QTime的本质上是一个int，QDateTime本质上是一个qint64

研究这个问题的起因发现使用<=比较时间的不准确，所以怀疑是一个浮点数（Delphi里的time就是一个浮点数）。结果却发现是一个int

class Q_CORE_EXPORT QTime
{
    explicit Q_DECL_CONSTEXPR QTime(int ms) : mds(ms)
#if defined(Q_OS_WINCE)
        , startTick(NullTime)
#endif
    {}
public:
    Q_DECL_CONSTEXPR QTime(): mds(NullTime)
#if defined(Q_OS_WINCE)
        , startTick(NullTime)
#endif
    {}
    QTime(int h, int m, int s = 0, int ms = 0);

    Q_DECL_CONSTEXPR bool isNull() const { return mds == NullTime; }
    bool isValid() const;

    int hour() const;
    int minute() const;
    int second() const;
    int msec() const;
#ifndef QT_NO_DATESTRING
    QString toString(Qt::DateFormat f = Qt::TextDate) const;
    QString toString(const QString &format) const;
#endif
    bool setHMS(int h, int m, int s, int ms = 0);

    QTime addSecs(int secs) const Q_REQUIRED_RESULT;
    int secsTo(const QTime &) const;
    QTime addMSecs(int ms) const Q_REQUIRED_RESULT;
    int msecsTo(const QTime &) const;

    Q_DECL_CONSTEXPR bool operator==(const QTime &other) const { return mds == other.mds; }
    Q_DECL_CONSTEXPR bool operator!=(const QTime &other) const { return mds != other.mds; }
    Q_DECL_CONSTEXPR bool operator< (const QTime &other) const { return mds <  other.mds; }
    Q_DECL_CONSTEXPR bool operator<=(const QTime &other) const { return mds <= other.mds; }
    Q_DECL_CONSTEXPR bool operator> (const QTime &other) const { return mds >  other.mds; }
    Q_DECL_CONSTEXPR bool operator>=(const QTime &other) const { return mds >= other.mds; }

    static Q_DECL_CONSTEXPR inline QTime fromMSecsSinceStartOfDay(int msecs) { return QTime(msecs); }
    Q_DECL_CONSTEXPR inline int msecsSinceStartOfDay() const { return mds == NullTime ? 0 : mds; }

    static QTime currentTime();
#ifndef QT_NO_DATESTRING
    static QTime fromString(const QString &s, Qt::DateFormat f = Qt::TextDate);
    static QTime fromString(const QString &s, const QString &format);
#endif
    static bool isValid(int h, int m, int s, int ms = 0);

    void start();
    int restart();
    int elapsed() const;
private:
    enum TimeFlag { NullTime = -1 };
    Q_DECL_CONSTEXPR inline int ds() const { return mds == -1 ? 0 : mds; }
    int mds;
#if defined(Q_OS_WINCE)
    int startTick;
#endif

    friend class QDateTime;
    friend class QDateTimePrivate;
#ifndef QT_NO_DATASTREAM
    friend Q_CORE_EXPORT QDataStream &operator<<(QDataStream &, const QTime &);
    friend Q_CORE_EXPORT QDataStream &operator>>(QDataStream &, QTime &);
#endif
};

但是整数的比较，为什么会不准确呢？不是很理解。。。非得减去一秒之后，比较才准确。让我再想想。。。

看到里面有一个QDateTimePrivate，略加研究之后发现

同时还在qdatetime.h里发现它使用了QDateTimePrivate，搜索之后（不在磁盘上搜索，只能看到它在qdatetime.h里有一个预定义class QDateTimePrivate），发现它定义在
C:QtQt5.6.25.6mingw49_32includeQtCore5.6.2QtCoreprivateqdatetime_p.h
C:QtQt5.6.25.6Srcqtbasesrccorelib oolsqdatetime.cpp

而不存在qdateime_p.cpp文件。

在qdatetime_p.h里发现如下定义：

class QDateTimePrivate : public QSharedData
{
public:
    // Never change or delete this enum, it is required for backwards compatible
    // serialization of QDateTime before 5.2, so is essentially public API
    enum Spec {
        LocalUnknown = -1,
        LocalStandard = 0,
        LocalDST = 1,
        UTC = 2,
        OffsetFromUTC = 3,
        TimeZone = 4
    };

    // Daylight Time Status
    enum DaylightStatus {
        NoDaylightTime = -2,
        UnknownDaylightTime = -1,
        StandardTime = 0,
        DaylightTime = 1
    };

    // Status of date/time
    enum StatusFlag {
        NullDate            = 0x01,
        NullTime            = 0x02,
        ValidDate           = 0x04, // just the date field
        ValidTime           = 0x08, // just the time field
        ValidDateTime       = 0x10, // the whole object (including timezone)
        SetToStandardTime   = 0x40,
        SetToDaylightTime   = 0x80
    };
    Q_DECLARE_FLAGS(StatusFlags, StatusFlag)

