QElapsedTimer Class

     QElapsedTimer timer;
     timer.start();

     slowOperation1();

     qDebug() << "The slow operation took" << timer.elapsed() << "milliseconds";

 void executeSlowOperations(int timeout)
 {
     QElapsedTimer timer;
     timer.start();
     slowOperation1();

     int remainingTime = timeout - timer.elapsed();
     if (remainingTime > 0)
         slowOperation2(remainingTime);
 }

 void executeOperationsForTime(int ms)
 {
     QElapsedTimer timer;
     timer.start();

     while (!timer.hasExpired(ms))
         slowOperation1();
 }

Member Function Documentation

[constexpr noexcept] QElapsedTimer::QElapsedTimer()

Constructs an invalid QElapsedTimer. A timer becomes valid once it has been started.

See also isValid() and start().

[static noexcept] QElapsedTimer::ClockType QElapsedTimer::clockType()

Returns the clock type that this QElapsedTimer implementation uses.

Since Qt 6.6, QElapsedTimer uses std::chrono::steady_clock, so the clock type is always MonotonicClock.

See also isMonotonic().

[noexcept, since 6.6] QElapsedTimer::Duration QElapsedTimer::durationElapsed() const

Returns a std::chrono::nanoseconds with the time since this QElapsedTimer was last started.

Calling this function on a QElapsedTimer that is invalid results in undefined behavior.

On platforms that do not provide nanosecond resolution, the value returned will be the best estimate available.

This function was introduced in Qt 6.6.

See also start(), restart(), hasExpired(), and invalidate().

[noexcept, since 6.6] QElapsedTimer::Duration QElapsedTimer::durationTo(const QElapsedTimer &other) const

Returns the time difference between this QElapsedTimer and other as a std::chrono::nanoseconds. If other was started before this object, the returned value will be negative. If it was started later, the returned value will be positive.

The return value is undefined if this object or other were invalidated.

This function was introduced in Qt 6.6.

See also secsTo() and elapsed().

[noexcept] qint64 QElapsedTimer::elapsed() const

Returns the number of milliseconds since this QElapsedTimer was last started.

Calling this function on a QElapsedTimer that is invalid results in undefined behavior.

See also start(), restart(), hasExpired(), isValid(), and invalidate().

[noexcept] bool QElapsedTimer::hasExpired(qint64 timeout) const

Returns true if elapsed() exceeds the given timeout, otherwise false.

A negative timeout is interpreted as infinite, so false is returned in this case. Otherwise, this is equivalent to elapsed() > timeout. You can do the same for a duration by comparing durationElapsed() to a duration timeout.

See also elapsed() and QDeadlineTimer.

[noexcept] void QElapsedTimer::invalidate()

Marks this QElapsedTimer object as invalid.

An invalid object can be checked with isValid(). Calculations of timer elapsed since invalid data are undefined and will likely produce bizarre results.

See also isValid(), start(), and restart().

[static noexcept] bool QElapsedTimer::isMonotonic()

Returns true if this is a monotonic clock, false otherwise. See the information on the different clock types to understand which ones are monotonic.

Since Qt 6.6, QElapsedTimer uses std::chrono::steady_clock, so this function now always returns true.

See also clockType() and QElapsedTimer::ClockType.

[noexcept] bool QElapsedTimer::isValid() const

Returns false if the timer has never been started or invalidated by a call to invalidate().

See also invalidate(), start(), and restart().

[noexcept] qint64 QElapsedTimer::msecsSinceReference() const

Returns the number of milliseconds between last time this QElapsedTimer object was started and its reference clock's start.

This number is usually arbitrary for all clocks except the QElapsedTimer::SystemTime clock. For that clock type, this number is the number of milliseconds since January 1st, 1970 at 0:00 UTC (that is, it is the Unix time expressed in milliseconds).

On Linux, Windows and Apple platforms, this value is usually the time since the system boot, though it usually does not include the time the system has spent in sleep states.

See also clockType() and elapsed().

[noexcept] qint64 QElapsedTimer::msecsTo(const QElapsedTimer &other) const

Returns the number of milliseconds between this QElapsedTimer and other. If other was started before this object, the returned value will be negative. If it was started later, the returned value will be positive.

The return value is undefined if this object or other were invalidated.

See also secsTo() and elapsed().

[noexcept] qint64 QElapsedTimer::nsecsElapsed() const

Returns the number of nanoseconds since this QElapsedTimer was last started.

Calling this function on a QElapsedTimer that is invalid results in undefined behavior.

On platforms that do not provide nanosecond resolution, the value returned will be the best estimate available.

See also start(), restart(), hasExpired(), and invalidate().

[noexcept] qint64 QElapsedTimer::restart()

Restarts the timer and returns the number of milliseconds elapsed since the previous start. This function is equivalent to obtaining the elapsed time with elapsed() and then starting the timer again with start(), but it does so in one single operation, avoiding the need to obtain the clock value twice.

Calling this function on a QElapsedTimer that is invalid results in undefined behavior.

The following example illustrates how to use this function to calibrate a parameter to a slow operation (for example, an iteration count) so that this operation takes at least 250 milliseconds:

     QElapsedTimer timer;

     int count = 1;
     timer.start();
     do {
         count *= 2;
         slowOperation2(count);
     } while (timer.restart() < 250);

     return count;

See also start(), invalidate(), elapsed(), and isValid().

[noexcept] qint64 QElapsedTimer::secsTo(const QElapsedTimer &other) const

Returns the number of seconds between this QElapsedTimer and other. If other was started before this object, the returned value will be negative. If it was started later, the returned value will be positive.

Calling this function on or with a QElapsedTimer that is invalid results in undefined behavior.

See also msecsTo() and elapsed().

[noexcept] void QElapsedTimer::start()

Starts this timer. Once started, a timer value can be checked with elapsed() or msecsSinceReference().

Normally, a timer is started just before a lengthy operation, such as:

     QElapsedTimer timer;
     timer.start();

     slowOperation1();

     qDebug() << "The slow operation took" << timer.elapsed() << "milliseconds";

Also, starting a timer makes it valid again.

See also restart(), invalidate(), and elapsed().