QDeadlineTimer Class

The QDeadlineTimer class marks a deadline in the future. More...

This class was introduced in Qt 5.8.

• List of all members, including inherited members

Note: All functions in this class are reentrant.

Public Types

Public Functions

Static Public Members

Related Non-Members

Detailed Description

The QDeadlineTimer class is usually used to calculate future deadlines and verify whether the deadline has expired. QDeadlineTimer can also be used for deadlines without expiration ("forever"). It forms a counterpart to QElapsedTimer, which calculates how much time has elapsed since QElapsedTimer::start() was called.

QDeadlineTimer provides a more convenient API compared to QElapsedTimer::hasExpired().

The typical use-case for the class is to create a QDeadlineTimer before the operation in question is started, and then use remainingTime() or hasExpired() to determine whether to continue trying the operation. QDeadlineTimer objects can be passed to functions being called to execute this operation so they know how long to still operate.

    void executeOperation(int msecs)
    {
        QDeadlineTimer deadline(msecs);
        do {
            if (readFromDevice(deadline.remainingTime())
                break;
            waitForReadyRead(deadline);
        } while (!deadline.hasExpired());
    }

Many QDeadlineTimer functions deal with time out values, which all are measured in milliseconds. There are two special values, the same as many other Qt functions named waitFor or similar:

• 0: no time left, expired
• -1: infinite time left, timer never expires

Reference Clocks

QDeadlineTimer will use the same clock as QElapsedTimer (see QElapsedTimer::clockType() and QElapsedTimer::isMonotonic()).

Timer types

Like QTimer, QDeadlineTimer can select among different levels of coarseness on the timers. You can select precise timing by passing Qt::PreciseTimer to the functions that set of change the timer, or you can select coarse timing by passing Qt::CoarseTimer. Qt::VeryCoarseTimer is currently interpreted the same way as Qt::CoarseTimer.

This feature is dependent on support from the operating system: if the OS does not support a coarse timer functionality, then QDeadlineTimer will behave like Qt::PreciseTimer was passed.

QDeadlineTimer defaults to Qt::CoarseTimer because on operating systems that do support coarse timing, making timing calls to that clock source is often much more efficient. The level of coarseness depends on the operating system, but should be in the order of a couple of milliseconds.

std::chrono Compatibility

QDeadlineTimer is compatible with the std::chrono API from C++11 and can be constructed from or compared to both std::chrono::duration and std::chrono::time_point objects. In addition, it is fully compatible with the time literals from C++14, which allow one to write code as:

    using namespace std::chrono;
    using namespace std::chrono_literals;

    QDeadlineTimer deadline(30s);
    device->waitForReadyRead(deadline);
    if (deadline.remainingTime<nanoseconds>() > 300ms)
        cleanup();

As can be seen in the example above, QDeadlineTimer offers a templated version of remainingTime() and deadline() that can be used to return std::chrono objects.

Note that comparing to time_point is not as efficient as comparing to duration, since QDeadlineTimer may need to convert from its own internal clock source to the clock source used by the time_point object. Also note that, due to this conversion, the deadlines will not be precise, so the following code is not expected to compare equally:

    using namespace std::chrono;
    using namespace std::chrono_literals;
    auto now = steady_clock::now();
    QDeadlineTimer deadline(now + 1s);
    Q_ASSERT(deadline == now + 1s);

See also QTime, QTimer, QDeadlineTimer, and Qt::TimerType.

Member Type Documentation

enum QDeadlineTimer::ForeverConstant

Member Function Documentation

template <typename Rep, typename Period> QDeadlineTimer::QDeadlineTimer(std::chrono::duration<Rep, Period> remaining, Qt::TimerType type = Qt::CoarseTimer)

Constructs a QDeadlineTimer object with a remaining time of remaining. If remaining is zero or negative, this QDeadlineTimer object will be mark as expired, whereas if remaining is equal to duration::max(), the object will be set to never expire.

The QDeadlineTimer object will be constructed with the specified timer type.

This constructor can be used with C++14's user-defined literals for time, such as in:

    using namespace std::chrono_literals;
    QDeadlineTimer deadline(250ms);

For optimization purposes, if remaining is zero or negative, this function may skip obtaining the current time and may instead use a value known to be in the past. If that happens, deadline() may return an unexpected value and this object cannot be used in calculation of how long it is overdue. If that functionality is required, use QDeadlineTimer::current() and add time to it.

