Re: Lunar, C++, and premature object destruction

[Watusimoto <watusimoto@...>]

Sam,
> One pattern for avoiding overcreation of userdata is to ensure you
> never have more than 1 userdata (luaship)
> for each C++ object (c++ship).
>
> When creating a luaship, store it in a global weak table, indexed by
> lightuserdata of the c++ship's pointer value.
>
> Before creating a luaship for a c++ship, look up the pointer value in
> the global table, and return the preexisting luaship if possible, only
> creating a new luaship if necessary.
>
>   
This is an interesting idea, and will probably work well for my 
situation.  I have been thinking of ways to do this on the C++ side, but 
hadn't considered using weak tables on the Lua side to fit it all 
together.  I'm assuming that the overhead of asking Lua to look up a 
value in a table is much lower than that of creating a relatively 
lightweight object directly in C++ (then garbage collecting it later).  
Presumably the weak table would have weak keys only.

So if my current code were:

// C++, in Ship
LuaShip *Ship::getLuaShip() {
  return new LuaShip(this);
}

The new code might be:

LuaShip *Ship::getLuaShip() {
  LuaShip *luaship = findShipInWeakTable(L, this);   // Use this as 
"key" to find LuaShip in Lua table
  if(luaship) {
     return luaship;
  } else {
     luaship = new LuaShip(this);
     storeShipInWeakTable(L, this, luaship );   // Store luaship in Lua 
table, using "this" as key
     return luaship;
  }
}

> In the c++ship's delete, you can then look up any possibly existing
> luaship that might have a reference to it, and both zero the pointer
> in the userdata, as you are now, and also delete the pointer->luaship
> mapping from the global weak table.
>   
Would I need to manually delete the pointer from the weak table, or 
would gc take care of that once the ship has been deleted?  I suppose it 
wouldn't, as that's an analogous situation to the problem I'm trying to 
solve.
> Lots of variations on this are possible, but they key is using weak
> tables with lightuserdata as keys to maintain a mapping from the C++
> world to the lua world.
>
> Sam
>
>   
Thanks for the good advice. 

Chris