Re: CString over LPARM?

Tech-Archive recommends: Repair Windows Errors & Optimize Windows Performance

From: Joseph M. Newcomer (newcomer_at_flounder.com)
Date: 02/16/04 

Date: Mon, 16 Feb 2004 02:06:19 -0500

You are probably wrong. If you use a char buffer that is a local variable, it will likely
be gone by the time the message is processed. If you use one which is a class member,
static, or global (the latter two being incredibly poor choices), then you have the
problem of what happpens if you post two messages.

For example
        
function()
  {
    char buffer[100];
    strcpy(buffer, "this is a test");
    wnd->PostMessage(UWM_WHATEVER. (WPARAM)buffer);
 }

will produce something between the string you give, complete nonsense, and an access fault
when you try to use the value. In rare and unusual circumstances, you might see the actual
value. Most of the time you will get trash, and sometimes you will generate access faults.
This is because the buffer is gone when the message is processed.

If you do

function(CString whatever)
   {
     static char buffer[100];
     strcpy(buffer, whatever);
     wnd->PostMessage(UWM_WHATEVER, (WPARAM)buffer);
   }

means the buffer address will be valid, but the contents will be trash; imagine calling
the function twice, once with the string "this" and once with the string "that". The
receiving thread of the post will most likely display "that" twice, because the address
points to the same place for both messages but the contents have been trashed.

The only safe solution I know of is to allocate a heap object for each request and delete
it when the message is received.

CString * s = new CString;
s->Format("%s %d", "The value is", n); // silly example, but shows the point
wnd->PostMessage(UWM_WHATEVER, (WPARAM)s);

and the receiving function would be

LRESULT classname::OnWhatever(WPARAM wParam, LPARAM)
   {
    CString * s = (CString *)wParam;
    ... do something with CString value
   delete s; // unless you have stored the pointer for some other reason
   return 0;
  }

                                        joe

On Fri, 13 Feb 2004 16:59:27 +0100, "Holger Kreißl" <hok@inormatik.tuchemnitz.de> wrote:

>I got it.
>
>I only have to use a char buffer instead of CString an it works.
>
>
>Holger
>

Joseph M. Newcomer [MVP]
email: newcomer@flounder.com
Web: http://www.flounder.com
MVP Tips: http://www.flounder.com/mvp_tips.htm

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