Best practices forward-declaring QStringList (and other containers) in Qt6?

Talkless

Hi,

QtService from qt-solutions (https://code.qt.io/cgit/qt-solutions/qt-solutions.git/) does not build with Qt6:

In file included from ../src/qtservice_unix.cpp:4:
../src/qtservice.h:25:7: error: using typedef-name ‘using QStringList = class QList<QString>’ after ‘class’
   25 | class QStringList;
      |       ^~~~~~~~~~~
In file included from /home/vincas/Qt/6.8.3/gcc_64/include/QtCore/qtypeinfo.h:9,
                 from /home/vincas/Qt/6.8.3/gcc_64/include/QtCore/qglobal.h:47,
                 from /home/vincas/Qt/6.8.3/gcc_64/include/QtCore/qcoreapplication.h:7,
                 from /home/vincas/Qt/6.8.3/gcc_64/include/QtCore/QCoreApplication:1,
                 from ../src/qtservice.h:7:
/home/vincas/Qt/6.8.3/gcc_64/include/QtCore/qcontainerfwd.h:40:7: note: ‘using QStringList = class QList<QString>’ has a previous declaration here
   40 | using QStringList = QList<QString>;
      |       ^~~~~~~~~~~

In Qt6, QStringList is an alias, not a class, and so class QStringList errors out in https://code.qt.io/cgit/qt-solutions/qt-solutions.git/tree/qtservice/src/qtservice.h?id=777e95ba69952f11eaec0adfb0cb987fabcdecb3#n25 .

What would be the "best practices" way to handle this? I could:

include QtCore/qcontainerfwd.h. Too-"implementation-detail"?
just include #include <QStringList>. Too-user-heavy?
do not include anything, as it seems QtCore/qcontainerfwd.h is already available via other Qt includes. Might break in the future?
???

And to make it work on both Qt5 and Qt6, I might need to:

#if QT_VERSION >= QT_VERSION_CHECK(6, 0, 0)
    #include <QtCore/qcontainerfwd.h>
#else
    class QStringList; // Forward declaration only valid in Qt5
#endif

Which kinda feels overkill... so maybe just #include <QStringList>?

What do you think?

Thanks!

P.S. I might try to push it via gerrit.

Christian Ehrlicher

Include QStringList header