The QElapsedTimer class provides a fast way to calculate elapsed times. More...

#include <qelapsedtimer.h>

Public Types
enum	ClockType { SystemTime , MonotonicClock , TickCounter , MachAbsoluteTime , PerformanceCounter }

Public Member Functions
constexpr	QElapsedTimer ()

void	start () noexcept

qint64	restart () noexcept

void	invalidate () noexcept

bool	isValid () const noexcept

qint64	nsecsElapsed () const noexcept

qint64	elapsed () const noexcept

bool	hasExpired (qint64 timeout) const noexcept

qint64	msecsSinceReference () const noexcept

qint64	msecsTo (const QElapsedTimer &other) const noexcept

qint64	secsTo (const QElapsedTimer &other) const noexcept

Static Public Member Functions
static ClockType	clockType () noexcept

static bool	isMonotonic () noexcept

Friends
bool	operator== (const QElapsedTimer &lhs, const QElapsedTimer &rhs) noexcept

bool	operator!= (const QElapsedTimer &lhs, const QElapsedTimer &rhs) noexcept

bool Q_CORE_EXPORT	operator< (const QElapsedTimer &lhs, const QElapsedTimer &rhs) noexcept

Related Functions
(Note that these are not member functions.)
bool	operator< (const QElapsedTimer &lhs, const QElapsedTimer &rhs) noexcept

Detailed Description

The QElapsedTimer class provides a fast way to calculate elapsed times.

\inmodule QtCore

Since: 4.7

\reentrant

The QElapsedTimer class is usually used to quickly calculate how much time has elapsed between two events. Its API is similar to that of QTime, so code that was using that can be ported quickly to the new class.

However, unlike QTime, QElapsedTimer tries to use monotonic clocks if possible. This means it's not possible to convert QElapsedTimer objects to a human-readable time.

The typical use-case for the class is to determine how much time was spent in a slow operation. The simplest example of such a case is for debugging purposes, as in the following example:

In this example, the timer is started by a call to start() and the elapsed time is calculated by the elapsed() function.

The time elapsed can also be used to recalculate the time available for another operation, after the first one is complete. This is useful when the execution must complete within a certain time period, but several steps are needed. The \tt{waitFor}-type functions in QIODevice and its subclasses are good examples of such need. In that case, the code could be as follows:

Another use-case is to execute a certain operation for a specific timeslice. For this, QElapsedTimer provides the hasExpired() convenience function, which can be used to determine if a certain number of milliseconds has already elapsed:

It is often more convenient to use \l{QDeadlineTimer} in this case, which counts towards a timeout in the future instead of tracking elapsed time.

Definition at line 48 of file qelapsedtimer.h.

Member Enumeration Documentation

◆ ClockType

enum QElapsedTimer::ClockType

This enum contains the different clock types that QElapsedTimer may use.

QElapsedTimer will always use the same clock type in a particular machine, so this value will not change during the lifetime of a program. It is provided so that QElapsedTimer can be used with other non-Qt implementations, to guarantee that the same reference clock is being used.

\value SystemTime The human-readable system time. This clock is not monotonic. \value MonotonicClock The system's monotonic clock, usually found in Unix systems. This clock is monotonic. \value TickCounter Not used anymore. \value MachAbsoluteTime The Mach kernel's absolute time (\macos and iOS). This clock is monotonic. \value PerformanceCounter The performance counter provided by Windows. This clock is monotonic.

Enumerator
SystemTime
MonotonicClock
TickCounter
MachAbsoluteTime
PerformanceCounter

Definition at line 51 of file qelapsedtimer.h.

Constructor & Destructor Documentation

◆ QElapsedTimer()

QElapsedTimer::QElapsedTimer ( )

inlineconstexpr

Since: 5.4

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

See also: isValid(), start()

Definition at line 59 of file qelapsedtimer.h.

Member Function Documentation

◆ clockType()

QElapsedTimer::ClockType QElapsedTimer::clockType ( )

staticnoexcept

Returns the clock type that this QElapsedTimer implementation uses.

See also: isMonotonic()

Definition at line 51 of file qelapsedtimer_generic.cpp.

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◆ elapsed()

qint64 QElapsedTimer::elapsed ( ) const

noexcept

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(), invalidate()

Definition at line 136 of file qelapsedtimer_generic.cpp.

