Why does this Curious Code work?

[Wolfram Steinke]

From Microsoft AppUI1.cpp int AFXAPI AfxMessageBox(LPCTSTR lpszText, UINT nType, UINT nIDHelp) { CWinApp* pApp = AfxGetApp(); if (pApp != NULL) return pApp->DoMessageBox(lpszText, nType, nIDHelp); else return pApp->CWinApp::DoMessageBox(lpszText, nType, nIDHelp); } We came across this code while fixing a bug where a in a TAPI call the message box reporting a failure crashed. On the surface of it this code would crash if pApp is NULL. Incidentally in our case it wasn't but invalid and hence the crash. OUCH!!! Happy programming!!

[Michael Dunn]

Because it's perfectly valid to call a method through a NULL pointer. The result is that, in the method, this is NULL. If you want to see something equally weird-looking, look at CWnd::GetSafeHwnd() --

inline CWnd::GetSafeHwnd() const
{
return this == NULL ? NULL : m_hWnd;
}

--Mike-- http://home.inreach.com/mdunn/ The preferred snack of 4 out of 5 Lounge readers.

[Jim Wuerch]

If CWinApp::DoMessageBox isn't virtual, and it never dereferences the this pointer (never accesses any class members) then there's no problems. Essentially, it means that CWinApp::DoMessageBox could be static (it may be, I didn't look at it's definition to see) Jim Wuerch www.miwasoft.com Quote from my readme files: "This is BETA software, and as such may completely destroy your computer, change the alignment of the planets and invert the structure of the universe."

[Lost User]

Actually, it is virtual. What this trick is trying to do (to the best of my understanding), is force the compiler to invoke the CWinApp::DoMessageBox virtual function directly instead of through the nonexistent v-table (NULL pointer).