C++ / WIndows - Can't catch exceptions

Question

I'm a beginner with the windows api so there must be something i don't understand here. In my main function i'm using a try-catch to catch all uncaught exceptions, but for some reason, exceptions i throw from somewhere else in the code are never caught. My application uses a single (main) thread.

I'm throwing like this :

throw "ClassName::methodName() - Error message";

And catching the exceptions outside of the message loop :

try {
    while(GetMessage(args...)) {
        TranslateMessage(args...);
        DispatchMessage(args...);
    }
}
catch( const char * sExc ) {
    ::MessageBox(args...);
}

I first thought it was a problem of types mismatching, but then i added a catch(...) with ellipsis and i still caught nothing. If you ask, yes, i'm sure the exception is thrown. Isn't it a problem related to some kind of asynchronousness or something like that ?

Thanks for your help !

Answer 1

It depends on the specific message that's getting dispatched. But no, not all of them allow the stack to be unwound through Windows internal code. Particularly the messages that involve the window manager, WM_CREATE for example. There's a backstop inside Windows that prevents the stack from being unwound past that critical code. There's also an issue with exceptions in 32-bit code that run on the 64-bit version of Windows 7, they can get swallowed when the message needs to traverse the Wow64 boundary several times. Fixed in Windows 8.

On later Windows versions this can also activate "self-healing" code, automatically activating an appcompat shim that swallows the exception. You'll get a notification for that, easy to dismiss. You'll then see the first-chance exception notification in the VS Output window but your program keeps running. Okayish for the user perhaps but not great while you are debugging of course. Run Regedit.exe and navigate to HKCU\Software\Microsoft\Windows NT\CurrentVersion\AppCompatFlags\Compatibility Assistant\Persisted and check if your program is listed there. Just delete the entry.

Long story short, it is not safe to catch exceptions outside of the message loop. You have to do it inside the window procedure.