C中的嵌套Lua元表

Question

In a 3D scene, I have an Object that has a position that I would like to move using Lua.

eg. box.position.x = 10

box has a metatable ("Object") and so has position ("Vec"). Object has __newindex and __index set to call C functions NewIndexObject and IndexObject respectively. Same with Vec ( NewIndexVec and IndexVec ).

Object has an id so it can be identified in a list that is stored in the scene and when box.position is accessed all is fine, the C function IndexObject is called and I can extract the id from the stack, it's just when box.position.x = 10 is executed 'NewIndexVec' is called and the only thing on the stack is {table, x, 10} so no way of identifying the object to change its x position.

Is there anyway of pushing values onto a local state? Help!

UPDATE: thank you for getting back to me quickly, below I have distilled the code as much as possible. If you run this code it'll appear to work but I have comments where I'm stuck, it's just getting the first object in the array but I need to choose it by it's ID, Thanks in advance

struct Obj
{
    std::string id;
    int x,y,z;
    Obj()
    {
        x = 10; y = 20; z = 30;
        id = "12345";
    }
};

//array of external objects
std::vector<Obj> objects;

int NewObject(lua_State * L)
{
    Obj obj;
    objects.push_back(obj);

    lua_newtable(L); 

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

    lua_pushstring(L, "id");
    lua_pushstring(L, obj.id.c_str());
    lua_settable(L, 1);

    lua_newtable(L); 
    luaL_getmetatable(L, "MT_Vec");
    lua_setmetatable(L, -2);

    lua_pushinteger(L, obj.x);
    lua_setfield(L, -2, "x"); 

    lua_pushinteger(L, obj.y);
    lua_setfield(L, -2, "y"); 

    lua_pushinteger(L, obj.z);
    lua_setfield(L, -2, "z"); 

    lua_setfield(L, -2, "position");



    return 1;
}

int IndexVec(lua_State * L)
{
    // How do I get the correct object so I can pass its value back
    Obj &dunnoObj =  objects[0];

    std::string key = luaL_checkstring(L,-1);
    if(key == "x")
        lua_pushinteger(L,dunnoObj.x);
    else if(key == "y")
        lua_pushinteger(L,dunnoObj.y);
    else if(key == "z")
        lua_pushinteger(L,dunnoObj.z);
    return 1;
}


int NewIndexVec(lua_State * L)
{
    // How do I know which object's value to update
    Obj &dunnoObj =  objects[0];

    std::string key = luaL_checkstring(L,-2);
    int value = luaL_checkinteger(L,-1);

    if(key == "x")
        dunnoObj.x = value;
    else if(key == "y")
        dunnoObj.y = value;
    else if(key == "z")
        dunnoObj.z = value;

    return 0;
}

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


    luaL_Reg objreg[] =
    {
        { "new", NewObject },   
        { NULL, NULL }
    };
    luaL_newmetatable(L, "MT_Object");
    luaL_register(L, 0, objreg);
    lua_setglobal(L, "Object");


    luaL_Reg reg[] =
    {
        { "__index", IndexVec },    
        { "__newindex", NewIndexVec },
        { NULL, NULL }
    };
    luaL_newmetatable(L, "MT_Vec");
    luaL_register(L, 0, reg);
    lua_setglobal(L, "Vec");


    int res = luaL_dostring(L, "box = Object.new()   box.position.x = 1000   print(box.id .. \" , \" ..box.position.x .. \" , \" ..  box.position.y .. \" , \" .. box.position.z)");
    if(res)
        printf("Error: %s\n", lua_tostring(L, -1));



    lua_close(L);

    return 0;
}

Answer 1

If I understand you correctly, you don't have to do anything. Tables are tracked by reference, so NewIndexVec doesn't need to know anything about box if its first argument is box.position .

If this answer can't work for some reason, then I'd need more information about your data structure to understand your problem.

Basically, box.position needs to return some obj for which obj.x = 10 is a valid operation and changes exactly what you want it to change.

---

The problem is that you're trying to keep the same data in two separate places. Keep all the data in the C++ struct, then have NewObject return a userdata that pretends to be a table. Both the Object and the position field should be the same Obj* , but they may have different metatables to simulate different sets of fields.

Answer 2

Thanks, I've posted the code that works

struct Obj
{
    unsigned int id;
    int x,y,z;
    Obj()
    {
        x = 10; y = 20; z = 30;
        id = rand();
    }
};

//array of external objects
std::map<unsigned int,Obj> objects;

int NewObject(lua_State * L)
{
    Obj obj;
    objects[obj.id] = obj;

    lua_pushinteger(L, obj.id);
    luaL_getmetatable(L, "MT_Object");
    lua_setmetatable(L, -2);

    return 1;
}


int IndexObj(lua_State * L)
{
    unsigned int objid = luaL_checkinteger(L,1);

    std::string key = luaL_checkstring(L,-1);

    if(key == "position" )
    {
        Obj *a = (Obj *)lua_newuserdata(L, sizeof(Obj));
        *a = objects[objid];

        luaL_getmetatable(L, "MT_Vec");
        lua_setmetatable(L, -2);
    }
    else if(key == "id")
        lua_pushinteger(L, objid);  
    else
        lua_pushnil(L);

    StackDump(L);
    return 1;
}

int IndexVec(lua_State * L)
{
    Obj *a = (Obj *)lua_touserdata(L, 1);

    std::string key = luaL_checkstring(L,-1);
    if(key == "x")
        lua_pushinteger(L,a->x);
    else if(key == "y")
        lua_pushinteger(L,a->y);
    else if(key == "z")
        lua_pushinteger(L,a->z);
    return 1;
}


int NewIndexVec(lua_State * L)
{
    Obj *a = (Obj *)lua_touserdata(L, 1);
    Obj &objRef =  objects[a->id];

    std::string key = luaL_checkstring(L,-2);
    int value = luaL_checkinteger(L,-1);

    if(key == "x")
        objRef.x = value;
    else if(key == "y")
        objRef.y = value;
    else if(key == "z")
        objRef.z = value;

    return 0;
}

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


    luaL_Reg objreg[] =
    {
        { "new", NewObject },   
        { "__index", IndexObj },
        { "__newindex", NewIndexObj },
        { NULL, NULL }
    };
    luaL_newmetatable(L, "MT_Object");
    luaL_register(L, 0, objreg);
    lua_setglobal(L, "Object");


    luaL_Reg reg[] =
    {
        { "__index", IndexVec },    
        { "__newindex", NewIndexVec },
        { NULL, NULL }
    };
    luaL_newmetatable(L, "MT_Vec");
    luaL_register(L, 0, reg);
    lua_setglobal(L, "Vec");

    int res = luaL_dostring(L,  "box = Object.new()   box.position.x = 1000 "
                                "print(box.id .. \" , \" ..box.position.x .. \" , \" ..  box.position.y .. \" , \" .. box.position.z)");
    if(res)
        printf("Error: %s\n", lua_tostring(L, -1));

    lua_close(L);

    return 0;
}