Singleton on Qt.

dcbasso

I have create a Singleton Class to handle with my DataBaseConnection, see:

SingletonDatabaseConnection.h
@
#ifndef SINGLETONDATABASECONNECTION_H
#define SINGLETONDATABASECONNECTION_H

#include <QMutex>
#include "QtSql/QtSql"

class SingletonDatabaseConnection
{
public:
static SingletonDatabaseConnection* getInstance()
{
static QMutex mutex;
if (!myInstance) {
mutex.lock();
if (!myInstance) {
database = QSqlDatabase::addDatabase("QSQLITE");
database.setDatabaseName( QDir::homePath() + QDir::separator() + "inoveDB.db3");
if (database.open()) {
// database.exec("");
readCreateTable();
}
myInstance = new SingletonDatabaseConnection();
}
mutex.unlock();
}
return myInstance;
}

static void drop()
{
    static QMutex mutex;
    mutex.lock();
    delete myInstance;
    myInstance = 0;
    mutex.unlock();
}

void openDatabase();
QSqlDatabase getDatabase();

private:
static SingletonDatabaseConnection* myInstance;
static QSqlDatabase database;

SingletonDatabaseConnection();
SingletonDatabaseConnection(const SingletonDatabaseConnection &);
SingletonDatabaseConnection& operator=(const SingletonDatabaseConnection &);

static void readCreateTable()
{
   QFile file&#40;":/database/CreateTable.sql"&#41;;
   if(!file.open(QIODevice::ReadOnly))
   {
       qDebug() << "error opening file: " << file.error();
       return;
   }
   QTextStream instream(&file);
   QString line = instream.readLine();
   qDebug() << "first line: " << line;
   file.close();
}

};

#endif // SINGLETONDATABASECONNECTION_H
@

SingletonDatabaseConnection.cpp
@
#include "singletondatabaseconnection.h"

//SingletonDatabaseConnection::SingletonDatabaseConnection()
//{
//}
SingletonDatabaseConnection* SingletonDatabaseConnection::myInstance = 0;
@

When I try to use this Singleton on my mainWindow, the compiler gives me a error:

@
void MainWindow::on_pushButton_clicked()
{
SingletonDatabaseConnection *singleton = SingletonDatabaseConnection::getInstance();
// singleton->getDatabase();

}
@

Error:
"undefinded reference to "QsqlDataBase::defaultConnection""
.... any other similar too...

Scylla

May be you miss "QT += sql" in your *.pro file?

dcbasso

Yes it's missing... But still not working!

andre

Not working how? Do you get the same error?

dcbasso

Yes, same error!

franku

Some changes, as the QSqlDatabase already contains static funtionality. In my opinion it is not neccessary to implement your class as a singleton since it could be static at all:

Private constructor is missing its implementation (used by the new operator):
@ SingletonDatabaseConnection() {}@

Do not try to store the QSqlDatabase object as a static variable:
@static QSqlDatabase database; //remove this line@

Instead use QSqlDatabase as already static class:
@QSqlDatabase getDatabase() { return QSqlDatabase::database(); }@

Change the initialization into:
@
...
if (!myInstance) {
QSqlDatabase database = QSqlDatabase::addDatabase("QSQLITE");
database.setDatabaseName("inoveDB.db3");
if (database.open()) { ...
@

This is already done in the initialization of the Singleton, so don't do it twice:
@void openDatabase(); @

Return the singleton already provided by the QSqlDatabase (you currently only use the default connection):
@QSqlDatabase getDatabase() { return QSqlDatabase::database(); }@

If you want to check easily if this is working, implement the following method that you call in the button-slot:
@QString getDatabaseName() { return getDatabase().databaseName(); }@

The database is closed when the application is finished. But you might want to close it manually in the drop() function.

dcbasso

franku, I will try change as you write.

Thanks.

dcbasso

@
...
if (!myInstance) {
QSqlDatabase database = QSqlDatabase::addDatabase("QSQLITE");
database.setDatabaseName("inoveDB.db3");
if (database.open()) { ...
@

Where I need to put that?
I change the method getDatabase(), appers to resolve the problem... I'm need to make this works fine.

FOGET THIS!

dcbasso

This fix my problem!
Thanks!