sqlite - driver not loaded

[Saurian]

Hello,

I'm using Qt to work with a sqlite database but I have an error. If I try to declare the handler for the QSqlDatabase object as a pointer I get the error: Driver not loaded. So if I use

QSqlDatabase db;
db = QSqlDatabase::addDatabase("QSQLITE");

QSqlDatabase db;
db = QSqlDatabase::addDatabase("QSQLITE");

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everything is ok but if I'm trying to use a pointer

QSqlDatabase *db;
db = new QSqlDatabase;
db ->addDatabase("QSQLITE");

QSqlDatabase *db;
db = new QSqlDatabase;
db ->addDatabase("QSQLITE");

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I get the error that the Driver is not loaded. Anyone has any idea why the second case doesn't work?

[d_stranz]

Have you deployed the SQL drivers into the appropriate subdirectory of your app directory, as specified in the docs?

[Saurian]

Well, I have no problem with the SQL driver for the first case (when I don't use a pointer) so I haven't changed the location for the SQL driver. But because I use a pointer you say I might need to have the dll file in the folder of my application?

[anda_skoa]

The second code snippet doesn't do anything with the return value of addDatabase().

Well, I have no problem with the SQL driver for the first case (when I don't use a pointer)

The first case also uses the initialized database connection returned by addDatabase() and stores it in the local variable.

Cheers,
_

[Saurian]

I don't understand what you mean. The code is a little more complex. So basically the code is something like that:

QSqlDatabase db;
int record = 0;
 
//inițializări
MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    QSqlQuery *query;
    QMessageBox *messageBox = new QMessageBox;
 
    ui->setupUi(this);
 
 
    //conectare la baza de date
    db = QSqlDatabase::addDatabase("QSQLITE");
    db.setDatabaseName("conturi.db");
    if (!db.open())
    {
        messageBox ->warning(this, tr("Database Error"), db.lastError().text());
        exit(0);
    }
    else
    {
        //adăugare listă conturi din baza de date
        query = new QSqlQuery(db);
        query ->exec("SELECT * FROM cont");
        if (!query ->isActive())
        {
            messageBox ->warning(this, tr("Database Error"), query ->lastError().text());
            exit(0);
        }
        else
        {
            while (query ->next())
              {
                ui ->comboBox ->addItem(query ->value(3).toString());
              }
        }
    }
}
 
//curățare tot
MainWindow::~MainWindow()
{
    db.close();
    delete ui;
}

QSqlDatabase db;
int record = 0;

//inițializări
MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    QSqlQuery *query;
    QMessageBox *messageBox = new QMessageBox;

    ui->setupUi(this);


    //conectare la baza de date
    db = QSqlDatabase::addDatabase("QSQLITE");
    db.setDatabaseName("conturi.db");
    if (!db.open())
    {
        messageBox ->warning(this, tr("Database Error"), db.lastError().text());
        exit(0);
    }
    else
    {
        //adăugare listă conturi din baza de date
        query = new QSqlQuery(db);
        query ->exec("SELECT * FROM cont");
        if (!query ->isActive())
        {
            messageBox ->warning(this, tr("Database Error"), query ->lastError().text());
            exit(0);
        }
        else
        {
            while (query ->next())
              {
                ui ->comboBox ->addItem(query ->value(3).toString());
              }
        }
    }
}

//curățare tot
MainWindow::~MainWindow()
{
    db.close();
    delete ui;
}

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And of course there are other line codes for other operations on the database. So what I changed was defining db as a pointer and made changes everywhere from "." to "->" and from "db" to "*db".

Later edit: So using the method addDatabase I didn't assigned the driver QSQLITE to the variable db? I must use another function or something like that?

---

Added after 15 minutes:

Ok, so I found which was the solution: *db = QSqlDatabase::addDatabase("QSQLITE");

[anda_skoa]

So using the method addDatabase I didn't assigned the driver QSQLITE to the variable db?

If you are still wondering, think about how the method could possibly be able to do that.
It does know nothing about "db" and prior to your fix you discarded its return value.

Cheers,
_

[jefftee]

I get the error that the Driver is not loaded. Anyone has any idea why the second case doesn't work?

To elaborate on what @anda_skoa said:

QSqlDatabase::addDatabase is a static method and you are not operating on the db instance you think you are, because static member methods don't have a "this" pointer and thus do not operate on an instance of an object.

QSqlDatabase::addDatabase returns a QSqlDatabase instance, but you're not assigning that to a variable, so your attempt to add the database connection is essentially a no-op. You are creating a database instance by calling QSqlDatabase::addDatabase, then you throw away the result, hence the no-op.

The instance you allocated via "new QSqlDatabase()" is still an invalid database object since you have only invoked its default constructor.

If you *must* create a QSqlDatabase instance on the heap, try something like this:

QSqlDatabase *db = new QSqlDatabase(QSqlDatabase::addDatabase("QSQLITE"));

QSqlDatabase *db = new QSqlDatabase(QSqlDatabase::addDatabase("QSQLITE"));

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Good luck.