See also hasExpired(), isForever(), remainingTime(), and setRemainingTime().

template <typename Clock, typename Duration> QDeadlineTimer::QDeadlineTimer(std::chrono::time_point<Clock, Duration> deadline, Qt::TimerType type = Qt::CoarseTimer)

Constructs a QDeadlineTimer object with a deadline at deadline time point, converting from the clock source Clock to Qt's internal clock source (see QElapsedTimer::clockType()).

If deadline is in the past, this QDeadlineTimer object is set to expired, whereas if deadline is equal to Duration::max(), then this object is set to never expire.

The QDeadlineTimer object will be constructed with the specified timer type.

See also hasExpired(), isForever(), remainingTime(), and setDeadline().

QDeadlineTimer::QDeadlineTimer(qint64 msecs, Qt::TimerType type = Qt::CoarseTimer)

Constructs a QDeadlineTimer object with an expiry time of msecs msecs from the moment of the creation of this object, if msecs is positive. If msecs is zero, this QDeadlineTimer will be marked as expired, causing remainingTime() to return zero and deadline() to return an indeterminate time point in the past. If msecs is -1, the timer will be set to never expire, causing remainingTime() to return -1 and deadline() to return the maximum value.

The QDeadlineTimer object will be constructed with the specified timer type.

For optimization purposes, if msecs is zero, this function may skip obtaining the current time and may instead use a value known to be in the past. If that happens, deadline() may return an unexpected value and this object cannot be used in calculation of how long it is overdue. If that functionality is required, use QDeadlineTimer::current() and add time to it.

See also hasExpired(), isForever(), remainingTime(), and setRemainingTime().

QDeadlineTimer::QDeadlineTimer(QDeadlineTimer::ForeverConstant, Qt::TimerType timerType = Qt::CoarseTimer)

QDeadlineTimer objects created with ForeverConstant never expire. For such objects, remainingTime() will return -1, deadline() will return the maximum value, and isForever() will return true.

The timer type timerType may be ignored, since the timer will never expire.

See also ForeverConstant, hasExpired(), isForever(), remainingTime(), and timerType().

QDeadlineTimer::QDeadlineTimer(Qt::TimerType timerType = Qt::CoarseTimer)

Constructs an expired QDeadlineTimer object. For this object, remainingTime() will return 0.

The timer type timerType may be ignored, since the timer is already expired. Similarly, for optimization purposes, this function will not attempt to obtain the current time and will use a value known to be in the past. Therefore, deadline() may return an unexpected value and this object cannot be used in calculation of how long it is overdue. If that functionality is required, use QDeadlineTimer::current().

See also hasExpired(), remainingTime(), Qt::TimerType, and current().

[static] QDeadlineTimer QDeadlineTimer::addNSecs(QDeadlineTimer dt, qint64 nsecs)

Returns a QDeadlineTimer object whose deadline is extended from dt's deadline by nsecs nanoseconds. If dt was set to never expire, this function returns a QDeadlineTimer that will not expire either.

Note: if dt was created as expired, its deadline is indeterminate and adding an amount of time may or may not cause it to become unexpired.

[static] QDeadlineTimer QDeadlineTimer::current(Qt::TimerType timerType = Qt::CoarseTimer)

Returns a QDeadlineTimer that is expired but is guaranteed to contain the current time. Objects created by this function can participate in the calculation of how long a timer is overdue, using the deadline() function.

The QDeadlineTimer object will be constructed with the specified timerType.

qint64 QDeadlineTimer::deadline() const

Returns the absolute time point for the deadline stored in QDeadlineTimer object, calculated in milliseconds relative to the reference clock, the same as QElapsedTimer::msecsSinceReference(). The value will be in the past if this QDeadlineTimer has expired.

If this QDeadlineTimer never expires, this function returns std::numeric_limits<qint64>::max().

This function can be used to calculate the amount of time a timer is overdue, by subtracting QDeadlineTimer::current() or QElapsedTimer::msecsSinceReference(), as in the following example:

    qint64 realTimeLeft = deadline.deadline();
    if (realTimeLeft != (std::numeric_limits<qint64>::max)()) {
        realTimeLeft -= QDeadlineTimer::current().deadline();
        // or:
        //QElapsedTimer timer;
        //timer.start();
        //realTimeLeft -= timer.msecsSinceReference();
    }

Note: Timers that were created as expired have an indetermine time point in the past as their deadline, so the above calculation may not work.

