passing 'const QSqlQuery' as 'this' argument discards qualifiers

[zeroptr]

Hi All,

I'm trying to migrate from Qt5.6 to Qt6.5 my project is heavily SQL based...

At my Class at header... I define a

QSqlQueryModel *modKomponent;

At my class constructor

modKomponent = new QSqlQueryModel;

on the form one of the button

modKomponent->query().next()

then I get error for "modKomponent->query().next()"

passing 'const QSqlQuery' as 'this' argument discards qualifiers

How can I solve it?

thanks

[JonB]

@zeroptr
const QSqlQuery &QSqlQueryModel::query() const

The const at the beginning means that the QSqlQuery it returns is const/read-only. You cannot/are not supposed to do anything to it which alters it. But QSqlQuery::next() does (or may) alter it, e.g. changing its internal state. Hence the error message.

You are clearly not intended to do anything to modify the query returned by QSqlQueryModel::query() (I imagine it manages it all internally). I don't know why you got away with this at Qt5, whether you were never supposed to or it is an actual change in behaviour at Qt6. I guess you should review why you are trying to call next() on it when you should be letting the QSqlQueryModel handle its query without interference.

[Christian Ehrlicher]

@JonB with Qt5 you get the same compiler error. QSqlQuery::next() was never const.

[JonB]

@zeroptr
Then per @Christian-Ehrlicher's comment it should never have worked at Qt5 either....

The QSqlQuery assigned to a QSqlTableModel is for its use internally, to fetch all the rows from a table. You should never either need or want to call next() on it to fetch any further rows. If you want to do your own next()ing to fill a result set, for some reason, use your own QSqlQuery to do so, not one in use by a table or query model.

[Christian Ehrlicher]

@JonB after reconsideration it might have worked when the const ref was copied to a new QSqlQuery instance
This copy was removed in Qt6 as it's impossible to copy a query without getting strange side effects in the underlying layer.