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

QElapsedTimer::QElapsedTimer()

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

This function was introduced in Qt 5.4.

See also isValid() and start().

[static] ClockType QElapsedTimer::clockType()

Returns the clock type that this QElapsedTimer implementation uses.

See also isMonotonic().

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().

bool QElapsedTimer::hasExpired(qint64 timeout) const

Returns true if this QElapsedTimer has already expired by timeout milliseconds (that is, more than timeout milliseconds have elapsed). The value of timeout can be -1 to indicate that this timer does not expire, in which case this function will always return false.

See also elapsed() and QDeadlineTimer.

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] 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.

See also clockType() and QElapsedTimer::ClockType.

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().

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().

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().

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.

This function was introduced in Qt 4.8.

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

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().

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().

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().

bool QElapsedTimer::operator!=(const QElapsedTimer &other) const

Returns true if this object and other contain different times.

bool QElapsedTimer::operator==(const QElapsedTimer &other) const

Returns true if this object and other contain the same time.