Re: C-objects with __gc *and* __index?

[Rici Lake <lua@...>]

On 6-Aug-05, at 8:23 PM, Uli Kusterer wrote:

>  yes, looks like this was unnecessary, because this still doesn't 
> change anything. Still the same error message. The code I'm running 
> is:
>
> for i=100,1,-1 do
>     aRoom = Room.new()
>     print( tostring( aRoom ) )
>     aRoom = nil
> end

Any particular reason you didn't choose to write:

for i = 100, 1, -1 do
  local aRoom = Room.new()
  print(tostring(aRoom))
end

-- or even
for i = 1, 100 do print(tostring(Room.new())) end
-- ;)

Leaving that aside, Room.new() must contain the following:

  RoomData *self = lua_pushuserdata(L, sizeof(RoomData));
  luaL_getmetatable(L, "PACEmaker.Room");
  lua_setmetatable(L, -2);

(with appropriate substitution of your xHungarianNamingConvention.) If 
you don't have this code, or something that looks like it, that's why 
it's not working. If you're using a lightuserdata rather than a 
pushuserdata, that is why it's not working (light user data don't have 
metatable). If none of the above is the reason, try checking the return 
values from luaL_getmetatable and lua_setmetatable, and perhaps you'll 
get a hint.

Since you mention c++ constructors, there are two common ways of 
dealing with the RoomData object.

The "simplest" (or at least most common) is to simply use a pointer to 
the actual object; the statement after self = lua_pushuserdata(...) 
would be *self = new Room; and RoomData would either be typedef'd or 
replace with Room*, depending on your style preferences.

The more elegant way, which works as long as you're ok with letting Lua 
manage your memory for you, is to use "placement new" syntax, in which 
case the self = statement would look something like this:

Room self = new(lua_pushuserdata(L, sizeof(Room)) Room(...);

One of the nice things about placement-new is that the __gc method will 
typically simply be an explicit call to the class destructor; 
furthermore, it can be omitted completely under a specific but not 
uncommon case, where no member variable of the class needs to be 
destructed. The placement expression can be supplied by a template 
function if you're careful; I think there is an example on the Lua 
Wiki.

R.