QLocale::toString() bug

AndyBrice

I'm not sure if this is a bug in Qt or if I am doing something stupid.

This code:

QLocale locale1( QLocale::German, QLocale::Germany );
 for ( int m = 1; m <= 12; m++ )
 {
     QDate date1( 2025, m, 1 );
     qDebug() << "locale1=" << locale1 << " date1=" << date1 << "locale1.toString( date1, \"MMMM\" )=" << locale1.toString( date1, "MMMM" );
 }

Gives the expected output:

locale1= QLocale(German, Latin, Germany)  date1= QDate("2025-01-01") locale1.toString( date1, "MMMM" )= "Januar"
locale1= QLocale(German, Latin, Germany)  date1= QDate("2025-02-01") locale1.toString( date1, "MMMM" )= "Februar"
locale1= QLocale(German, Latin, Germany)  date1= QDate("2025-03-01") locale1.toString( date1, "MMMM" )= "März"
locale1= QLocale(German, Latin, Germany)  date1= QDate("2025-04-01") locale1.toString( date1, "MMMM" )= "April"
...

But this code:

foreach ( const QLocale& locale2, QLocale::matchingLocales( QLocale::AnyLanguage, QLocale::AnyScript, QLocale::AnyCountry ) )
 {
     if ( locale2.country() == QLocale::Germany && locale2.language() == QLocale::German )
     {
        for ( int m = 1; m <= 12; m++ )
        {
            QDate date2( 2025, m, 1 );
            qDebug() << "locale2=" << locale2 << " date2=" << date2 << "locale2.toString( date2, \"MMMM\" )=" << locale2.toString( date2, "MMMM" );
        }
 }

Gives different output:

locale2= QLocale(German, Latin, Germany)  date2= QDate("2025-01-01") locale2.toString( date2, "MMMM" )= "January"
locale2= QLocale(German, Latin, Germany)  date2= QDate("2025-02-01") locale2.toString( date2, "MMMM" )= "February"
locale2= QLocale(German, Latin, Germany)  date2= QDate("2025-03-01") locale2.toString( date2, "MMMM" )= "March"
locale2= QLocale(German, Latin, Germany)  date2= QDate("2025-04-01") locale2.toString( date2, "MMMM" )= "April"
...

I am completely baffled why the month output isn't the same. I've looked at it for over hour! Any ideas?

AndyBrice

I am using Qt 5.15.18 with MSVC2019 on Windows 11.

Christian Ehrlicher

Works fine for me with a recent Qt version:

int main(int argc, char* argv[])
{
    QApplication a(argc, argv);

    QLocale locale1(QLocale::German, QLocale::Germany);
    for (int m = 1; m <= 12; m++) {
        QDate date1(2025, m, 1);
        qDebug() << "locale1=" << locale1 << " date1=" << date1 << "locale1.toString( date1, \"MMMM\" )=" << locale1.toString(date1, "MMMM");
    }
    const auto locales = QLocale::matchingLocales(QLocale::AnyLanguage, QLocale::AnyScript, QLocale::AnyCountry);
    for (const QLocale& locale2 : locales) {
        if (locale2.country() == QLocale::Germany && locale2.language() == QLocale::German) {
            for (int m = 1; m <= 12; m++) {
                QDate date2(2025, m, 1);
                qDebug() << "locale2=" << locale2 << " date2=" << date2 << "locale2.toString( date2, \"MMMM\" )=" << locale2.toString(date2, "MMMM");
            }
        }
    }
    return 0;
}

AndyBrice

So it returns the German version of the month in all cases?

AndyBrice

Stranger still, if I change "MMMM" to "dddd-MMMM" it returns the German day name, but not the German month name, inside the loop:

locale1= QLocale(German, Latin, Germany)  date1= QDate("2025-01-01") locale1.toString( date1, "dddd-MMMM" )= "Mittwoch-Januar"
locale1= QLocale(German, Latin, Germany)  date1= QDate("2025-02-01") locale1.toString( date1, "dddd-MMMM" )= "Samstag-Februar"
locale1= QLocale(German, Latin, Germany)  date1= QDate("2025-03-01") locale1.toString( date1, "dddd-MMMM" )= "Samstag-März"
locale1= QLocale(German, Latin, Germany)  date1= QDate("2025-04-01") locale1.toString( date1, "dddd-MMMM" )= "Dienstag-April"
...

locale2= QLocale(German, Latin, Germany)  date2= QDate("2025-01-01") locale2.toString( date2, "dddd-MMMM" )= "Mittwoch-January"
locale2= QLocale(German, Latin, Germany)  date2= QDate("2025-02-01") locale2.toString( date2, "dddd-MMMM" )= "Samstag-February"
locale2= QLocale(German, Latin, Germany)  date2= QDate("2025-03-01") locale2.toString( date2, "dddd-MMMM" )= "Samstag-March"
locale2= QLocale(German, Latin, Germany)  date2= QDate("2025-04-01") locale2.toString( date2, "dddd-MMMM" )= "Dienstag-April"
...

