Question

I want to have C++ objects that I can read/write in both C++ & Lua.

I have looked at: http://www.lua.org/pil/28.html

However, I do not like that solution, since my objects have constructors & destructors (and they are important as I use RAII and these take care of reference counts).

What I don’t like in the PIL solution is that the object is allocated in Lua’s heap.

What i want instead, is to create hte C++ object on my own, and just have lua have a way to do get/set on them.

Does anyone have a good tutorial/link on this?

Tanks!

Answer 1

One option is to use a light userdata, which allows you to allocate the object on the C++ heap. See the documentation for lua_pushlightuserdata. Unfortunately a light userdata has no metatable. Even if you are willing to access it using Lua functions to do get/set you still have to do something like this:

static int myget(lua_State *L) {
   myclass thing = lua_touserdata(L, 1);
   lua_pushnumber(thing::getsize());
   return 1;
}

Unfortunately, because it’s a light userdata, there’s no real way to make this operation type-safe—all light userdata are treated the same, and they have no metatable.

The better solution is to allocate a full userdata on the Lua heap, with a proper metatable, whose contents are a single pointer to the object allocated on the C++ heap. Then you can follow the model in Programming in Lua. For examples you can look at the Lua io library to see how FILE * is handled. This way you can write your C interfaces using the luaL_checkudata function, and they will be safe, but you still will have the right to allocate your objects on the C++ heap instead of on the Lua heap.