Question

I have a QMultiMap<QDateTime, SomeOwnDataType> from which I’d like to retrieve all values with a certain timestamp. This is what I do:

QMap<QDateTime, Appointment>::iterator it = _reminders.find(now);

where now has a value of di 6. mrt 12:07:00 2012. This is my loop condition:

while (it != _reminders.end() && it.key() == now) {

This was the state of the _reminders object:

Contrary to my expectations, the loop was skipped entirely. How come?

Answer 1

I believe that the problem is that the two timestamps are not equal. If you check the == operator code of QDateTime you will see that the equality holds if both time and date are equal.

bool QDateTime::operator==(const QDateTime &other) const
{
    if (d->spec == other.d->spec && d->utcOffset == other.d->utcOffset)
        return d->time == other.d->time && d->date == other.d->date;
    else {
        QDate date1, date2;
        QTime time1, time2;

        d->getUTC(date1, time1);
        other.d->getUTC(date2, time2);
        return time1 == time2 && date1 == date2;
    }
}

But the time equal operator compares miliseconds:

bool operator==(const QTime &other) const { return mds == other.mds; }

where mds is the time in miliseconds. In the QTime constructor mds is calculated as follows:

 mds = (h*SECS_PER_HOUR + m*SECS_PER_MIN + s)*1000 + ms;

It would be safer if you just checked if the difference between two timestamps is within a limit. For example :

while (it != _reminders.end() && abs(now.msecsTo(it.key())) < aLimitInMsecs) {