What is the correct way of QSqlDatabase & QSqlQuery?

I got confused with the manual , should i work like this:

{
 QSqlDatabase db = QSqlDatabase::addDatabase (...);
 QSqlQuery query (db);
 query.exec (...);
}

QSqlDatabase::removeDatabase (...);

As the document points out, query or db will be deconstructed automatically. But is that efficient ?

Well , if i cache db inside a class , like the following:

class Dummy {
  Dummy() { 
    db = QSqlDatabase::addDatabase (...);
  }
  ~Dummy() {
    db.close();
  }

  bool run() {
    QSqlQuery query (db);
    bool retval = query.exec (...);
    blabla ...
  }

  private:
    QSqlDatabase db;
};

Sometimes i could see warnings like:

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

Even if i didn't call run().

When you create a QSqlDatabase object with addDatabase or when you call removeDatabase, you are merely associating or disassociating a tuple (driver, hostname:port, database name, username/password) to a name (or to the default connection name if you don't specify a connection name).
The SQL driver is instantiated, but the database will only be opened when you call QSqlDatabase::open.

That connection name is defined application-wide. So if you call addDatabase in each of the objects that use it, you are changing all QSqlDatabase objects that use the same connection name and invalidating all queries that were active on them.

The first code example you cited shows how to correctly disassociate the connection name, by ensuring that:

• all QSqlQuery are detached from the QSqlDatabase before closing the database by calling QSqlQuery::finish(), which is automatic when the QSqlQuery object goes out of scope,
• all QSqlDatabase with the same connection name are close()d when you call QSqlDatabase::removeDatabase (close() is also called automatically when the last QSqlDatabase object of the same connection goes out of scope).

When you create the QSqlDatabase, depending on whether you want the connection to stay open for the application lifetime (1) or just when needed (2), you can:

1. keep a single QSqlDatabase instance in one single class (for example, in your mainwindow), and use it in other objects that needs it, either by passing the QSqlDatabase directly or just the connection name that you pass to QSqlDatabase::database to get the QSqlDatabase instance back. QSqlDatabase::database uses QHash to retrieve a QSqlDatabase from its name, so it is probably negligibly slower than passing the QSqlDatabase object directly between objects and functions. If you use the default connection, you don't even have to pass anything anywhere, just call QSqlDatabase::database() without any parameter.

   // In an object that has the same lifetime as your application
   // (or as a global variable, since it has almost the same goal here)
   QSqlDatabase db;
   
   // In the constructor or initialization function of that object       
   db = QSqlDatabase::addDatabase("QSQLDRIVER", "connection-name"); 
   db.setHostname(...);
   // ...
   if(!this->db.open())  // open it and keep it opened
   {
       // Error handling...
   }
   
   // --------
   // Anywhere you need it, you can use the "global" db object 
   // or get the database connection from the connection name        
   QSqlDatabase db = QSqlDatabase::database("connection-name"); 
   QSqlQuery query(db);             
   

2. configure the QSqlDatabase once, open it to test that the parameters are correct, and ditch the instance. The connection name, will still be accessible anywhere, but the database will have to be reopened:

   {
       // Allocated on the stack
       QSqlDatabase db = QSqlDatabase::addDatabase("QSQLDRIVER", "connection-name"); 
       db.setHostname(...);
       // ...
       if(!this->db.open()) // test the connection
       {
          // Error handling
       }
   // db is closed when it goes out of scope
   } 
   
   {
       // Same thing as for (1), but by default database() opens 
       // the connection if it isn't already opened 
       QSqlDatabase db = QSqlDatabase::database("connection-name"); 
       QSqlQuery query(db);
   
   // if there is no other connection open with that connection name,
   // the connection is closed when db goes out of scope
   } 
   

In that case, note that you shouldn't close the database explicitly, because you can have multiple objects using the same database connection in a reentrant manner (for example, if a function A uses the connection and calls B which also use the connection. If B closes the connection before returning control to A, the connection will also be closed for A, which is probably a bad thing).