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Am 08.09.2014 um 10:31 schröbte Procházka Lukáš Ing. - Pontex s. r. o.:

Hello,


Hi!



I'm trying to understand Lunar<T> code.


In case anyone else is interested, the Lunar source code is here[1] ...



There is still one point I can't understand:

---
     static int push(lua_State *L, T *obj, bool gc=false) {
         if (!obj) { lua_pushnil(L); return 0; }
         luaL_getmetatable(L, T::className);  // lookup metatable in Lua
registry
         if (lua_isnil(L, -1)) luaL_error(L, "%s missing metatable",
T::className);*/
         int mt = lua_gettop(L);
         subtable(L, mt, "userdata", "v"); // WHY THIS???
...
---

Lunar creates a metatable for each T; that's clear.

(
This metatable has also subtable named "do not trash", which contains
keys=userdata for userdata which are to be ignored during GC cycle;
keys in "do not trash" table are weak; obviously.
)

But - why is there a subtable "userdata" with weak values?
It seems to me it is not used anywhere later, so why?
It is used later in the `pushuserdata` function to map pointers to objects (as lightuserdata) to full userdata wrapping those object pointers. This way you can reuse the full userdata and avoid memory allocation if a suitable full userdata already exists in the Lua state. The table is weak-valued so that it doesn't prevent the full userdata from being garbage collected if it isn't referenced anywhere else. 



Thanks in advance for any hint or explanation.

Best regards,

Lukas



HTH,
Philipp

  [1]: http://lua-users.org/wiki/CppBindingWithLunar