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From: risto@tuura.UUCP (Risto Lankinen)
Newsgroups: comp.windows.ms.programmer
Subject: Re: Class Extra and Window Extra bytes
Message-ID: <1167@tuura.UUCP>
Date: 13 May 91 08:20:00 GMT
References: <1636@msa3b.UUCP>
Organization: Nokia Data Systems Oy
Lines: 36

kevin@msa3b.UUCP (Kevin P. Kleinfelter) writes:

>Is it safe to assume that window class "extra" bytes and window instance
>"extra" bytes are initialized to 0 by Windows?

>(i.e. if I RegisterClass with the cbClsExtra field set to 2, will I
>get a word of 0 or a word of random value. I always seem to get a zero word,
>but can I COUNT on this.)

Hi!

I WOULDN'T COUNT on it.  I would also like to DISCOURAGE ANYONE FROM DOING
SO, because I'd like to see the following as a documented way to use the
RegisterClass() in forthcoming versions of Windows' SDK (Mike Stella, Mike
Stella, Robin Illinois Stella Tango Oliver calling do you read me, over ;-)

  typedef struct
  {
    ...
    // current contents of WNDCLASS
    ...
    BYTE bClsExtra[]; // or WORD, or DWORD or whatever as long as something
  } WNDCLASS,*PWNDCLASS, ...

... so you could use a construct LocalAlloc( ...,sizeof(WNDCLASS)+CLS_XTRA )
and 'npWndCls->bClsExtra[INDEX] = MyData' within the registration itself, to
get the extra class bytes defined right when the class is being registered.
To make them initialize to zero, one would *use* zero, thus the behaviour in
question wouldn't be voided, nor the current actual RegisterClass() usage.

Terveisin: Risto Lankinen
-- 
Risto Lankinen / product specialist ***************************************
Nokia Data Systems, Technology Dept *  2                              2   *
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