Re: Lua and C++ memory

[Matt Campbell <mattcampbell@...>]

Memory for userdata is allocated and freed by Lua.  There's nothing 
special about userdata in tables, at least from the perspective of the 
Lua C API.  Lua has no knowledge of C++ constructors and destructors, 
but it does support __gc metamethods in the metatables of userdata.  So 
to wrap an instance of a class called Foo in Lua userdata, you'd write 
code like this:

Foo **pp = (Foo *) lua_newuserdata(L, sizeof(Foo *));
*pp = new Foo;

Then you'd set the new userdata's metatable to a table with a __gc 
metamethod like this:

static int foo_gc(lua_State *L) {
  Foo **pp = (Foo **) lua_touserdata(L, 1);
  if (*pp) {
    delete *pp;
    *pp = NULL;
  }
  return 0;
}

Actually, to be foolproof, any C functions which take userdata as 
arguments, including __gc metamethods, should check for the correct 
metatable.  But that's beside the point here.

Your C++ code need not be concerned about how many references to the 
userdata are passed around within Lua.  Just implement a __gc metamethod 
as shown above, and Lua will take care of the rest.

Matt