Christian Ehrlicher

Update to a recent version.

AndyBrice

I don't understand how the same bit of code can be returning different answers inside and outside a loop. It doesn't make any sense.

SGaist

Hi,

Just for the sake of research, what do you get if you compare the two QLocale objects for equality ?

Christian Ehrlicher

Please provide a minimal, compileable example which does not work with a recent Qt version. I provided one which does work fine.

AndyBrice

Code:

#include <QCoreApplication>
#include <QLocale>
#include <QDate>
#include <QtDebug>

int main(int argc, char* argv[])
{
    QCoreApplication a(argc, argv);

    QLocale locale1(QLocale::German, QLocale::Germany);
    for (int m = 1; m <= 3; m++)
    {
        QDate date1(2025, m, 1);
        qDebug() << "locale1=" << locale1 << " date1=" << date1 << "locale1.toString( date1, \"MMMM\" )=" << locale1.toString(date1, "MMMM");
    }
    const auto locales = QLocale::matchingLocales(QLocale::AnyLanguage, QLocale::AnyScript, QLocale::AnyCountry);
    for (const QLocale& locale2 : locales)
    {
        if (locale2.country() == QLocale::Germany && locale2.language() == QLocale::German)
        {
            qDebug() << "locale1 == locale2 =" << ( locale1 == locale2 );
            for (int m = 1; m <= 3; m++)
            {
                QDate date2(2025, m, 1);
                qDebug() << "locale2=" << locale2 << " date2=" << date2 << "locale2.toString( date2, \"MMMM\" )=" << locale2.toString(date2, "MMMM");
            }
        }
    }
    return 0;
}

Results:

Qt 5.15.18/MSVC2019/Windows:
Qt 5.15.18/XCode/macOS:

locale1= QLocale(German, Latin, Germany)  date1= QDate("2025-01-01") locale1.toString( date1, "MMMM" )= "Januar"
locale1= QLocale(German, Latin, Germany)  date1= QDate("2025-02-01") locale1.toString( date1, "MMMM" )= "Februar"
locale1= QLocale(German, Latin, Germany)  date1= QDate("2025-03-01") locale1.toString( date1, "MMMM" )= "März"
locale1 == locale2 = true
locale2= QLocale(German, Latin, Germany)  date2= QDate("2025-01-01") locale2.toString( date2, "MMMM" )= "January"
locale2= QLocale(German, Latin, Germany)  date2= QDate("2025-02-01") locale2.toString( date2, "MMMM" )= "February"
locale2= QLocale(German, Latin, Germany)  date2= QDate("2025-03-01") locale2.toString( date2, "MMMM" )= "March"

^^^Month is incorrect for locale2^^^

Qt 6.7.3/MSVC2019/Windows:
Qt 6.7.3/XCode/macOS:

locale1= QLocale(German, Latin, Germany)  date1= QDate("2025-01-01") locale1.toString( date1, "MMMM" )= "Januar"
locale1= QLocale(German, Latin, Germany)  date1= QDate("2025-02-01") locale1.toString( date1, "MMMM" )= "Februar"
locale1= QLocale(German, Latin, Germany)  date1= QDate("2025-03-01") locale1.toString( date1, "MMMM" )= "März"
locale1 == locale2 = true
locale2= QLocale(German, Latin, Germany)  date2= QDate("2025-01-01") locale2.toString( date2, "MMMM" )= "Januar"
locale2= QLocale(German, Latin, Germany)  date2= QDate("2025-02-01") locale2.toString( date2, "MMMM" )= "Februar"
locale2= QLocale(German, Latin, Germany)  date2= QDate("2025-03-01") locale2.toString( date2, "MMMM" )= "März"

^^^ Month is as expected^^^

So the localised month is wrong (but only inside the loop) for Qt 5.15.18.

Weirdly, day of week ("ddd" or "dddd") is correct in both cases.

I can work around it, but it is a very odd bug.