The window class defines general characteristics of a window. By using that window class we can create many different windows. We can create a window by calling CreateWindow.

Programmers new to Windows are sometimes confused about the distinction between the window class and the window and why all the characteristics of a window can’t be specified in one shot. Actually, dividing the information in this way is quite convenient. For example, all push-button windows are created based on the same window class. The window procedure associated with this window class is located inside Windows itself, and it is responsible for processing keyboard and mouse input to the push button and defining the button’s visual appearance on the screen. All push buttons work the same way in this respect. But not all push buttons are the same. They almost certainly have different sizes, different locations on the screen, and different text strings. These latter characteristics are part of the window definition rather than the window class definition.

While the information passed to the RegisterClass function is specified in a data structure, the information passed to the CreateWindow function is specified as separate arguments to the function. Here’s the CreateWindow call

hwnd = CreateWindow (szAppName,                  // window class name
TEXT (“The Hello Program”), // window caption
WS_OVERLAPPEDWINDOW,        // window style
CW_USEDEFAULT,              // initial x position
CW_USEDEFAULT,              // initial y position
CW_USEDEFAULT,              // initial x size
CW_USEDEFAULT,              // initial y size
NULL,                       // parent window handle
NULL,                       // window menu handle
hInstance,                  // program instance handle
NULL) ;                     // creation parameters

szAppName — > “window class name”. The name of the window class the program just registered

The window created by this program is a normal overlapped window. It will have a title bar; a system menu button to the left of the title bar; a thick window-sizing border; and minimize, maximize, and close buttons to the right of the title bar. That’s a standard style for windows, and it has the name WS_OVERLAPPEDWINDOW, which appears as the “window style” parameter in CreateWindow. If you look in WINUSER.H, you’ll find that this style is a combination of several bit flags:

#define WS_OVERLAPPEDWINDOW (WS_OVERLAPPED     | \
WS_CAPTION        | \
WS_SYSMENU        | \
WS_THICKFRAME     | \
WS_MINIMIZEBOX    | \
WS_MAXIMIZEBOX)
The “window caption” is the text that will appear in the title bar of the window.

The arguments marked “initial x position” and “initial y position” specify the initial position of the upper left corner of the window relative to the upper left corner of the screen. By using the identifier CW_USEDEFAULT for these parameters, we are indicating that we want Windows to use the default position for an overlapped window. (CW_USEDEFAULT is defined as 0×80000000.) By default, Windows positions successive newly created windows at stepped horizontal and vertical offsets from the upper left corner of the display. Similarly, the “initial x size” and “initial y size” arguments specify the initial width and height of the window. The CW_USEDEFAULT identifier again indicates that we want Windows to use a default size for the window.

The argument marked “parent window handle” is set to NULL when creating a “top-level” window, such as an application window. Normally, when a parent-child relationship exists between two windows, the child window always appears on the surface of its parent. An application window appears on the surface of the desktop window, but you don’t need to find out the desktop window’s handle to call CreateWindow.

The “window menu handle” is also set to NULL because the window has no menu. The “program instance handle” is set to the instance handle passed to the program as a parameter of WinMain. Finally, a “creation parameters” pointer is set to NULL. You could use this parameter to point to some data that you might later want to reference in your program.

The CreateWindow call returns a handle to the created window. This handle is saved in the variable hwnd, which is defined to be of type HWND (“handle to a window”). Every window in Windows has a handle. Your program uses the handle to refer to the window. Many Windows functions require hwnd as an argument so that Windows knows which window the function applies to. If a program creates many windows, each has a different handle. The handle to a window is one of the most important handles that a Windows program (pardon the expression) handles.