A year ago, I explained how the IMVU client automatically reports unexpected Python exceptions (crashes) to us. I intended that post to be the first of a long series that covered all of the tricks we use to detect and report abnormal situations. Clearly, my intentions have not played out yet, so I am going to pick up that series by describing how we catch exceptions that occur in our C++ code. Without further ado,

Reporting C++ Exceptions

As discussed earlier, IMVU's error handling system can handle any Python exception that bubbles out of the client's main loop and automatically report the failure back to us so that we can fix it for the next release. However, our application is a combination of Python and C++, so what happens if our C++ code has a bug and raises an uncaught C++ exception, such as std::bad_alloc or std::out_of_range?

Most of our C++ code is exposed to Python via the excellent Boost.Python library, which automatically catches C++ exceptions at the boundary and translates them to Python exceptions. The translation layer looks something like this: 

bool handle_errors(function fn) {
    try {
        fn();
        return false; // no error
    }
    catch (const std::runtime_error& e) {
        // raise RuntimeError into Python
        PyErr_SetString(PyExc_RuntimeError, e.what());
    }
    catch (const std::bad_alloc&) {
        // raise MemoryError into Python
        PyErr_SetString(PyExc_MemoryError, "out of memory");
    }
    catch (const std::exception& e) {
        // raise Exception into Python
        PyErr_SetString(PyExc_Exception, e.what());
    }
    catch (...) {
        PyErr_SetString(PyExc_Exception, "Unknown C++ exception");
    }
    return true;
}

Thus, any C++ exception that's thrown by the C++ function is caught by Boost.Python and reraised as the appropriate Python exception, which will already be handled by the previously-discussed crash reporting system.

Let's take another look at the client's main loop: 

def mainLoop():
    while running:
        pumpWindowsMessages()
        updateAnimations()
        redrawWindows()

def main():
    try:
        mainLoop()
    except:
        # includes exception type, exception value, and python stack trace
        error_information = sys.exc_info()
        if OK == askUserForPermission():
            submitError(error_information)

If the C++ functions called from updateAnimations() or redrawWindows() raise a C++ exception, it will be caught by the Python error-handling code and reported to us the same way Python exceptions are.

Great! But is this a complete solution to the problem? Exercise for the reader: what else could go wrong here? (Hint: we use Visual Studio 2005 and there was a bug in catch (...) that Microsoft fixed in Visual Studio 2008...)