QSqlDatabase don't close automatically

Sentret_C

The doc says: (http://doc.qt.io/qt-5/qsqldatabase.html#dtor.QSqlDatabase)
"When the last connection is destroyed, the destructor implicitly calls close() to release the database connection."

I thought that as long as I only create local instances of QSqlDatabase, I don't need to call close() manually. However, one day I found that my app is always connected to the database, so I created a simple test:

#include <QSqlDatabase>
#include <QDebug>

int main(int argc, char *argv[])
{
    {
        QSqlDatabase dbIn = QSqlDatabase::addDatabase("QMYSQL", "test");
        dbIn.setHostName("localhost");
        dbIn.setPort(3306);
        dbIn.setUserName("username");
        dbIn.setPassword("password");
        dbIn.setDatabaseName("database");
        dbIn.open();
        qDebug() << dbIn.isOpen();
    }
    QSqlDatabase dbOut = QSqlDatabase::database("test", false);
    qDebug() << dbOut.isOpen();
    return 0;
}

and it prints "true true".

I'm using Qt 5.11.2 MinGW. Is it a bug, or maybe I misunderstood the doc?

kshegunov

@Sentret_C said in QSqlDatabase don't close automatically:

I thought that as long as I only create local instances of QSqlDatabase, I don't need to call close() manually.

Nope. This would imply, beside other things, that every time you call QSqlDatabase::database a new connection is to be created. QSqlDatabase is a value object, you need to remove the connection from the global registry for the database to be actually closed.

Sentret_C

@kshegunov said in QSqlDatabase don't close automatically:

remove the connection from the global registry

You mean by calling QSqlDatabase::removeDatabase? What about the sentence I quoted?
"When the last connection is destroyed, the destructor implicitly calls close() to release the database connection."
Under what circumstances will the destructor implicitly call close()?

I made the test clearer:

#include <QSqlDatabase>
#include <QDebug>
void addAndOpen(QString connectionName) {
    QSqlDatabase db = QSqlDatabase::addDatabase("QMYSQL", connectionName);
    db.setHostName("localhost");
    db.setPort(3306);
    db.setUserName("username");
    db.setPassword("password");
    db.setDatabaseName("database");
    db.open();
    qDebug() << db.isOpen();
}
void check(QString connectionName) {
    QSqlDatabase db = QSqlDatabase::database(connectionName, false);
    qDebug() << db.isOpen();
}
int main(int argc, char *argv[])
{
    addAndOpen("test");
    check("test");
    return 0;
}

Before calling check(), the last connection is destroyed, right?

Christian Ehrlicher

@Sentret_C said in QSqlDatabase don't close automatically:

When the last connection is destroyed

You destroy the last conection when you call QSqlDatabase::removeDatabase()

Sentret_C

I read the source code and finally realized that a copy of the QSqlDatabase instance is stored in the global static object dbDict when QSqlDatabase::addDatabase is called, and that instance is the last connection, destroyed in QSqlDatabase::removeDatabase.

Is that right?

BTW, I'm working on an app which interacts with the database about once a second. Should I close the connection everytime the (group of) query is finished? What if the interaction is more or less often?

kshegunov

@Sentret_C said in QSqlDatabase don't close automatically:

Should I close the connection everytime the (group of) query is finished?

Of course not. That's why you have a "connection" to the database - to keep it alive as long as needed. What would be the point of closing and reopening it every time? Say you are reading a book, do you close the book after you read a passage and then reopen it again so you can read the next one?