Re: error code

On Jan 5, 12:00 pm, rahul <hyrahu...@gmail.com> wrote:


As explained by Giovanni, last error is probably eaten by cleanup
code. A quick inspection of ::CreateEx shows that you can see
GetLastError() in _DEBUG builds.

My guess is that the MFC people thinking goes like this: there's two
reasons for CreateEx to fail, either there is a resource shortage, in
which case result FALSE means OutOfResources, either there is a bug in
client code, in which case it's enough to show last error only in
_DEBUG builds.

Of course, that's not very nice. It's better to employ scope guard^^^^
and preserve original error code, e.g.

void ___RestoreLastError(DWORD e)
{ SetLastError(e); }

void ___UnhookWindowCreate(CWnd& wnd)
{
  if (AfxUnhookWindowCreate())
    wnd.PostNcDestroy();
}

BOOL CWnd::CreateEx(....)
{
  .......

  DWORD dwLastError=NOERROR;
  ScopeGuard GuardLastError = MakeGuard(&__RestoreLastError, ByRef
(dwLastError));

  AfxHookWindowCreate(this); // wincore.cpp, line 705 (VS2008 MFC
sources)
  ON_BLOCK_EXIT(&___UnhookWindowCreate, *this);

  if (::AfxCtxCreateWindowEx(cs.dwExStyle, cs.lpszClass,
            cs.lpszName, cs.style, cs.x, cs.y, cs.cx, cs.cy,
            cs.hwndParent, cs.hMenu, cs.hInstance, cs.lpCreateParams) == NULL)
  {
    dwLastCreationError = GetLastError();
    return FALSE;
  }
  ASSERT(hWnd == m_hWnd); // should have been set in send msg hook

  GuardLastError.dismiss(); // Creation worked, don't touch last
error.
  return TRUE;
}

Goran.

P.S. (^^^^) he who knows not of ScopeGuard, Google it; if you know C+
+, it will forever change the way you code, and for the better.

P.P.S How does the above work? (Provided it does, since it's compiled
with head-compiler and debugged with head-debugger). In short: we
register last error just after the failed call to
AfxCtxCreateWindowEx. GuardLastError object forces a call to
SetLastError. Call to SetLastError happens just before CWnd::CreateEx
exits, during the destruction of GuardLastError object. One has to be
a crafty bastard and place GuardLastError in a good place so that it
this is indeed last to be destroyed, which I am (crafty bastard, that
is). If creation works, GuardLastError is "dismissed" (in scope guard
parlance), that is, we don't touch last error value on success. Call
to AfxHookWindowCreate is matched with Unhook/PostDestroy in lines
718/719. That's assured through ON_BLOCK_EXIT with
___UnhookWindowCreate.

P.P.P.S. This seems complicated, and indeed it is in a simple
situation like this. However, judicious application of scope guard in
anything a more complicated easily makes for simpler code. And when
exceptions are present (and they are even if you don't see any throw
statements), code gets unwieldy even easier. For example, just ask
yourself how would correct code look like if AfxCtxCreateWindowEx
could throw. (BTW, I guess it can't; if it can, CWnd:::CreateEx does
not satisfy "Basic exception safety" from
http://en.wikipedia.org/wiki/Exception_handling#Exception_safety. That
wouldn't be a surprise, though; quick glance at MFC sources shows that
MFC is all but exception safe. But luckily for MFC, next contender,
Qt, ain't any better in that regard either).