See also remainingTime(), deadlineNSecs(), and setDeadline().

qint64 QDeadlineTimer::deadlineNSecs() const

Returns the absolute time point for the deadline stored in QDeadlineTimer object, calculated in nanoseconds relative to the reference clock, the same as QElapsedTimer::msecsSinceReference(). The value will be in the past if this QDeadlineTimer has expired.

If this QDeadlineTimer never expires or the number of nanoseconds until the deadline can't be accommodated in the return type, this function returns std::numeric_limits<qint64>::max().

This function can be used to calculate the amount of time a timer is overdue, by subtracting QDeadlineTimer::current(), as in the following example:

    qint64 realTimeLeft = deadline.deadlineNSecs();
    if (realTimeLeft != std::numeric_limits<qint64>::max())
        realTimeLeft -= QDeadlineTimer::current().deadlineNSecs();

Note: Timers that were created as expired have an indetermine time point in the past as their deadline, so the above calculation may not work.

See also remainingTime() and deadlineNSecs().

bool QDeadlineTimer::hasExpired() const

Returns true if this QDeadlineTimer object has expired, false if there remains time left. For objects that have expired, remainingTime() will return zero and deadline() will return a time point in the past.

QDeadlineTimer objects created with the ForeverConstant never expire and this function always returns false for them.

See also isForever() and remainingTime().

bool QDeadlineTimer::isForever() const

Returns true if this QDeadlineTimer object never expires, false otherwise. For timers that never expire, remainingTime() always returns -1 and deadline() returns the maximum value.

See also ForeverConstant, hasExpired(), and remainingTime().

qint64 QDeadlineTimer::remainingTime() const

Returns the remaining time in this QDeadlineTimer object in milliseconds. If the timer has already expired, this function will return zero and it is not possible to obtain the amount of time overdue with this function (to do that, see deadline()). If the timer was set to never expire, this function returns -1.

This function is suitable for use in Qt APIs that take a millisecond timeout, such as the many QIODevice waitFor functions or the timed lock functions in QMutex, QWaitCondition, QSemaphore, or QReadWriteLock. For example:

    mutex.tryLock(deadline.remainingTime());

See also setRemainingTime(), remainingTimeNSecs(), isForever(), and hasExpired().

std::chrono::nanoseconds QDeadlineTimer::remainingTimeAsDuration() const

Returns the time remaining before the deadline.

qint64 QDeadlineTimer::remainingTimeNSecs() const

Returns the remaining time in this QDeadlineTimer object in nanoseconds. If the timer has already expired, this function will return zero and it is not possible to obtain the amount of time overdue with this function. If the timer was set to never expire, this function returns -1.

See also remainingTime(), isForever(), and hasExpired().

void QDeadlineTimer::setDeadline(qint64 msecs, Qt::TimerType timerType = Qt::CoarseTimer)

Sets the deadline for this QDeadlineTimer object to be the msecs absolute time point, counted in milliseconds since the reference clock (the same as QElapsedTimer::msecsSinceReference()), and the timer type to timerType. If the value is in the past, this QDeadlineTimer will be marked as expired.

If msecs is std::numeric_limits<qint64>::max() or the deadline is beyond a representable point in the future, this QDeadlineTimer will be set to never expire.

See also setPreciseDeadline(), deadline(), deadlineNSecs(), and setRemainingTime().

template <typename Clock, typename Duration> void QDeadlineTimer::setDeadline(std::chrono::time_point<Clock, Duration> deadline, Qt::TimerType type = Qt::CoarseTimer)

Sets this QDeadlineTimer to the deadline marked by deadline time point, converting from the clock source Clock to Qt's internal clock source (see QElapsedTimer::clockType()).

If deadline is in the past, this QDeadlineTimer object is set to expired, whereas if deadline is equal to Duration::max(), then this object is set to never expire.

The timer type for this QDeadlineTimer object will be set to the specified type.

See also hasExpired(), isForever(), and remainingTime().

void QDeadlineTimer::setPreciseDeadline(qint64 secs, qint64 nsecs = 0, Qt::TimerType timerType = Qt::CoarseTimer)

Sets the deadline for this QDeadlineTimer object to be secs seconds and nsecs nanoseconds since the reference clock epoch (the same as QElapsedTimer::msecsSinceReference()), and the timer type to timerType. If the value is in the past, this QDeadlineTimer will be marked as expired.

