c++/Qt: How to get a reference to database in the MainWindow class?

Question

In Qt-creator, I created SQLite database in a class called databaseManager, as follow:

QString DatabaseManager::open_db()
{
    QSqlDatabase db;
    QString path = "/Users/me/Documents/workspace/Muasaa/";
    db = QSqlDatabase::addDatabase("QSQLITE");
    db.setDatabaseName(path+"Database v.1");
    if (db.open()){
        return "Database is created, open, and ready ...";
    } else {
        return db.lastError().text();
    }
}

Then I define the following in the header file of the MainWindow class:

Public:
   DatabaseManager *db_manager;

In the source file, I call it as follow:

db_manager->open_db();

which creates and open the database.

However, I would like to use a reference to same database to use it in many functions in the MainWindow source file. How can I do that ?!

score 0 · Answer 1 · answered Nov 30 '15 at 16:21

0

Move the QSqlDatabase db variable into your class header, and add a method for getting it. As long as you create an instance of your databaseManager class and maintain a pointer to it in MainWindow you'll be able to retrieve it.

answered Nov 30 '15 at 16:21

Nicholas Smith

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if I moved `QSqlDatabase db` to the header file, the application exit with **_the program has unexpectedly finished_** !? – McLan Nov 30 '15 at 16:42

score 0 · Answer 2 · answered Nov 30 '15 at 16:24

0

QSqlDatabase::database() is a static function that returns a QSqlDatabase. If you have more than one database, you have to provide a connection name in addDatabase() and in database()

answered Nov 30 '15 at 16:24

hmuelner

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I have only one database that I would like to use it in the MainWindow class. I want to display it in my GUI using QSqlTableModel. – McLan Nov 30 '15 at 16:47
In your MainWindow class use QSqlDatabase db = QSqlDatabase::database(). This represents the database connected that you established with addDatabase(). I recommend reading the documentation and some example programs. – hmuelner Dec 02 '15 at 13:21

score 0 · Answer 3 · edited May 23 '17 at 11:52

0

Maybe a design solution may help?

Whatever your goal is, consider turning your DatabaseManager class into a Singleton. This way you'll be able to use your manager in all the gui classes. Something like DatabaseManager::instance()->your_method()

Advantages:

You are sure the connections are managed the right way
Less oportunities for build problems

By the way, I'm not sure, but your program crash may be the result of using your db_manager pointer before it was initialized, in a slot maybe, or (more probable) it's an internal connection error. Have you checked the same connection attributes in a minimum possible example?

edited May 23 '17 at 11:52

Community

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answered Nov 30 '15 at 17:30

MasterAler

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QSqlDatabase::addDatabase(...) and QSqlDatabase::database(...) are static functions, and if you study the documentation you will see that the internal list of database connections already is a singleton. – hmuelner Dec 03 '15 at 09:48

c++/Qt: How to get a reference to database in the MainWindow class?

3 Answers3