Segmentation fault

[jpn]

I think most of the QtSql examples I've seen simply access database connection via the static method QSqlDatabase::database() whenever needed, without the need of storing connection as member variable anywhere. QSqlDatabase::database() seems to be basically a call to QHash::value() in a thread-safe manner so it's not that inefficient at least.

[MarkoSan]

Ok, I know, I've complicated things a bit here but I simply wanted to have A SINGLE INSTANCE of object that connects to database, i.e., an instance of application is allowed to connect to database ONLY ONCE. Is this right policy or not?

[Frozenjim]

This thread has really helped me out, but I am still somewhat confused about what appears to be the foundation of the problem (I am fairly new to C++):
How do you know if you should be creating a pointer or not? How do I know when to use *model or model?

The documentation from Trolltech (http://doc.trolltech.com/4.3/qsqlquerymodel.html) actually makes it even worse for a beginner because they seem to use the two interchangeably - but when I do so, I get Segmentation faults if I can compile at all:

QSqlQueryModel *model = new QSqlQueryModel;
     model->setQuery("SELECT name, salary FROM employee");
     model->setHeaderData(0, Qt::Horizontal, tr("Name"));
     model->setHeaderData(1, Qt::Horizontal, tr("Salary"));
 
     QTableView *view = new QTableView;
     view->setModel(model);
     view->show();
 
We set the model's query, then we set up the labels displayed in the view header.
 
QSqlQueryModel can also be used to access a database programmatically, without binding it to a view:
 
     QSqlQueryModel model;
     model.setQuery("SELECT * FROM employee");
     int salary = model.record(4).value("salary").toInt();

QSqlQueryModel *model = new QSqlQueryModel;
     model->setQuery("SELECT name, salary FROM employee");
     model->setHeaderData(0, Qt::Horizontal, tr("Name"));
     model->setHeaderData(1, Qt::Horizontal, tr("Salary"));

     QTableView *view = new QTableView;
     view->setModel(model);
     view->show();

We set the model's query, then we set up the labels displayed in the view header.

QSqlQueryModel can also be used to access a database programmatically, without binding it to a view:

     QSqlQueryModel model;
     model.setQuery("SELECT * FROM employee");
     int salary = model.record(4).value("salary").toInt();

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[jacek]

How do you know if you should be creating a pointer or not?

It depends on how long you need the object. If you create it on the stack it will be destroyed when it goes out of scope, so all objects that need to exist longer should be created using new operator. If you use a model only within one function, you can create it on the stack. If you pass a model to a view, it has to exist as long as the view, so you have to use new.

The documentation from Trolltech actually makes it even worse for a beginner because they seem to use the two interchangeably

The docs assume you know C++.