The Application class handles the message loop of the application. The message loop waits for the next message from the Windows system and sends it to the right window. The Application class also defines the Windows classes (which are not C++ classes) for the Window, Document, StandardDocument, and Dialog C++ classes. The fields of the classes are static since Application is not intended to be instantiated.
From this point in Small Windows, every part of the Small Windows implementation is included in the SmallWindows namespace. A namespace is a C++ feature that encapsulates classes and functions. The declaration of MainWindow, we saw earlier, is not included in the Smallwindows namespace since the C++ language rules stipulate that it cannot be included in a namespace. The WinMain definition is also not included in the namespace, since it needs to be placed outside the namespace to be called by the Windows system.
Application.h
namespace SmallWindows {
class...
