[Development] Calendar Systems proposal

[Soroush Rabiei]

>
> Can you elaborate on the reasons that prevent any change of that kind in
> QDate? Maybe they can be worked around?
>
>
According to
https://community.kde.org/Policies/Binary_Compatibility_Issues_With_C%2B%2B#The_Do.27s_and_Don.27ts
Adding a member to struct or class is not possible without breaking ABI. On
the other hand https://wiki.qt.io/Qt-Version-Compatibility suggests that Qt
minor releases are backwards binary and source compatible. So there is no
chance of touching QDate in Qt5 series.

I'm working on a solution to provide calendar functionality without
breaking ABI, while considering possibilities for Qt6 (keep minimum effort
for converting current, temporary solution to futures Qt6 one). This may
fail of course, There are too many details that need to be discussed. How
we are supposed to change underlying calendar without adding information to
QDate? Can we add arguments to all methods with a default value? Something
like:


    QDate d;
    qDebug() << d.year();               // prints 2016
    qDebug() << d.year(QCalendar::Jalali); // prints 1395

And then force relevant widgets/views to show/edit date and time on a
specific calendar system:

    QDateEdit de;
    de.setDate(d);
    de.setCalendarSystem(QCalendar::Hebrew); // Is this possible?
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