If secs or nsecs is std::numeric_limits<qint64>::max(), this QDeadlineTimer will be set to never expire. If nsecs is more than 1 billion nanoseconds (1 second), then secs will be adjusted accordingly.

See also setDeadline(), deadline(), deadlineNSecs(), and setRemainingTime().

void QDeadlineTimer::setPreciseRemainingTime(qint64 secs, qint64 nsecs = 0, Qt::TimerType timerType = Qt::CoarseTimer)

Sets the remaining time for this QDeadlineTimer object to secs seconds plus nsecs nanoseconds from now, if secs has a positive value. If secs is -1, this QDeadlineTimer will be set it to never expire. If both parameters are zero, this QDeadlineTimer will be marked as expired.

The timer type for this QDeadlineTimer object will be set to the specified timerType.

See also setRemainingTime(), hasExpired(), isForever(), and remainingTime().

void QDeadlineTimer::setRemainingTime(qint64 msecs, Qt::TimerType timerType = Qt::CoarseTimer)

Sets the remaining time for this QDeadlineTimer object to msecs milliseconds from now, if msecs has a positive value. If msecs is zero, this QDeadlineTimer object will be marked as expired, whereas a value of -1 will set it to never expire.

The timer type for this QDeadlineTimer object will be set to the specified timerType.

See also setPreciseRemainingTime(), hasExpired(), isForever(), and remainingTime().

template <typename Rep, typename Period> void QDeadlineTimer::setRemainingTime(std::chrono::duration<Rep, Period> remaining, Qt::TimerType type = Qt::CoarseTimer)

This is an overloaded function.

Sets the remaining time for this QDeadlineTimer object to remaining. If remaining is zero or negative, this QDeadlineTimer object will be mark as expired, whereas if remaining is equal to duration::max(), the object will be set to never expire.

The timer type for this QDeadlineTimer object will be set to the specified type.

This function can be used with C++14's user-defined literals for time, such as in:

    using namespace std::chrono_literals;
    deadline.setRemainingTime(250ms);

Note: Qt detects the necessary C++14 compiler support by way of the feature test recommendations from C++ Committee's Standing Document 6.

See also setDeadline(), remainingTime(), hasExpired(), and isForever().

void QDeadlineTimer::setTimerType(Qt::TimerType timerType)

Changes the timer type for this object to timerType.

The behavior for each possible value of timerType is operating-system dependent. Qt::PreciseTimer will use the most precise timer that Qt can find, with resolution of 1 millisecond or better, whereas QDeadlineTimer will try to use a more coarse timer for Qt::CoarseTimer and Qt::VeryCoarseTimer.

See also timerType() and Qt::TimerType.

void QDeadlineTimer::swap(QDeadlineTimer &other)

Swaps this deadline timer with the other deadline timer.

Qt::TimerType QDeadlineTimer::timerType() const

Returns the timer type is active for this object.

See also setTimerType().

QDeadlineTimer &QDeadlineTimer::operator+=(qint64 msecs)

Extends this QDeadlineTimer object by msecs milliseconds and returns itself. If this object is set to never expire, this function does nothing.

To add times of precision greater than 1 millisecond, use addNSecs().

QDeadlineTimer &QDeadlineTimer::operator-=(qint64 msecs)

Shortens this QDeadlineTimer object by msecs milliseconds and returns itself. If this object is set to never expire, this function does nothing.

To subtract times of precision greater than 1 millisecond, use addNSecs().

template <typename Clock, typename Duration> QDeadlineTimer &QDeadlineTimer::operator=(std::chrono::time_point<Clock, Duration> deadline_)

Assigns deadline_ to this deadline timer.

template <typename Rep, typename Period> QDeadlineTimer &QDeadlineTimer::operator=(std::chrono::duration<Rep, Period> remaining)

Sets this deadline timer to the remaining time.

    return d1.deadlineNSecs() != d2.deadlineNSecs();

    return d1.deadlineNSecs() < d2.deadlineNSecs();

    return d1.deadlineNSecs() <= d2.deadlineNSecs();

    return d1.deadlineNSecs() == d2.deadlineNSecs();

    return d1.deadlineNSecs() > d2.deadlineNSecs();

    return d1.deadlineNSecs() >= d2.deadlineNSecs();