How to register Lua userdata correct from Delphi?

Question

I am still confused about registering Delphi userdata to Lua. To teach me the principle I tried to implement a Date(Time) type.

At the beginning this type should have three functions accessible to Lua:

1. A new function to create variables of this type.
2. A getdate function.
3. and a setdate function.

At the end this little Lua-Script should work:

DT = DateTime.new()
DT:setdate(1, 1, 2011)
day, month, year = DT:getdate()
print("Day: " .. day .. " Month: " .. month .." Year: " .. year)

I tried to implement it by myself (using the Programming in Lua book) but I receive an error saying: _attempt to index global 'DT' (a userdata value)_ on Line 2. I probably did something wrong with the userdata registration but I'm having trouble locating the error.

I hope you can help me finding it, here is what already I got:

Const
  MetaPosDateTime = 'DateTime';

Type
  tLuaDateTime = tDateTime;
  pLuaDateTime = ^tLuaDateTime;

  Function newdatetime(aState : pLua_State) : longint; cdecl;
  Var
    NewData : pLuaDateTime;
  Begin
    Result := 0;
    NewData := lua_newuserdata(aState, SizeOf(tLuaDateTime)); 
    NewData^ := now; 
    luaL_newmetatable(aState, MetaPosDateTime);
    lua_setmetatable(aState, -2);    
    Result := 1;
  End;

  Function setdate(aState : pLua_State) : longint; cdecl;
  Var
    DT : pLuaDateTime;
    ParamType : integer;
    day, month, year : lua_Integer;
  Begin
    Result := 0;
    DT := luaL_checkudata(aState, 1, MetaPosDateTime);
    luaL_argcheck(aState, DT <> Nil, 1, 'DataTime expected');
    ParamType := lua_type(aState, 2); 
    If (ParamType = LUA_TTABLE) Then 
      Begin
        { GetData from Table }
      End
    Else
      Begin // param order must be: day, month, year
        day := luaL_checkinteger(aState, 2);
        month := luaL_checkinteger(aState, 3);
        year := luaL_checkinteger(aState, 4);
      End;
    DT^:= EncodeDate(year, month, day);
  End;

  Function getdate(aState : pLua_State) : longint; cdecl;
  Var
    DT : pLuaDateTime;
    Day, Month, Year : Word;
  Begin
    DT := luaL_checkudata(aState, 1, MetaPosDateTime);
    luaL_argcheck(aState, DT <> Nil, 1, 'DataTime expected');
    DecodeDate(DT^, Year, Month, Day);
    lua_pushinteger(aState, Day);
    lua_pushinteger(aState, Month);
    lua_pushinteger(aState, Year);
  End;

Procedure RegisterDateTime(aState : pLua_State; aName: string);
Var
  Funcs : packed Array[0..3] of luaL_reg;
Begin
  Funcs[0].name := 'new';
  Funcs[0].func := newdatetime;
  Funcs[1].name := 'setdate';
  Funcs[1].func := setdate;
  Funcs[2].name := 'getdate';
  Funcs[2].func := getdate;
  Funcs[3].name := Nil;
  Funcs[3].func := Nil;
  luaL_register(aState, PAnsiChar(aName), Funcs[0]);
End;

Because I'm not sure about the luaL_register function (does it only work by creating a library which must be called with require?) I also tried to replace the RegisterDateTime function with this:

Type
  tLuaFuncDef = Record
    FuncName : string;
    Func : Lua_CFunction;
  End;

tLuaFuncList = Array of tLuaFuncDef;

Procedure RegisterLuaObject(aState : pLua_State; aObjectName: string; aFuncList: tLuaFuncList);
Var
  i : Integer;
Begin
  If (aObjectName = '') Or (High(aFuncList) < 0) Then
    Exit;

  lua_newtable(aState); 
  For i := Low(aFuncList) To High(aFuncList) Do 
    If Assigned(aFuncList[i].Func) And Not (aFuncList[i].FuncName = '') Then
      Begin
        lua_pushcfunction(aState, aFuncList[i].Func);
        lua_setfield(aState, -2, pAnsiChar(aFuncList[i].FuncName));
      End;
  lua_SetGlobal(aState, pAnsiChar(aObjectName));
End;

Procedure RegisterDateTime(aState : pLua_State, aName: string);
Var
  FuncList : tLuaFuncList;
Begin
  SetLength(FuncList, 3);
  FuncList[0].FuncName := 'new';
  FuncList[0].Func := newdatetime;
  FuncList[1].FuncName := 'setdate';
  FuncList[1].Func := setdate;
  FuncList[2].FuncName := 'getdate';
  FuncList[2].Func := getdate;
  RegisterLuaObject(aState, aName, FuncList);
End;

Unfortunately the effect (errormessage ;)) with both versions of RegisterDateTime is the same. They are called direct in my Delphi Program, before the script starts (I ensured this by setting Breakpoints in "RegisterDateTime" and "newdatetime". Both functions are called in this order. So my error must be in one of this two functions. I'm almost sure it is a simple thing but I'm to blind to see it. :(

Answer 1

Today I pushed the big reset Button of this project and completely restarted the implementation of my LuaDateTime-Type and this day I made it correct. Now I want to post my solution as example for anybody else who has the same problem.

