• To save, you need to iterate the list calling QString::number() on each one to build up a comma (or other character)-separated string, then pass that to settings.
• To restore, you need to call QString.split() to split the string into items and call QString::toInt/Float() on each one to convert back to numbers.

Of course, if you have the numbers stored as a separated string in your text edit or elsewhere and you only want to deal with that then you don't need to do joining/splitting/converting from/to numbers.

UPDATE

@sonichy said in How to change QVector to QString:

@mpergand I accept your code

You have gone back and changed your post to say "I accept your code" now, maybe you mean you have no problem, I don't know.

how to change the dash value ? So I need QLineEdit and QVector QString exchange.

You can create a float vector like this:

QString text="1.2, 2.4, 5.75, 10.25";
QStringList list=text.split(',');
QVector<float> fVec;

for(QString& s : list) fVec<<s.toFloat();
	
qDebug()<<fVec;  // --> QVector(1.2, 2.4, 5.75, 10.25)

I need set the value by QLineEdit:

Why?

If you are using Qt6 then:

QVector<qreal> pattern { 5,5 };
constexpr auto PatternKey = QLatin1String("DashPattern");
settings.setValue(PatternKey, QVariant::fromValue(pattern));

will save something to QSettings that can be restored using:

auto pattern = settings.value(PatternKey).value<QVector<qreal>>();

If the QSetting backing store is an INI file then it might look like this:

$ cat test.ini 
[General]
DashPattern=@Variant(\0\0\0\x7f\0\0\0\xeQList<double>\0\0\0\0\x2@\x14\0\0\0\0\0\0@\x14\0\0\0\0\0\0)

but you really do not need to worry about that.

constexpr auto PatternKey = QLatin1String("DashPattern");
settings.setValue(PatternKey, QVariant::fromValue(pattern));
// ...
auto pattern = settings.value(PatternKey).value<QVector<qreal>>();