blag/post/qt-throw-exceptions-from-signals-and-slots.mdwn

   1 [[!meta title="Qt: Throw exceptions from signals and slots"]]
   2 [[!meta date="2010-07-08"]]
   3 [[!meta author="rohieb"]]
   4 [[!meta license="CC-BY-SA 3.0"]]
   5
   6 By default, you can not throw exceptions from signals and slots, and if
   7 you try it, you get the following message:
   8
   9 > Qt has caught an exception thrown from an event handler. Throwing
  10 > exceptions from an event handler is not supported in Qt. You must
  11 > reimplement QApplication::notify() and catch all exceptions there.
  12
  13 So, what to do? The answer is simple: Overwrite the function `bool
  14 QApplication::notify(QObject * receiver, QEvent * event)` so that it
  15 catches all thrown exceptions. Here is some sample code:
  16
  17 [[!format c """
  18 #include <QtGui>
  19 #include <QApplication>
  20 class MyApplication : public QApplication {
  21 public:
  22     MyApplication(int& argc, char ** argv) :
  23         QApplication(argc, argv) { }
  24     virtual ~MyApplication() { }
  25
  26     // reimplemented from QApplication so we can throw exceptions in slots
  27     virtual bool notify(QObject * receiver, QEvent * event) {
  28         try {
  29             return QApplication::notify(receiver, event);
  30         } catch(std::exception& e) {
  31             qCritical() << "Exception thrown:" << e.what();
  32         }
  33         return false;
  34     }
  35 };
  36 int main(int argc, char* argv[]) {
  37     MyApplication app(argc, argv);
  38     // ...
  39 }
  40 """]]
  41
  42 Of course, you can also inherit from `QCoreApplication` to get rid of
  43 the `QtGui` dependency, or display a nice dialog box instead of printing
  44 the messages to the console, or…
  45
  46 Found at: [Stack Overflow: Qt and error handling strategy][1]
  47
  48 [1]: http://stackoverflow.com/questions/1578331/qt-and-error-handling-strategy
  49
  50 [[!tag howto programming exceptions Qt signals slots]]