My biggest mistake yesterday was forgetting to set the __index field of my metatable. A working Delphi implementation for Lua userdata looks like this:

implementation
Uses
  LuaLib,
  LauXLib,    
  SysUtils;

Type
  tLuaDateTime = tDateTime;
  pLuaDateTime = ^tLuaDateTime;

Const
  PosMetaTaleLuaDateTime = 'metatables.LuaDateTime';
  PosLuaDateTime = 'datetime';    

Function checkLuaDateTime(L : Plua_State) : pLuaDateTime; // raises error if first (self) parameter is not of type metatables.LuaDateTime
Begin
  Result := luaL_checkudata(L, 1, PosMetaTaleLuaDateTime);
End;

Function newLuaDateTime(L : pLua_State) : LongInt; cdecl;
Var
  a : pLuaDateTime;
Begin
  a := lua_newuserdata(L, SizeOf(tLuaDateTime)); // Get Mem of Usertype
  a^ := now; // Init Value
  lua_getfield(L, LUA_REGISTRYINDEX, PosMetaTaleLuaDateTime); 
  lua_setmetatable(L, -2);

  Result := 1;
End;

Function setLuaDateTime(L : pLua_State) : LongInt; cdecl;
Var
  a : pLuaDateTime;
  day, month, year : Integer;
Begin
  a := checkLuaDateTime(L);
  // get params day, month and year
  day := luaL_checkint(L, 2);
  month := luaL_checkint(L, 3);
  year := luaL_checkint(L, 4);

  // check Param Values
  luaL_argcheck(L, (day >= 1) and (day < 32), 2, 'day out of range');
  luaL_argcheck(L, (month >= 1) and (month < 13), 3, 'month out of range');

  a^ := EncodeDate(year, month, day);

  Result := 0;
End;

Function getLuaDateTime(L : pLua_State) : LongInt; cdecl;
Var
  a : pLuaDateTime;
  day, month, year : Word;
Begin
  a := checkLuaDateTime(L);
  DecodeDate(a^, year, month, day);

  // push 3 results of function
  lua_pushinteger(L, day);
  lua_pushinteger(L, month);
  lua_pushinteger(L, year);

  Result := 3;
End;

Function LuaDateTime2string(L : pLua_State) : LongInt; cdecl;
Var
  a : pLuaDateTime;
Begin
  a := checkLuaDateTime(L);
  lua_pushstring(L, pAnsiChar(FormatDateTime('c', a^)));
  Result := 1;
End;

Const
  LuaDateTimeLib_f : packed Array[0..1] of luaL_reg = // Normal functions (no self)
    (
      (name: 'new'; func: newLuaDateTime),
      (name: Nil; func: Nil)
    );

  LuaDateTimeLib_m : packed Array[0..3] of luaL_reg = // methods of class (need self)
    (
      (name: '__tostring'; func: LuaDateTime2string),
      (name: 'set'; func: setLuaDateTime),
      (name: 'get'; func: getLuaDateTime),
      (name: Nil; func: Nil)
    );

Function luaopen_LuaDateTime(L : pLua_State) : LongInt; cdecl;
Begin
  luaL_newmetatable(L, PosMetaTaleLuaDateTime);
  // Metatable.__index = Metatable
  lua_pushvalue(L, -1);
  lua_setfield(L, -2, '__index');
  luaL_register(L, Nil, LuaDateTimeLib_m[0]);

  luaL_register(L,PosLuaDateTime, LuaDateTimeLib_f[0]);
  Result := 1;
End;

You have to call luaopen_LuaDateTime from Delphi to register the type in your Lua-State. After doing that you can run a Lua-Script like this:

 dt = datetime.new()
 day, month, year = dt:get()
 print ("Day: " .. day .. " Month: " .. month .. " Year: " .. year)
 dt:set(1, 2, 1903)
 day, month, year = dt:get()
 print ("Day: " .. day .. " Month: " .. month .. " Year: " .. year)

I hope this is helpful for someone else.