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

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

From stackoverflow

• 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;
  }
  

• 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!

  Watusimoto : Wow --it's possible to be a totally different user on a different computer with the same user name! Who knew?