How do I pass a list of objects from C++ to Lua?

Question

I'm the lead dev for Bitfighter, and am adding user-scripted bots using Lua. I'm working with C++ and Lua using Lunar to glue them together.

I'm trying to do something that I think should be pretty simple: I have an C++ object in Lua (bot in the code below), and I call a method on it that (findItems) which causes C++ to search the area around the robot and return a list of objects it finds (TestItems and others not shown here). My question is simply how do I assemble and return the list of found items in C++, and then iterate over them in Lua?

Basically, I want to fill in the <<<< Create list of items, return it to lua >>>> block below, and make any corrections I may need in the Lua code itself, included below that.

I've tried to keep the code simple but complete. Hope there's not too much here! Thanks!

C++ Header file

class TestItem : public LuaObject
{

public:
   TestItem();     // C++ constructor

   ///// Lua Interface

   TestItem(lua_State *L) { } ;             //  Lua constructor

   static const char className[];
   static Lunar<TestItem>::RegType methods[];

   S32 getClassID(lua_State *L) { return returnInt(L, TestItemType); }
};


class LuaRobot : public Robot
{
   LuaRobot();     // C++ constructor

   ///// Lua Interface

   LuaRobot(lua_State *L) { } ;             //  Lua constructor

   static const char className[];
   static Lunar<LuaRobot>::RegType methods[];

   S32 findItems(lua_State *L);
}

C++ .cpp file

const char LuaRobot::className[] = "Robot";      // Class name in Lua
// Define the methods we will expose to Lua
Lunar<LuaRobot>::RegType LuaRobot::methods[] =
{
   method(LuaRobot, findItems),
   {0,0}    // End method list
};


S32 LuaRobot::findItems(lua_State *L)
{
   range = getIntFromStack(L, 1);    // Pop range from the stack
   thisRobot->findObjects(fillVector, range);  // Put items in fillVector

   <<<< Create list of items, return it to lua >>>>

   for(int i=0; i < fillVector.size(); i++)
      do something(fillVector[i]);    // Do... what, exactly?

   return something;
}


/////
const char TestItem::className[] = "TestItem";      // Class name in Lua

// Define the methods we will expose to Lua
Lunar<TestItem>::RegType TestItem::methods[] =
{
   // Standard gameItem methods
   method(TestItem, getClassID),
   {0,0}    // End method list
};

Lua Code

bot = LuaRobot( Robot ) -- This is a reference to our bot

range = 10
items = bot:findItems( range )

for i, v in ipairs( items ) do
    print( "Item Type: " .. v:getClassID() )
end

Answer 1

So you need to fill a vector and push that to Lua. Some example code follows. Applications is a std::list.

typedef std::list<std::string> Applications;

I create a table and fill it with the data in my list.

int ReturnArray(lua_State* L) {
    lua_createtable(L, applications.size(), 0);
    int newTable = lua_gettop(L);
    int index = 1;
    Applications::const_iterator iter = applications.begin();
    while(iter != applications.end()) {
        lua_pushstring(L, (*iter).c_str());
        lua_rawseti(L, newTable, index);
        ++iter;
        ++index;
    }
    return 1;
}

This leaves me with an array in the stack. If it were returned to Lua, then I could write the following:

for k,v in ipairs( ReturnArray() ) do
    print(v)
end

Of course so far, this just gets me a Lua array of strings. To get an array of Lua objects we just tweak your method a bit:

S32 LuaRobot::findItems(lua_State *L)
{
    range = getIntFromStack(L, 1);    // Pop range from the stack
    thisRobot->findObjects(fillVector, range);  // Put items in fillVector

    // <<<< Create list of items, return it to lua >>>>

    lua_createtable(L, fillVector.size(), 0);
    int newTable = lua_gettop(L);
    for(int i=0; i < fillVector.size(); i++) {
        TestItem* item = fillVector[i];
        item->push(L);  // put an object, not a string, in Lua array
        lua_rawseti(L, newTable, i + 1);
    }
    return 1;
}

Answer 2

This works perfectly. To clarify to others who are reading this, the method

item->push(L)

is

void push(lua_State *L) {  Lunar<TestItem>::push(L, this); }

By encapsulating this in a method, it's possible to make the findItems agnostic to what it's finding.

Thank you for the help!