Should I check QEvent dynamic_cast result if I already checked event->type()?

Question

QEvent has a type() member function which returns the event type as enum value. Should I check dynamic_cast result for QEvent* if I already checked event->type(). E.g.

bool ClassName::eventFilter(QObject* obj, QEvent* event)
{
    if (event->type() == QEvent::KeyPress) 
    {
        auto ke = dynamic_cast<QKeyEvent*>(event);
        if (ke)                                     // <----------- needed?
        {
            // ...
        }
    }
    // ...
}

Is if (ke) condition needed here?

If you already know the type you can safely use `static_cast` instead without checking the result against null pointer. This is faster approach. — vahancho, Nov 21 '18 at 14:17
That's what I meant. And `static_cast` is more preferable in this case. — vahancho, Nov 21 '18 at 14:29
@vahancho maybe some reference to Qt documentation/implementation? Such info should be placed in answer area and accepted :) — αλεχολυτ, Nov 21 '18 at 14:34
Have a look at Qt doc.: [Event Filters](http://doc.qt.io/qt-5/eventsandfilters.html#event-filters). There you find an example which uses a `static_cast` like mentioned by @vahancho. — Scheff's Cat, Nov 21 '18 at 15:16

score 3 · Accepted Answer · answered Aug 18 '22 at 04:09

As a commenter suggested, checking out the Qt docs regarding this has the following example:

bool FilterObject::eventFilter(QObject *object, QEvent *event)
{
    if (object == target && event->type() == QEvent::KeyPress) {
        QKeyEvent *keyEvent = static_cast<QKeyEvent *>(event);
        if (keyEvent->key() == Qt::Key_Tab) {
            // Special tab handling
            return true;
        } else
            return false;
    }
    return false;
}

Because specific events will always be guaranteed to return the same type, checking against QEvent::type() is sufficient to allow for a safe static_cast to the target type. This will avoids the RTTI cost that a dynamic_cast would entail.

Should I check QEvent dynamic_cast result if I already checked event->type()?

1 Answers1