    QDateTimePrivate() : m_msecs(0),
                         m_spec(Qt::LocalTime),
                         m_offsetFromUtc(0),
                         m_status(NullDate | NullTime)
    {}

    QDateTimePrivate(const QDate &toDate, const QTime &toTime, Qt::TimeSpec toSpec,
                     int offsetSeconds);

#ifndef QT_BOOTSTRAPPED
    QDateTimePrivate(const QDate &toDate, const QTime &toTime, const QTimeZone & timeZone);
#endif // QT_BOOTSTRAPPED

    // ### XXX: when the tooling situation improves, look at fixing the padding.
    // 4 bytes padding

    qint64 m_msecs;
    Qt::TimeSpec m_spec;
    int m_offsetFromUtc;
#ifndef QT_BOOTSTRAPPED
    QTimeZone m_timeZone;
#endif // QT_BOOTSTRAPPED
    StatusFlags m_status;

    void setTimeSpec(Qt::TimeSpec spec, int offsetSeconds);
    void setDateTime(const QDate &date, const QTime &time);
    QPair<QDate, QTime> getDateTime() const;

    void setDaylightStatus(DaylightStatus status);
    DaylightStatus daylightStatus() const;

    // Returns msecs since epoch, assumes offset value is current
    inline qint64 toMSecsSinceEpoch() const;

    void checkValidDateTime();
    void refreshDateTime();

    // Get/set date and time status
    inline bool isNullDate() const { return m_status & NullDate; }
    inline bool isNullTime() const { return m_status & NullTime; }
    inline bool isValidDate() const { return m_status & ValidDate; }
    inline bool isValidTime() const { return m_status & ValidTime; }
    inline bool isValidDateTime() const { return m_status & ValidDateTime; }
    inline void setValidDateTime() { m_status |= ValidDateTime; }
    inline void clearValidDateTime() { m_status &= ~ValidDateTime; }
    inline void clearSetToDaylightStatus() { m_status &= ~(SetToStandardTime | SetToDaylightTime); }

#ifndef QT_BOOTSTRAPPED
    static qint64 zoneMSecsToEpochMSecs(qint64 msecs, const QTimeZone &zone,
                                        QDate *localDate = 0, QTime *localTime = 0);
#endif // QT_BOOTSTRAPPED

    static inline qint64 minJd() { return QDate::minJd(); }
    static inline qint64 maxJd() { return QDate::maxJd(); }
};

可见的此时又变成了一个qint64，它可能是QDateTime要用到的数据。于是又看了一下qdatetime的定义：

class QDateTimePrivate;

class Q_CORE_EXPORT QDateTime
{
public:
    QDateTime();
    explicit QDateTime(const QDate &);
    QDateTime(const QDate &, const QTime &, Qt::TimeSpec spec = Qt::LocalTime);
    // ### Qt 6: Merge with above with default offsetSeconds = 0
    QDateTime(const QDate &date, const QTime &time, Qt::TimeSpec spec, int offsetSeconds);
#ifndef QT_BOOTSTRAPPED
    QDateTime(const QDate &date, const QTime &time, const QTimeZone &timeZone);
#endif // QT_BOOTSTRAPPED
    QDateTime(const QDateTime &other);
    ~QDateTime();

#ifdef Q_COMPILER_RVALUE_REFS
    QDateTime &operator=(QDateTime &&other) Q_DECL_NOTHROW { swap(other); return *this; }
#endif
    QDateTime &operator=(const QDateTime &other);

    void swap(QDateTime &other) Q_DECL_NOTHROW { qSwap(d, other.d); }

    bool isNull() const;
    bool isValid() const;

    QDate date() const;
    QTime time() const;
    Qt::TimeSpec timeSpec() const;
    int offsetFromUtc() const;
#ifndef QT_BOOTSTRAPPED
    QTimeZone timeZone() const;
#endif // QT_BOOTSTRAPPED
    QString timeZoneAbbreviation() const;
    bool isDaylightTime() const;

    qint64 toMSecsSinceEpoch() const;
    // ### Qt 6: use quint64 instead of uint
    uint toTime_t() const;

    void setDate(const QDate &date);
    void setTime(const QTime &time);
    void setTimeSpec(Qt::TimeSpec spec);
    void setOffsetFromUtc(int offsetSeconds);
#ifndef QT_BOOTSTRAPPED
    void setTimeZone(const QTimeZone &toZone);
#endif // QT_BOOTSTRAPPED
    void setMSecsSinceEpoch(qint64 msecs);
    // ### Qt 6: use quint64 instead of uint
    void setTime_t(uint secsSince1Jan1970UTC);

