typedef struct _WNDCLASS {  
    UINT    style; 
    WNDPROC lpfnWndProc; 
    int     cbClsExtra; 
    int     cbWndExtra; 
    HANDLE  hInstance; 
    HICON   hIcon; 
    HCURSOR hCursor; 
    HBRUSH  hbrBackground; 
    LPCTSTR lpszMenuName; 
    LPCTSTR lpszClassName; 
} WNDCLASS;

//using Delphi types
type
  PWndClassA = ^TWndClassA;
  PWndClass = PWndClassA;

  tagWNDCLASSA = packed record
    style: Cardinal;
    lpfnWndProc: TFNWndProc;  //Pointer
    cbClsExtra: Integer;
    cbWndExtra: Integer;
    hInstance: THandle;
    hIcon: THandle;
    hCursor: THandle;
    hbrBackground: THandle;
    lpszMenuName: PChar;
    lpszClassName: PChar;
  end;

All windows that are created must use a registered windows Class, because the Class sets the function (Window Proc) that processes the messages to that window.
To register a Class, you first need to set it's parameters. The two parameters that must be set in order to have CreateWindow succeed are lpszClassName and lpfnWndProc. Set the windows Class hInstance to the memory locacation of your apps hInstance, this associates that Class with your programs thread execution. The registered Class will be then unregistered when that hInstance is terminated. The hIcon and hCursor will define the Icon and Cursor used by windows of this Class. The hbrBackground is the brush handle used to paint the background of windows of this class, you can also use a system color reference number here. The lpszMenuName sets a menu for this class. The "style" sets how the window will update after moving it, how to process double-clicks, the way to allocate space for it's device context, and other properties of the window. Look at the Class styles in WNDCLASS API help. We will not be using a class style in this first example. The lpfnWndProc is set to the memory Address of the process (function) where windows messages are sent, often called a "Window Proc".

Later in this chapter, you'll find the code for a Windows GUI application called "First Window GUI Application" where you can see how the class parameters are set up and then registered, look below the Program's BEGIN for wClass.
Since we set lpfnWndProc := @WndMessageProc; we need to have a WndMessageProc function with the (hWnd: HWnd; Msg: UINT; WParam: WPARAM; LParam: LPARAM): UINT; stdcall; parameters to receive the messages. If there is no message function or that message function does not "Handle" (use) any messages, then a working GUI can not be created. The lpszMenuName is for a PChar name of a Resource menu, we will not use this parameter here and set it to an empty string. The cbClsExtra and cbWndExtra params are for "Extra" information, you can place in the Registered class to read or change later. These will not be used here.

There are "Standard" Win32 classes like "Button", "Edit", "Static", "Listbox" and others, so you don't have to create and register those. Look at the Win32 help for "CreateWindow" and review the "Button" class, which will be used in the "First Window GUI Application" program code later.

Before we proceed, let's take a look at Windows messaging...

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