void MyQObjectBasedClass::valueChanged( QDateTime dt ) {
    qDebug() << "value:" << &dt;
}

value: 0x0

QDateTimeEdit * edit = new QDateTimeEdit;
...
MyQObjectBasedClass * myobject = new MyQObjectBasedClass;
...
connect( edit, SIGNAL( dateTimeChanged(QDateTime) ),
    myobject, SLOT( valueChanged(QDateTime) ) );

edit->setDateTime( .../*тут например QDateTime::currentDateTime(), хотя писал и константы*/ );

connect(dateTimeEdit, SIGNAL(dateTimeChanged(QDateTime)), this, SLOT(valueChanged1(QDateTime)));
connect(dateTimeEdit, SIGNAL(dateTimeChanged(const QDateTime &)), this, SLOT( valueChanged2(const QDateTime &)));

.....

void Test::valueChanged1(QDateTime dt)
{
    qDebug() << "valueChanged1:" << &dt;
}

void Test::valueChanged2( const QDateTime &dt )
{
    qDebug() << "valueChanged2:" << &dt;
}

QDateTimeEdit * edit = new QDateTimeEdit;
MyQObjectBasedClass * myobject = new MyQObjectBasedClass;

void f(int i) {
  cout<<&i<<endl;
}
void ff(const int& i) {
  f(i);
}
void fff(const int* i) {
 ff(*i);
}
int main() {
  fff(0);
}