#ifndef QT_NO_DATESTRING
    QString toString(Qt::DateFormat f = Qt::TextDate) const;
    QString toString(const QString &format) const;
#endif
    QDateTime addDays(qint64 days) const Q_REQUIRED_RESULT;
    QDateTime addMonths(int months) const Q_REQUIRED_RESULT;
    QDateTime addYears(int years) const Q_REQUIRED_RESULT;
    QDateTime addSecs(qint64 secs) const Q_REQUIRED_RESULT;
    QDateTime addMSecs(qint64 msecs) const Q_REQUIRED_RESULT;

    QDateTime toTimeSpec(Qt::TimeSpec spec) const;
    inline QDateTime toLocalTime() const { return toTimeSpec(Qt::LocalTime); }
    inline QDateTime toUTC() const { return toTimeSpec(Qt::UTC); }
    QDateTime toOffsetFromUtc(int offsetSeconds) const;
#ifndef QT_BOOTSTRAPPED
    QDateTime toTimeZone(const QTimeZone &toZone) const;
#endif // QT_BOOTSTRAPPED

    qint64 daysTo(const QDateTime &) const;
    qint64 secsTo(const QDateTime &) const;
    qint64 msecsTo(const QDateTime &) const;

    bool operator==(const QDateTime &other) const;
    inline bool operator!=(const QDateTime &other) const { return !(*this == other); }
    bool operator<(const QDateTime &other) const;
    inline bool operator<=(const QDateTime &other) const { return !(other < *this); }
    inline bool operator>(const QDateTime &other) const { return other < *this; }
    inline bool operator>=(const QDateTime &other) const { return !(*this < other); }

#if QT_DEPRECATED_SINCE(5, 2)
    QT_DEPRECATED void setUtcOffset(int seconds);
    QT_DEPRECATED int utcOffset() const;
#endif // QT_DEPRECATED_SINCE

    static QDateTime currentDateTime();
    static QDateTime currentDateTimeUtc();
#ifndef QT_NO_DATESTRING
    static QDateTime fromString(const QString &s, Qt::DateFormat f = Qt::TextDate);
    static QDateTime fromString(const QString &s, const QString &format);
#endif
    // ### Qt 6: use quint64 instead of uint
    static QDateTime fromTime_t(uint secsSince1Jan1970UTC);
    // ### Qt 6: Merge with above with default spec = Qt::LocalTime
    static QDateTime fromTime_t(uint secsSince1Jan1970UTC, Qt::TimeSpec spec,
                                int offsetFromUtc = 0);
#ifndef QT_BOOTSTRAPPED
    static QDateTime fromTime_t(uint secsSince1Jan1970UTC, const QTimeZone &timeZone);
#endif
    static QDateTime fromMSecsSinceEpoch(qint64 msecs);
    // ### Qt 6: Merge with above with default spec = Qt::LocalTime
    static QDateTime fromMSecsSinceEpoch(qint64 msecs, Qt::TimeSpec spec, int offsetFromUtc = 0);
#ifndef QT_BOOTSTRAPPED
    static QDateTime fromMSecsSinceEpoch(qint64 msecs, const QTimeZone &timeZone);
#endif
    static qint64 currentMSecsSinceEpoch() Q_DECL_NOTHROW;

#if defined(Q_OS_MAC) || defined(Q_QDOC)
    static QDateTime fromCFDate(CFDateRef date);
    CFDateRef toCFDate() const Q_DECL_CF_RETURNS_RETAINED;
#  if defined(__OBJC__) || defined(Q_QDOC)
    static QDateTime fromNSDate(const NSDate *date);
    NSDate *toNSDate() const Q_DECL_NS_RETURNS_AUTORELEASED;
#  endif
#endif

private:
    friend class QDateTimePrivate;

    // ### Qt6: Using a private here has high impact on runtime
    // on users such as QFileInfo. In Qt 6, the data members
    // should be inlined.
    QSharedDataPointer<QDateTimePrivate> d;

#ifndef QT_NO_DATASTREAM
    friend Q_CORE_EXPORT QDataStream &operator<<(QDataStream &, const QDateTime &);
    friend Q_CORE_EXPORT QDataStream &operator>>(QDataStream &, QDateTime &);
#endif

#if !defined(QT_NO_DEBUG_STREAM) && !defined(QT_NO_DATESTRING)
    friend Q_CORE_EXPORT QDebug operator<<(QDebug, const QDateTime &);
#endif
};
Q_DECLARE_SHARED(QDateTime)

其中最关键的是<操作符的比较，它代表了使用的真正数据，而且其它几个比较函数实质上都依赖这个操作符：

bool QDateTime::operator<(const QDateTime &other) const
{
    if (d->m_spec == Qt::LocalTime
        && other.d->m_spec == Qt::LocalTime
        && d->m_status == other.d->m_status) {
        return (d->m_msecs < other.d->m_msecs);
    }
    // Convert to UTC and compare
    return (toMSecsSinceEpoch() < other.toMSecsSinceEpoch());
}

其中最关键的是toMSecsSinceEpoch函数，结果发现如下定义：

    qint64 toMSecsSinceEpoch() const;

可见的QDateTime本质上是一个qint64