Question

How to return C++ object to lua?

My C++ code is following:

class MyClass
{
public:
  void say()
  {
    print("Hello\r\n");
  }
};

int test(lua_State* l)
{
  MyClass* obj = new MyClass();
  lua_pushlightuserdata(l, obj);
  return 1;
}

Lua Test is following:

local a = MyClass:new()
a:say()  <--- OK, beacause I set metatable!!
local b = test()
b:say()  <--- ERROR: attempt to index local 'b' (a userdata value)

How to modify test() function to work fine?
obj will auto destory by lua ?

PS: I has set MyClass metatable is following

void l_registerClass()
{
  lua_newtable(l);
  int methods = lua_gettop(l);
  luaL_newmetatable(l, "MyClass");
  int metatable = lua_gettop(l);
  lua_pushvalue(l, methods);
  lua_setglobal(l, "MyClass");

  lua_pushvalue(l, methods);  
  l_set(l, metatable, "__metatable");

  //set metatable __index
  lua_pushvalue(l, methods);
  l_set(l, metatable, "__index");

  //set metatable __gc
  lua_pushcfunction(l, l_destructor);  
  l_set(l, metatable, "__gc");

  //set method table
  lua_newtable(l);                // mt for method table  
  lua_pushcfunction(l, l_constructor);  
  lua_pushvalue(l, -1);           // dup new_T function  
  l_set(l, methods, "new");         // add new_T to method table  
  l_set(l, -3, "__call");           // mt.__call = new_T  
  lua_setmetatable(l, methods);  

  // set methods metatable   
  lua_pushstring(l, "say");
  lua_pushcclosure(l, l_proxy, 1);  
  lua_settable(l, methods);

  lua_pop(l, 2);
}

int l_proxy(lua_State* l)
{
  int i = (int)lua_tonumber(l, lua_upvalueindex(1));
  lua_remove(l, 1);  // remove self so member function args start at index 1
  //call real function
  MyClass* obj = getInstance();
  obj->say();
  return 1;
}

I should don’t lost step ?
I don’t use any Lua Binding Framework, I am using pure Lua Library.

==== update 1 ====

Thanks for user1520427’s answer, but….

int test(lua_State* l)
{
  MyClass** c = (MyClass**)lua_newuserdata(l, sizeof(MyClass*));
  *c = new MyClass();       // we manage this
  lua_getglobal(l, "MyClass");
  lua_setmetatable(l, -2);
  return 1;
}

and I test it in Lua

local b = test()
print( type(b) )
local meta = getmetatable(b)
for k,v in pairs(meta) do
  print("    ", k, v)
end

Lua show metatable is correct.

userdata
  say     function: 00602860
  new     function: 00493665

But lua still shows the same error in

b:say()   <-- attempt to index local 'b' (a userdata value)

=== update 2 ===

int test(lua_State* l)
{
  MyClass** c = (MyClass**)lua_newuserdata(l, sizeof(MyClass*));
  *c = new MyClass();       // we manage this
  luaL_getmetatable(l, "MyClass");  //
  lua_getglobal(l, "MyClass");
  lua_setmetatable(l, -2);
  return 1;
}

the lua test result:

b:say()   <-- attempt to call method 'say' (a nil value)

=== update 3 ===

int test(lua_State* l)
{
  MyClass** c = (MyClass**)lua_newuserdata(l, sizeof(MyClass*));
  *c = new MyClass();       // we manage this
  luaL_getmetatable(l, "MyClass");  
  luaL_setmetatable(l, "MyClass");  //modify
  return 1;
}

Lua test result:

b:say()   <-- calling 'say' on bad self

Answer 1

thanks for user1520427 and lhf
correct code sould is following:

int test(lua_State* l)
{
  MyClass** c = (MyClass**)lua_newuserdata(l, sizeof(MyClass*));
  *c = new MyClass();       // we manage this
  luaL_setmetatable(l, "MyClass");  //assign MyClass metatable
  return 1;
}

Lua test code is work fine.