where should I declare a database object - c++

Question

I am using the Qt libraries in a C++ project but I have a design question: where should a database be declared? I would prefer not to declare global variables.
Currently I am dealing with this problem in this way. I have a mainwindow and I have declared the DB in there so I perform the queries in the main window and pass the results to the dialogs using different signals and slots.
I start the DB when the main window starts and close it when the window has been closed. I don't know if this is ok
Now I need the DB connection in another class as well so I can pass a reference to the DB or make the DB global
I don't like these solutions.. is there a standard pattern to deal with this situation?
edit:
My class now looks like:
class Database
{
public:
bool open(void);
bool close(void);
static Database* getDatabase(void);
// various methods like loadThisTable(), saveThisTable() etc
private:
Database(); // disable constructor
~Database(); // disable destructor
Database(const Database&); // disable copy constructor
Database& operator=(const Database&); // disable assignment
static Database* instance_; // database instance
QSqlDatabase qtDB; // qt db database
}
If I want I can add the add and remove methods but I have a single DB instance.

Answer 1

If you're using QSqlDatabase, you don't really need to make it a global variable. Just set up the connection when you first start your application, then use the static QSqlDatabase methods to access the connection when you need it in different modules.
Example
QSqlDatabase db; // set up the default connection
// alternative: set up a named connection
// QSqlDatabase db("conn-name");
// set the connection params and open the connection
// ... later on
QSqlDatabase db = QSqlDatabase::database(); // retrieve the default connection
// alternative: retrieve the named connection
// QSqlDatabase db = QSqlDatabase::database("conn-name");
From the docs:
QSqlDatabase is a value class. Changes made to a database connection via one instance of QSqlDatabase will affect other instances of QSqlDatabase that represent the same connection. Use cloneDatabase() to create an independent database connection based on an existing one.
Note: If you're application is multi-threaded, you have to be careful to only use a connection in the thread in which it was created.

Answer 2

You need a singleton pattern. It's a global class which have only one instance. Someone calls it antipattern (and sometimes it is), but it is the best way to handle resources like database connections. And dont forget that you can use QSqlDatabase QSqlDatabase::database ( const QString & connectionName = QLatin1String( defaultConnection ), bool open = true ) [static] method to get QSqlDatabase instance by name (name can be set via QSqlDatabase QSqlDatabase::addDatabase ( QSqlDriver * driver, const QString & connectionName = QLatin1String( defaultConnection ) ) [static] method) to avoid creating singleton just for storing QSqlDatabase instances.