closing database connection

bart.hollis

I have a function in my program that creates or opens a SqLite database:

QSqlError createDb()
{
    QSqlDatabase db;
    db = QSqlDatabase::addDatabase("QSQLITE");
    db.setDatabaseName("tracker.db");
    db.open();
    return db.lastError();
}

It works fine.
In the function I call to quit the program is a call to this function:

/QSqlError closeDb()
{
    QSqlDatabase db;
    db = QSqlDatabase::database();

// debug text:
    if ( db.isOpen() )
        std::cout << "the database is:  open" << std::endl;
    else
        std::cout << "the database is:  Closed" << std::endl;

    db.close();

// debug text.
    if ( db.isOpen() )
        std::cout << "the database is:  open" << std::endl;
    else
        std::cout << "the database is:  Closed" << std::endl;

    db.removeDatabase( QSqlDatabase::defaultConnection );
    return db.lastError();
}

It also seems to work, based on the debug couts in place.

In various functions in the program, I create a QSqlQuery, but these are created within the function and should, as I understand it, go away or become non-existant when the function ends.

When I run the program, and close it, after the debug messages showing the database is closed, I get an error message stating :

QSqlDatabasePrivate::removeDatabase: connection 'qt_sql_default_connection' is still in use, all queries will cease to work.

I read that message to mean that the database is closed but that I still have a connection to it somewhere. Further, there are some querys hanging around and that means memory leaks!

Looking at the docs for QSqlQuery, I see QSqlQuery::clear() with the statement that I should rarely if ever need it. I see ~QSqlQuery() which causes a crash. I see QSqlQuery::finish() which there's normally no need to call it.

So, what do I do? ( Read that as what do I not understand? )

Christian Ehrlicher

You have to make sure that no QSqlQuery exits using this connection when you want to close it to avoid this warning. Or just ignore it :)

bart.hollis

But if a query is created within a function, does it not cease to exist when the function returns?

Christian Ehrlicher

Yes but there must be an open QSqlQuery anywhere - at least this is what the warning tells you.
/edit: maybe a QSqlQuery as member value somewhere?

aha_1980

@bart.hollis said in closing database connection:

But if a query is created within a function, does it not cease to exist when the function returns?

if you create the query as a stack variable, the it is destroyed when the surrounding scope is leaved.

if you create the query as heap variable (with new) then it exists until delete or program end.

bart.hollis

I Found it!

In the closeDB function I wrote, I called the line:

QSqlDatabase db = QSqlDatabase::database();

And then closed it. This obviously created an instance of the connection which was left dangling when I removed it. The new function reads as follows:

// >>>>>>>>>>>>>>>>  Close the database when quitting  <<<<<<<<<<<<<<<<<<<<<
void closeDb()
{
    {
        QSqlDatabase db = QSqlDatabase::database();
        db.close();
    }
    QSqlDatabase::removeDatabase( QSqlDatabase::defaultConnection );
    return;
}

This has eliminated the warning.

Thanks all for your assistance.