[Interest] how to properly close a SQLite database?

[Constantin Makshin]

QSqlDatabase::close() closes the database backend connection but leaves
QtSql one intact, i.e. even after close() your QSqlDatabase object
occupies its slot in the [global] list of existing database objects. So
in order to let QSqlDatabase::removeDatabase() do its job you have to
destroy all QSqlDatabase objects referring to that connection.

Here is a fragment from one of my applications:

void Application::closeDatabase ()
{
    QString connectionName = m_db.connectionName();

    m_db.close();
    m_db = QSqlDatabase();
    QSqlDatabase::removeDatabase(connectionName);
}

On 06/24/2014 09:25 PM, Francisco Ares wrote:
> Hi,
> 
> Before of anything else, thanks to anyone reading this message.  I'm
> using Qt 4.8.5 and QtCreator.
> 
> I use SQLite for data storage in an embedded system, and that is OK. 
> There is a class, named "CDataBase" that holds the "QSqlDatabase" object
> and all of the "QSqlQuery" objects used on this program, and all access
> to the database itself is made through member functions of this class,
> providing a non-SQL interface for the rest of the program. The class has
> an "init" function to receive the database file name:
> 
> void CDataBase::init(QString fileName)
> {
>     mDB = QSqlDatabase::addDatabase("QSQLITE");
>     mDB.setDatabaseName(fileName);
>     mDB.open();
>      ...
> }
> 
> Now I need to implement a backup for that data in a removable drive. For
> that, I have created a second "CDataBase" object to handle the backup
> data base file. The objects that use "CDataBase" have means implemented
> to switch from a "CDataBase" object to another one (at least I believe
> so, as I could not test it).
> 
> When the program tries open the new object's database connection, the
> program stops (in debug mode) on the first instruction of the "init"
> function; hitting F10, a warning shows up on the program output pane:
> 
> QSqlDatabasePrivate::removeDatabase: connection
> 'qt_sql_default_connection' is still in use, all queries will cease to work.
> 
> So, I've implemented a "finish()" member function for this class:
> 
> void CDataBase::finish()
> {
>     if (mDB.isOpen())
>     {
>         mDB.commit();
>         mDB.close();
>         mDB.removeDatabase(mDB.connectionName());
>     }
> }
> 
> So, when I try this:
> 
>     mDataBaseWork->finish();
> 
> 
> the same warning message shows up on trying to execute the
> "removeDatabase" instruction.
> 
> All the QSqlQueries are temporary, they are all dead at this point. I
> even tried to do:
> 
> {
>     QSqlQuery query (mDB, query);
>     ...
>     ...
>     query.finish();
>     query.clear();
> }
> 
> And it didn't work as well.
> 
> What am I doing wrong?
> 
> Thanks again,
> Francisco

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