a.cpp

                struct UserData
                {
                    void*         data;
                    unsigned char type;
                };

                template <class T>
                struct UserData_Value : UserData
                {
                    T value;
                };

                // Creates a new UserData with your own data embedded within it
                template <class T>
                inline void PushUserType_Value( const T& val, int iType )
                {
                    // The UserData allocated by CLuaInterface is only guaranteed to have a data alignment of 8
                    static_assert( std::alignment_of<UserData_Value<T>>::value <= 8,
                        "PushUserType_Value given type with unsupported alignment requirement" );

                    // Don't give this function objects that can't be trivially destructed
                    // You could ignore this limitation if you implement object destruction in `__gc`
                    static_assert( std::is_trivially_destructible<UserData_Value<T>>::value,
                        "PushUserType_Value given type that is not trivially destructible" );

                    auto* ud = static_cast<UserData_Value<T>*>( NewUserdata( sizeof( UserData_Value<T> ) ) );
                    ud->data = new( &ud->value ) T ( val );
                    ud->type = iType;

                    // Set the metatable
                    if ( PushMetaTable( iType ) ) SetMetaTable( -2 );
                }