c ++ / Qt：如何在MainWindow类中获取对数据库的引用？

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 ?!

Answer 1

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.

Answer 2

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()

Answer 3

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?