wndclass and registerclass()

In the following code, registerclass(wndclass) is called, what exactly are we doing in this call conceptually? am new to GUI application!!!!!!!!!

typedef struct tagAPPINFO {
   INT      iVersion;
   INT      iFiller;

   HWND     hAppInst;
   INT      iAppType;

   HWND     hWndFrame;
   HWND     hWndMDIClient;
   HWND     hWndToolbar;
   HWND     hWndStatbar;

   UINT     uMemBlocks;
   UINT     uMemBlockSize;

   char     szAppName  [MAXOBJNAME      + 2];
   char     szFileName [FILENAME_LENGTH + 2];
   char     szHelpFile [PATHNAME_LENGTH + 2];
   char     szPathName [PATHNAME_LENGTH + 2];
   BOOL     bDeltaMode;
   char     szSysPathName[PATHNAME_LENGTH + 2];

int WINAPI TNGMain (LPSTR lpszCmdLine, ULONG TNGContext, int nCmdShow, HWND hParent)
/* clear application information structure */
   memset  (&appInfo, '\0', sizeof (APPINFO));

   /* save instance handle in structure */
   hAPPINST = hResourceModule;
   hWndParent = hParent;

   /* set other application information in structure */
   appInfo.iVersion      = iVERSION;
   appInfo.uMemBlocks    = NUMMEMBLOCKS;
   appInfo.uMemBlockSize = MEMBLOCKSIZE;
   strncpy(appInfo.szAppName,  szAPPNAME, MAXOBJNAME);
   strcpy(appInfo.szFileName, szFILENAME);
   strcpy(appInfo.szHelpFile, sz2DHELPFILE);

   /* register window classes if this is the first instance */
   if (!RegisterClasses (hAPPINST))
      return FALSE;

static BOOL PASCAL RegisterClasses (hInstance)
HANDLE      hInstance;
   WNDCLASS   wndclass;

   SetFunctionName (RegisterClasses);

/* register class for frame window */
   wndclass.style         = CS_HREDRAW | CS_VREDRAW;  
   wndclass.lpfnWndProc   = GRFFrameWndProc;            
   wndclass.cbClsExtra    = 0;
   wndclass.cbWndExtra    = 0;
   wndclass.hInstance     = hInstance;
   wndclass.hIcon         = LoadIcon   (hInstance, "GrafBASEIcon");
   wndclass.hCursor       = LoadCursor (NULL, IDC_ARROW);
   wndclass.hbrBackground = (HBRUSH) (COLOR_WINDOW + 1);
   wndclass.lpszMenuName  = NULL;
   wndclass.lpszClassName = appInfo.szFileName;

   if (!RegisterClass (&wndclass))
      return FALSE;
/* register class for MDI child windows */
/*   wndclass.style         = CS_HREDRAW | CS_VREDRAW | CS_DBLCLKS; */
   wndclass.style         = CS_DBLCLKS;
   wndclass.lpfnWndProc   = GRFChildWndProc;
   wndclass.hIcon         = LoadIcon   (hInstance, "CALOGOICON");
   wndclass.lpszClassName = szWndClass;
   wndclass.lpszMenuName  = NULL;

   return (RegisterClass (&wndclass));


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alb66Connect With a Mentor Commented:
From the link of my first post...

Each window class has an associated window procedure shared by all windows of the same class. The window procedure processes messages for all windows of that class and therefore controls their behavior and appearance. For more information, see Window Procedures.

A process must register a window class before it can create a window of that class. Registering a window class associates a window procedure, class styles, and other class attributes with a class name. When a process specifies a class name in the CreateWindow or CreateWindowEx function, the system creates a window with the window procedure, styles, and other attributes associated with that class name.
sham_ibmgsAuthor Commented:
Why are we running RegisterClass() with wndclass as parameter?

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"wndclass" is the structure where you define all the window class behavior ( for example the background color).
RegisterClass registers in the system the "wndclass" that you have defined.
You can sort of think as RegisterClass as the definition of an *object's class* definition in an OO language.

The WNDCLASS defines this template,  RegisterClass let's the system know about it once you've defined it, and the WndProc defines the behaviour of objects of that type - think of it as a single parameterised method definition.

Once you've registered a class, you can then create as many instances of objects of that class using CreateWindow etc.
sham_ibmgsAuthor Commented:
@peet: definition of an object's class definition?

sham_ibmgsAuthor Commented:
Like in a language like C++ ..

class Foo

So now you can have any number of Foos

Foo a;
Foo b;


The WNDCLASS is very similar in so many ways.

WNDCLASS definition somewhat analogous to ...

class appInfo_szFileName

    // No constructor - handled by WM_CREATE and CreateWindow().

    long GRFFrameWndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam);

So now you can ...

HWND hWnd = CreateWindow(appInfo_szFileName, ...)

CreateWindow's creating a appInfo_szFileName type window and returning a polymorphic pointer to it here.

// Single method that handles all message traffic for objects of type appInfo_szFileName
HWND appInfo_szFileName::GRFFrameWndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)

Hope this hasn't confused more than it's helped??

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It really pi***s me off when someone asks an additional question, and you answer it - just to find they've already accepted a solution!
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