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◆ hasExpired()

bool QElapsedTimer::hasExpired ( qint64 timeout ) const

noexcept

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(), QDeadlineTimer

Definition at line 239 of file qelapsedtimer.cpp.

◆ invalidate()

void QElapsedTimer::invalidate ( )

noexcept

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(), restart()

Definition at line 215 of file qelapsedtimer.cpp.

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◆ isMonotonic()

bool QElapsedTimer::isMonotonic ( )

staticnoexcept

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(), QElapsedTimer::ClockType

Definition at line 63 of file qelapsedtimer_generic.cpp.

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◆ isValid()

bool QElapsedTimer::isValid ( ) const

noexcept

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

See also: invalidate(), start(), restart()

Definition at line 226 of file qelapsedtimer.cpp.

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◆ msecsSinceReference()

qint64 QElapsedTimer::msecsSinceReference ( ) const

noexcept

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(), elapsed()

Definition at line 156 of file qelapsedtimer_generic.cpp.

◆ msecsTo()

qint64 QElapsedTimer::msecsTo ( const QElapsedTimer & other ) const

noexcept

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(), elapsed()

Definition at line 171 of file qelapsedtimer_generic.cpp.

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◆ nsecsElapsed()

qint64 QElapsedTimer::nsecsElapsed ( ) const

noexcept

Since: 4.8

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(), invalidate()

Definition at line 122 of file qelapsedtimer_generic.cpp.

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◆ restart()

qint64 QElapsedTimer::restart ( )

noexcept

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:

See also: start(), invalidate(), elapsed(), isValid()

Definition at line 101 of file qelapsedtimer_generic.cpp.

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◆ secsTo()

qint64 QElapsedTimer::secsTo ( const QElapsedTimer & other ) const

noexcept

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(), elapsed()

Definition at line 187 of file qelapsedtimer_generic.cpp.

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◆ start()

void QElapsedTimer::start ( )

noexcept

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:

Also, starting a timer makes it valid again.

See also: restart(), invalidate(), elapsed()

Definition at line 78 of file qelapsedtimer_generic.cpp.

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Friends And Related Function Documentation

◆ operator!=

bool QElapsedTimer::operator!=	(	const QElapsedTimer &	lhs,
		const QElapsedTimer &	rhs
	)

friend

Returns true if lhs and rhs contain different times, false otherwise.

Definition at line 83 of file qelapsedtimer.h.

◆ operator<() [1/2]

bool operator<	(	const QElapsedTimer &	lhs,
		const QElapsedTimer &	rhs
	)

The returned value is undefined if one of the two parameters is invalid and the other isn't. However, two invalid timers are equal and thus this function will return false.

Definition at line 192 of file qelapsedtimer_generic.cpp.

◆ operator< [2/2]

bool operator<	(	const QElapsedTimer &	lhs,
		const QElapsedTimer &	rhs
	)

friend

Returns true if lhs was started before rhs, false otherwise.

The returned value is undefined if one of the two parameters is invalid and the other isn't. However, two invalid timers are equal and thus this function will return false.

Definition at line 192 of file qelapsedtimer_generic.cpp.

◆ operator==

bool QElapsedTimer::operator==	(	const QElapsedTimer &	lhs,
		const QElapsedTimer &	rhs
	)

friend

Returns true if lhs and rhs contain the same time, false otherwise.

Definition at line 81 of file qelapsedtimer.h.

The documentation for this class was generated from the following files:

src/corelib/kernel/qelapsedtimer.h
src/corelib/kernel/qelapsedtimer.cpp
src/corelib/kernel/qelapsedtimer_generic.cpp
src/corelib/kernel/qelapsedtimer_mac.cpp
src/corelib/kernel/qelapsedtimer_unix.cpp
src/corelib/kernel/qelapsedtimer_win.cpp

Public Types

Public Member Functions

Static Public Member Functions

Friends

Related Functions

Detailed Description

Member Enumeration Documentation

◆ ClockType

Constructor & Destructor Documentation

◆ QElapsedTimer()

Member Function Documentation

◆ clockType()

◆ elapsed()

◆ hasExpired()

◆ invalidate()

◆ isMonotonic()

◆ isValid()

◆ msecsSinceReference()

◆ msecsTo()

◆ nsecsElapsed()

◆ restart()

◆ secsTo()

◆ start()

Friends And Related Function Documentation

◆ operator!=

◆ operator<() [1/2]

◆ operator< [2/2]

◆ operator==