CMPT 212 Object-Oriented Applications Design in C++ Spring 1998

Microsoft Windows Programming

• An application programming interface (API) is a specification of the way an application programmer can use an OS or GUI library.

• An API generally consists of
  • constants,
  • variables,
  • types,
  • functions.

• In MS-Windows, you will typically include the header file <windows.h>, which contains the declarations of the constants, variables, types, and functions in the MS-Windows API.

• The MS-Windows API is not object-oriented.
  • The API must be usable with different programming languages.

• There are object-oriented C++ class libraries that make MS-Windows programming easier. These include:
  • Microsoft Foundation Classes (MFC),
  • Borland Object Windows Library (OWL),
  • Visual Age C++ from IBM.

• We will be using the raw C API in order to learn the basics. Learning these class libraries is fairly easy once we have learned the API basics.

• MS-Windows has several APIs:
  • Win16 is used for 16-bit programs that will run with Windows 3.x. Windows 3.x is 16-bit.

  • Win32 is a 32-bit API for programs that will run with Windows NT. Windows NT is 32-bit.

  • Win32s is a subset of Win32. Programs that use this API can be recompiled with no change to the source code to run on Windows NT or Windows 3.x (provided a special run-time library is loaded for Windows 3.x).

  • Win95 is an API for Windows 95. It contains most of Win32 and additionally has some things that Win32 does not have.

A Simple MS-Windows Program

• See the example program on pages 20-21 of Schildt's Windows 95 Programming in C and C++, by Herbert Schildt.

• You must include <windows.h>.

• A MS-Windows program does not have a main function. Instead it must have a WinMain function.

• The variable wcl is a structure which describes the windows class used by the program.
  • A "class" in this sense is not related to a C++ class.

  • This simple program has only one window class.

  • The name of a window class (szWinName is this case) must not be stored on the stack. It must be declared outside of any function or be declared with static.

  • The window class must be registered before it can be used.

• There are 5 steps to producing a window:
  1. Define the window class.

  2. Register it with the OS: RegisterClass().

  3. Create an instance of the window class: CreateWindow().

  4. Display the window: ShowWindow().

  5. Update the contents of the window: UpdateWindow().

  (Note that the words "class" and "instance" are used in a general sense, and not in the specific object-oriented meaning.)

The Message Loop

• Any messages sent to a window are first sent to the application which owns the window.
  • The application has the opportunity to act on these messages.

  • The application usually passes the messages on to the appropriate window using the DispatchMessage API function.

• MS-Windows maintains queues for messages, since messages may be generated faster than they can be processed.

• Here is a diagram showing the interaction between the application and MS-Windows, the routing of messages, and the role of the message loop:

• The function GetMessage blocks until a message is available.
  • It returns a non-zero value if the message is not the "Quit" message (WM_QUIT).

The Callback Function

• The purpose of the callback function is to handle messages dispatched from the message loop.

• A message structure (MSG) has a field which contains the handle (HWND) of the window in which the event occurred.
  • MS-Windows puts the message in the queue for that window.

  • When it gets time, MS-Windows will call the callback function for that window so that the message can be processed.

• Normally a callback function contains just a switch statement where each case handles a different message.

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