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Hello Vyacheslav,

when you sue userdata, the Lua VM will allocate (and free) the memory.
You may possibly go better with light userdata, which is actually a pointer to something you maintain. __gc will still be called when the Lua VM does no longer references the data, but it will not free then memory. This way you get the chance to let your objects live longe than Lua uses them, and share the same object among several variables (even in different Lua VMs if you handle concurrency correctly)


Am 07.01.2015 um 15:42 schrieb Vyacheslav Napadovsky:

I'm trying to integrate lua into my program. I have a bunch of objects and their userdata's. Each object's userdata only holds a pointer to a C++ object (4 bytes on a 32-bit OS). I use the reference counting system to determine when to delete the C++ objects (Addref/Release)
(The lifetime of C++ object is longer than Lua userdata)

Every C++ object has its own userdata in Lua. I create userdatas when needed (and call Addref on the C++ object). Moreover, if I want to load C++ object into Lua once more I need to use the same userdata (to make '==' operator work correctly). For this purpose I've created a table in LUA_REGISTRYINDEX.
The table has __mode=='v', and when I load object into Lua I check
if (table[this] ~= nil) then -- actually I use lua_rawgetp(L, table_idx, this) in C++ code
        I use userdata from this table
        I create lua_newuserdata and save it in this table (lua_rawsetp)

When Lua's userdata __gc metamethod is called I have to decrement the object reference (Release) and (probably) clear the reference for the userdata from the table: table[this] = nil.

The question is: should I do this:  table[this] = nil ?
And is that overall a correct way to propagate C++ objects to Lua?

P.S. If I do (table[this] = nil), I sometimes get 2 userdatas pointing to the same C++ object. P.P.S. How to get to know that __gc metamethod has been called for this particular userdata from C code?

I use Lua 5.2.3.

Best regards,