QElapsedTimer 类

QElapsedTimer 类提供计算消耗时间的快速方式。 更多...

头：	#include <QElapsedTimer>
CMake:	find_package(Qt6 REQUIRED COMPONENTS Core) target_link_libraries(mytarget PRIVATE Qt6::Core)
qmake:	QT += core

• 所有成员列表，包括继承成员

注意： 此类的所有函数 可重入 .

公共类型

enum	ClockType { SystemTime, MonotonicClock, TickCounter, MachAbsoluteTime, PerformanceCounter }
	Duration
	TimePoint

公共函数

	QElapsedTimer ()
(从 6.6 起) QElapsedTimer::Duration	durationElapsed () const
(从 6.6 起) QElapsedTimer::Duration	durationTo (const QElapsedTimer & other ) const
qint64	elapsed () const
bool	hasExpired (qint64 timeout ) const
void	invalidate ()
bool	isValid () const
qint64	msecsSinceReference () const
qint64	msecsTo (const QElapsedTimer & other ) const
qint64	nsecsElapsed () const
qint64	restart ()
qint64	secsTo (const QElapsedTimer & other ) const
void	start ()

静态公共成员

QElapsedTimer::ClockType	clockType ()
bool	isMonotonic ()

相关非成员

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

详细描述

QElapsedTimer 类通常用于快速计算 2 事件之间消耗了多少时间。其 API 类似于 QTime ，因此，正使用代码可以快速移植到新类。

不管怎样，不像 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:

    QElapsedTimer timer;
    timer.start();
    slowOperation1();
    qDebug() << "The slow operation took" << timer.elapsed() << "milliseconds";
					

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

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 waitFor -type functions in QIODevice and its subclasses are good examples of such need. In that case, the code could be as follows:

void executeSlowOperations(int timeout)
{
    QElapsedTimer timer;
    timer.start();
    slowOperation1();
    int remainingTime = timeout - timer.elapsed();
    if (remainingTime > 0)
        slowOperation2(remainingTime);
}
					

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:

void executeOperationsForTime(int ms)
{
    QElapsedTimer timer;
    timer.start();
    while (!timer.hasExpired(ms))
        slowOperation1();
}
					

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

参考时钟

QElapsedTimer will use the platform's monotonic reference clock in all platforms that support it (see QElapsedTimer::isMonotonic ()). This has the added benefit that QElapsedTimer is immune to time adjustments, such as the user correcting the time. Also unlike QTime , QElapsedTimer is immune to changes in the timezone settings, such as daylight-saving periods.

On the other hand, this means QElapsedTimer values can only be compared with other values that use the same reference. This is especially true if the time since the reference is extracted from the QElapsedTimer object ( QElapsedTimer::msecsSinceReference ()) and serialised. These values should never be exchanged across the network or saved to disk, since there's no telling whether the computer node receiving the data is the same as the one originating it or if it has rebooted since.

It is, however, possible to exchange the value with other processes running on the same machine, provided that they also use the same reference clock. QElapsedTimer will always use the same clock, so it's safe to compare with the value coming from another process in the same machine. If comparing to values produced by other APIs, you should check that the clock used is the same as QElapsedTimer (see QElapsedTimer::clockType ()).

另请参阅 QTime , QTimer ，和 QDeadlineTimer .

成员类型文档编制

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.

常量	值	描述
QElapsedTimer::SystemTime	0	The human-readable system time. This clock is not monotonic.
QElapsedTimer::MonotonicClock	1	The system's monotonic clock, usually found in Unix systems. This clock is monotonic.
QElapsedTimer::TickCounter	2	Not used anymore.
QElapsedTimer::MachAbsoluteTime	3	The Mach kernel's absolute time (macOS and iOS). This clock is monotonic.
QElapsedTimer::PerformanceCounter	4	The performance counter provided by Windows. This clock is monotonic.

SystemTime

The system time clock is purely the real time, expressed in milliseconds since Jan 1, 1970 at 0:00 UTC. It's equivalent to the value returned by the C and POSIX time function, with the milliseconds added. This clock type is currently only used on Unix systems that do not support monotonic clocks (see below).

This is the only non-monotonic clock that QElapsedTimer may use.

MonotonicClock

This is the system's monotonic clock, expressed in milliseconds since an arbitrary point in the past. This clock type is used on Unix systems which support POSIX monotonic clocks ( _POSIX_MONOTONIC_CLOCK ).

MachAbsoluteTime

This clock type is based on the absolute time presented by Mach kernels, such as that found on macOS. This clock type is presented separately from MonotonicClock since macOS and iOS are also Unix systems and may support a POSIX monotonic clock with values differing from the Mach absolute time.

This clock is monotonic.

PerformanceCounter

This clock uses the Windows functions QueryPerformanceCounter and QueryPerformanceFrequency to access the system's performance counter.

This clock is monotonic.

另请参阅 clockType () 和 isMonotonic ().

[alias] QElapsedTimer:: Duration

同义词 std::chrono::nanoseconds .

[alias] QElapsedTimer:: TimePoint

同义词 std::chrono::time_point<std::chrono::steady_clock, Duration> .

成员函数文档编制

[constexpr noexcept] QElapsedTimer:: QElapsedTimer ()

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

另请参阅 isValid () 和 start ().

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

Returns the clock type that this QElapsedTimer implementation uses.

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

另请参阅 isMonotonic ().

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

返回 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.

该函数在 Qt 6.6 引入。

另请参阅 start (), restart (), hasExpired ()，和 invalidate ().

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

Returns the time difference between this QElapsedTimer and other 作为 std::chrono::nanoseconds 。若 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.

该函数在 Qt 6.6 引入。

另请参阅 secsTo () 和 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.

另请参阅 start (), restart (), hasExpired (), isValid ()，和 invalidate ().

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

返回 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.

另请参阅 elapsed () 和 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.

另请参阅 isValid (), start ()，和 restart ().

[static noexcept] bool QElapsedTimer:: isMonotonic ()

返回 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 使用 std::chrono::steady_clock , so this function now always returns true.

另请参阅 clockType () 和 QElapsedTimer::ClockType .

[noexcept] bool QElapsedTimer:: isValid () const

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

另请参阅 invalidate (), start ()，和 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.

另请参阅 clockType () 和 elapsed ().

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

Returns the number of milliseconds between this QElapsedTimer and other 。若 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.

另请参阅 secsTo () 和 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.

另请参阅 start (), restart (), hasExpired ()，和 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;
					

另请参阅 start (), invalidate (), elapsed ()，和 isValid ().

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

Returns the number of seconds between this QElapsedTimer and other 。若 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.

另请参阅 msecsTo () 和 elapsed ().

[noexcept] void QElapsedTimer:: start ()

启动此计时器。一旦被启动，可以校验计时器值采用 elapsed () 或 msecsSinceReference ().

通常，仅仅启动计时器在长时间操作之前，譬如：

    QElapsedTimer timer;
    timer.start();
    slowOperation1();
    qDebug() << "The slow operation took" << timer.elapsed() << "milliseconds";
					

此外，启动计时器使之再次有效。

另请参阅 restart (), invalidate ()，和 elapsed ().

相关非成员

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

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

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

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

内容

• 公共类型

• 公共函数

• 静态公共成员

• 相关非成员

• 详细描述

• 参考时钟