Array of c++ objects that Lua can access and call member functions

Question

I am pulling my hair out here, I can share an array between Lua and c++, I can even create an array of objects in Lua (using the code below) and access their member functions (eg obj[10]:setPosition(0,0,0) ). What I can't do is send c++ objects to Lua that already exist and have Lua call their respective member functions. For example:

objects = Scene.getAllObjects()
objects[5]:setPosition(0,0,0)

...doesn't work, however, The code below works

for i=1,10 do
  objects[i] = Object.new("Box")
  objects[i]:setPosition(0,0,0)
end

...which calls the c++ function below

int luaAddNewEditableObject(lua_State * L)
{
    const char * name = luaL_checkstring(L, 1);

    EditableObject ** udata = (EditableObject **)lua_newuserdata(L, sizeof(void*));
    EditableObject *obj = scene->addNewEditableObject(name);
    *udata = obj;

    luaL_getmetatable(L, "luaL_EditableObject");

    lua_setmetatable(L, -2);

    return 1;
}

So basically, if it's created in Lua then no problems, but if there are objects that already exist in c++ then I need to get it into a Lua table/array so Lua can do magic to them

Please Help

struct ObjectArray 
{
    int size;
    EditableObject *objects;  /* The cpp objects */
};

static int getAllObjects (lua_State *L) 
{
    //This creates the metatable for array notation
    size_t nbytes = sizeof(ObjectArray) + numObjects * sizeof(ObjectArray*);
    ObjectArray *objectArray = (ObjectArray*)lua_newuserdata(L, nbytes);
    objectArray->size = numObjects;

    for (int i = 0; i < numFoos; i++)
    {
            //This sets the c++ pointers to the lua_newuserdata 
                objectArray->objects[i] = objects[i];

        //So maybe here I need to assign the 'luaL_EditableObject' metatable for each object
        //so I can call it's member functions ??
    }
    luaL_getmetatable(L, "ObjectArray");
    lua_setmetatable(L, -2);

    return 1; 
}

Can you clarify how it doesn't work? Do you get an error? Does it crash? What happens? What does Scene.getAllObjects return? Are you sure it's returning a table of your objects (eg. if you compare metatable of the udata returned by Scene.getAllObjects is it the same as Object.new "Box")? — greatwolf
– greatwolf, Commented Dec 31, 2013 at 8:57
The metatable is as follows void registerEditableObject() { luaL_Reg regs[] = { { "new", luaAddNewEditableObject }, { "setPosition", luaSetObjectPosition }, { NULL, NULL } }; luaL_newmetatable(L, "luaL_EditableObject"); luaL_register(L, NULL, regs); lua_pushvalue(L, -1); lua_setfield(L, -1, "__index"); lua_setglobal(L, "Object"); } — Ken MacLean
– Ken MacLean, Commented Jan 1, 2014 at 0:58
getAllObjects() should return a table/array of what Object.new("Box") returns, but of already existing objects. But I have no idea of how to go about it. Thanks in advance — Ken MacLean
– Ken MacLean, Commented Jan 1, 2014 at 1:08
To clarify further, Object.new("Box") returns a metatable of a single object but what I need for getAllObjects() is a metatable of object metatables... if that makes sense — Ken MacLean
– Ken MacLean, Commented Jan 1, 2014 at 1:14
Can you add the C++ snippet for Scene.getAllobjects to your question? — greatwolf
– greatwolf, Commented Jan 1, 2014 at 3:20

Ken MacLean · Accepted Answer · 2014-01-01 22:21:02Z

Ok I figured it out and will answer my own question in case anyone else has had this problem

extern "C"
{
#include <lua.h>
#include <lauxlib.h>
#include <lualib.h>
}

#include <map>

class Object 
{
public:
    Object()
    {
        x = 0;
        y = 0;
    }
    Object(int x, int y)
    {
        this->x = x;
        this->y = y;
    }
    int x, y;
};


std::map<std::string,Object> objects;

static int luaGetObjects(lua_State *L)
{
    lua_newtable(L);
    int i=0;
    for (auto it = objects.begin(); it != objects.end(); it++, i++) 
    {

        lua_pushstring(L, it->first.c_str());

        luaL_getmetatable(L, "luaL_Object");
        lua_setmetatable(L, -2);

        lua_rawseti(L, -2, i+1);
        stackDump(L);
    }

    return 1;   
}

static int luaSetPosition(lua_State* L) 
{
    const char* key = luaL_checkstring(L,1);

    int x = luaL_checknumber(L,2);
    int y = luaL_checknumber(L,3);

    objects[key].x = x;
    objects[key].y = y;

    return 0; 
}

static int luaGetPosition(lua_State* L) 
{
    const char* key = luaL_checkstring(L,1);

    lua_pushnumber(L, objects[key].x);
    lua_pushnumber(L, objects[key].y);

    return 2; 
}

void registerObject(lua_State *L)
{
    luaL_Reg regs[] =
    {
        { "setPosition", luaSetPosition },
        { "getPosition", luaGetPosition },
        { NULL, NULL }
    };

    luaL_newmetatable(L, "luaL_Object");
    luaL_register(L, NULL, regs);
    lua_pushvalue(L, -1);
    lua_setfield(L, -1, "__index");
}

int main()
{
    lua_State * L = luaL_newstate();
    luaL_openlibs(L);

    lua_pushcfunction(L, luaGetObjects);
    lua_setglobal(L, "getObjects");

    registerObject(L);

    objects["id001"] = Object(1,2);
    objects["id002"] = Object(3,4);
    objects["id003"] = Object(5,6);

    int erred = luaL_dofile(L, "hello.lua");
    if(erred)
        std::cout << "Lua error: " << luaL_checkstring(L, -1) << std::endl;

    lua_close(L); 

    return 0;
}

The 'hello.lua' code:

objects = getObjects()

for i=1,#objects do
   x,y = objects[i]:getPosition();
   print(string.format("object[%d] x = %d y = %d",i,x,y))

   objects[i]:setPosition(x*100, y*100);

   x,y = objects[i]:getPosition();
   print(string.format("object[%d] x = %d y = %d after mul",i,x,y))
end

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