error C2181: illegal else without matching if

(Another Visual C++ 2008 Express Edition related post.)

If something is wrong within the if expression, you may wrongly receive the indication "error C2181: illegal else without matching if".

Example:

class primary_thread_t
{
  public:
    LRESULT message (HWND const & window, UINT const & message, WPARAM const & W, LPARAM const & L)
    {
      LRESULT result;

      switch (message)
      {
        case WM_PAINT:
        {
          PAINTSTRUCT PS;
          HDC const DC (BeginPaint (window, &PS));

          if (DC)
          {
          }

          BOOL (EndPaint (window, &PS)); // Just to show there's a return value.

          result = 0;
          break;
        }
        case WM_DESTROY:
        {
          PostQuitMessage (0);
          result = 0;
          break;
        }
        case WM_CREATE:
        {
          result = 0;
          break;
        }
        default:
        {
          result = DefWindowProc (window, message, W, L);
          break;
        }
      }

      return result;
    }
};

static LRESULT CALLBACK window_procedure (HWND const window, UINT const message, WPARAM const W, LPARAM const L)
{
  LRESULT result; // Initialized before return statement.

  if (old_t* const old = reinterpret_cast <old_t*> (GetWindowLongPtr (window, GWLP_USERDATA)))
  {
    result = old->message (window, message, W, L);
  }
  else if (message == WM_NCCREATE)
  {
    SetWindowLongPtr (window, GWLP_USERDATA, reinterpret_cast <LONG_PTR> (reinterpret_cast <CREATESTRUCT*> (L)->lpCreateParams));
    result = DefWindowProc (window, message, W, L);
  }
  else
  {
    result = DefWindowProc (window, message, W, L);
  }

  return result;
}

This is my typical copy-paste scenario. I copy code from one source which uses another name for a class, namely old_t, to a new project which uses another name for a class of the same function, namely primary_thread_t. Although the error lies with the expression within the if statement (the identifier is not known), the compiler claims that the error lies with the structure of the if--else if--else structure.

To resolve this problem, do a search for 'old' and replace it with 'primary_thread'.