cannot get QDataStream::setVersion to work

Poldi

Maybe my expectation is wrong but I expect that when I set the version, that that version is used.
I'm using Qt Creator 11.0.3 and cannot when I check the version after I created a new file, the version is always the default (the latest),

#include <QCoreApplication>
#include <QDebug>
#include <QDataStream>
#include <QFile>

bool saveFile(QString path)
{
    QFile file(path);
    if (!file.open(QIODevice::WriteOnly))
        return false;

    QDataStream out(&file);
    out.setVersion(QDataStream::Qt_6_5);
    out << (qint32) 0xA0B0C0D0;
    out << (qint32) 123;

    qInfo() << "The version is" << out.version();

    QString title = "The anser is 42";
    qint64 num = 42;

    out << title;
    out << num;

    file.close();
    return true;
}

bool readFile(QString path)
{
    QFile file(path);
    if (!file.open(QIODevice::ReadOnly))
        return false;

    QDataStream in(&file);
    qInfo() << "Reading version " << in.version();

    file.close();
    return true;
}

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

    QString file = "test.txt";

    if (saveFile(file)) {
        qInfo() << "Saved";
        readFile(file);
    }
    return a.exec();
}

Shows the following output

The version is 20
Saved
Reading version 21

What am I missing?

Christian Ehrlicher

@Poldi said in cannot get QDataStream::setVersion to work:

What am I missing?

Nothing - the version is not saved in the output. You have to define your version you want to use on both sides.

JonB

@Poldi
I'm not sure I understand either. I expected it to write a version number to the stream, which you would check at reader side. I think @Christian-Ehrlicher is saying that is not how QDataStream::setVersion() works, it does not actually write anything into the stream, so it's an in-memory thing only, affecting what code is executed.

Have you read https://doc.qt.io/qt-6/qdatastream.html#versioning? If I understand right, the example code there shows how you can/should introduce your own, arbitrary field in the stream which the reader can use to recognise the writer's version for this purpose.