C++ (Qt)
connect( ui.comboBox, SIGNAL( currentIndexChanged( const QString & ) ), this, SLOT( XXX() ) );

C++ (Qt)
connect( ui.comboBox, SIGNAL(currentIndexChanged(QString)), this, SLOT(XXX()));
 

C++ (Qt)
void MainWindow::on_pushButton_clicked()
{
    qDebug() << &str;
    emit someSignal(str);
}
 

C++ (Qt)
void MainWindow::someSlot(const QString &str)
{
    qDebug() << &str;
}
 

C++ (Qt)
QObject::connect(this, SIGNAL(someSignal(QString)), SLOT(someSlot(QString))/*, Qt::QueuedConnection*/);
 

C++ (Qt)
private slots:
    void someSlot(QString);
signals:
    void someSignal(QString str);
 

C++ (Qt)
QObject::connect(this, SIGNAL(someSignal(QString)), SLOT(someSlot(QString)));
 

C++ (Qt)
signals:
    void someSignal(const QString& str);
    void someSignal(QString str);
 

C++ (Qt)
А еще в некоторых случаях, я могу захотеть сделать следующее:
 

C++ (Qt)
QObject::connect(this, SIGNAL(someSignal(QString)), SLOT(someSlot(QString))/*, Qt::QueuedConnection*/);
 

C++ (Qt)
QObject::connect(this, SIGNAL(someSignal(QString&)), SLOT(someSlot(QString&))/*, Qt::QueuedConnection*/);
 

Object::connect: No such signal MainWindow::someSignal(QString)

QObject::connect: Cannot queue arguments of type 'QString&'
(Make sure 'QString&' is registered using qRegisterMetaType().)