Question

#include <QtGui>
#include <QtSql>
#include <QApplication>

class   ABC {
public:
    QSqlDatabase db;
    QSqlQuery memberQuery;

    ABC() {
        db = QSqlDatabase::addDatabase("QSQLITE");
        db.setDatabaseName("test");
        qDebug() << "Database open test : " << db.open();

    }

    void    database() {
        qDebug() << "Inside database method..." << endl;
        QSqlQuery localQuery ;
        qDebug() << "Using local Query : " << localQuery.exec("create table if not exists alu (ini int)");
        localQuery.clear();

        qDebug() << "Using memeber query object: " << memberQuery.exec("create table if not exists alu (ini int)");
        memberQuery.clear();
    }
};

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    ABC ob;
    ob.database();

    return a.exec();
}

Following result is found after executing this code.

Database open test :  true 
Inside database method... 

Using local Query :  true 
QSqlQuery::exec: database not open
Using memeber query object:  false 

My question is why can’t I use a member object QSqlQuery instead of a local one?

Answer 1

You need to initialise your QSqlQuery using the QSqlQuery(QSqlDatabase db) constructor.

To do this you will need to use initialiser lists in the constructor and have already setup the DB connection, or use pointers to initialise the QSqlQuery later.

With an ABC constructor taking an initialised DB (create the DB in main):

ABC(QSqlDatabase _db) :
  db(_db),
  memberQuery(db)
{
  Q_ASSERT(db.isOpen());
}

Or

class ABC
{
  QSqlDatabase db;
  QSqlQuery* memberQueryPtr;

  public:
    ABC() : db(QSqlDatabase::addDatabase("QSQLITE")),memberQueryPtr(0)
    {
      db.setDatabaseName("test");
      if (!db.open())
        return;
      memberQueryPtr = new QSqlQuery(db);
    }

    ~ABC()
    {
      delete memberQueryPtr; memberQueryPtr = 0;
    }

    // ...
};