object A is unreachable, then if the only references to B are via A (and hence B is also unreachable), both are GC candidates and can be collected in any order.
They are collected in the reverse order that they were marked for finalization[1] (usually by setting the metatable), so B will always be collected before A if both become garbage at the same time, because it was created/marked last.
The only real solution is to change the order in which they are created, but you can try to decouple the finalization of the two objects via `luaL_ref`: You `luaL_unref` the B object in A's `__gc` method. The registry has a strong reference to B until A's finalizer has run, so B's finalizer will run in the next cycle. *But* this won't work at `lua_close` when the registry becomes garbage as well, all strong references are gone, and the usual finalization order applies ...
If you really need cleanup to occur in a defined order you will need to either release the reference to B first or make the __gc() metamethods co-ordinate the ordering of cleanup. —Tim
Philipp [1]: http://www.lua.org/manual/5.2/manual.html#2.5.1