Usage of QTimer::singleShot(int msec, Func1 slot)?

Thuan_Firelight

Hi,

I recently stumbled upon this particular variant of QTimer::singleShot. What is this meant to do? I stepped through the code and it ends up calling QTimer::singleShot(int msec, Qt::TimerType timerType, const QObject *receiver, const char *member) with receiver being 0 and it does nothing. This is Qt 5.4.1 by the way.

JKSH

Hi,

@Thuan_Firelight said:

I stepped through the code and it ends up calling QTimer::singleShot(int msec, Qt::TimerType timerType, const QObject *receiver, const char *member)

That sounds odd to me. How are you ensuring that you are using this overload?

Func1 is a function pointer. You can use this overload of QTimer::singleShot() to call a standalone function or a lambda expression:

void standaloneFunc()
{
    qDebug() << "Hello from standaloneFunc()";
}

int main(int argc, char **argv)
{
    // ...

    QTimer::singleShot(1000, &standaloneFunc);

    QTimer::singleShot(2000, []{
        qDebug("Hello from lambda");
    });

    // ...
}

This will print "Hello from standaloneFunc()" after 1 second, and "Hello from lambda" after 2 seconds.

Thuan_Firelight

@JKSH said:

QTimer::singleShot(1000, &standaloneFunc);

Sorry for the late response. Just realized someone has replied to my question. In my code, I used the old syntax,

QTimer::singleShot(0, SLOT(standaloneFunc()));

Both your code and my code goes into singleShot(int msec, Func1 slot) but seems to do different things. In my case I was really meant to call a function on a particular receiver, so passing the receiver fixed my problem. Just curious regarding the behavior. If I step through the code, it goes to:

qtimer.h

    // singleShot to a functor or function pointer (without context)
    template <typename Func1>
    static inline void singleShot(int msec, Func1 slot)
    {
        singleShot(msec, msec >= 2000 ? Qt::CoarseTimer : Qt::PreciseTimer, Q_NULLPTR, slot);
    }

qtimer.cpp

void QTimer::singleShot(int msec, Qt::TimerType timerType, const QObject *receiver, const char *member)
{
    if (Q_UNLIKELY(msec < 0)) {
        qWarning("QTimer::singleShot: Timers cannot have negative timeouts");
        return;
    }
    if (receiver && member) {
        if (msec == 0) {
            // special code shortpath for 0-timers
            const char* bracketPosition = strchr(member, '(');
            if (!bracketPosition || !(member[0] >= '0' && member[0] <= '2')) {
                qWarning("QTimer::singleShot: Invalid slot specification");
                return;
            }
            QByteArray methodName(member+1, bracketPosition - 1 - member); // extract method name
            QMetaObject::invokeMethod(const_cast<QObject *>(receiver), methodName.constData(), Qt::QueuedConnection);
            return;
        }
        (void) new QSingleShotTimer(msec, timerType, receiver, member);
    }
}

which just bails out without a warning.