QT signal slot behavior with respect to const ref arguments - c++

Question

I am working on some code where I see the following:
In header file:
private slot:
void OnNotifySomeSlot(const QList<bool>& someList); // Note the argument is pass by const ref
In implementation file:
connect(&m_serviceObj,SIGNAL(NotifySomeSlot(QList<bool>)), this, SLOT(OnNotifySomeSlot(QList<bool>)); // Note that arguments are pass by value
Slot function definition:
void OnNotifySomeSlot(const QList<bool>& someList)
{
m_list = someList;
} // Note that argument is passed by const ref as in the header
Now my doubt is that:
1: Why is such a code getting compiled as there is difference in signature between connect statement and slot definition?
2: During runtime if the value is getting passed a const ref , will it still not cause problems in a multithreaded environment. Because the reference to the list (be it const) can still be modified by some other code referring the same list in a multithreaded environment.
3: Can we just "pass by value" everywhere to avoid any issue in a multi threaded environment. Will it make the application slower?

Answer 1

Answering your questions in order:
The Qt macro machinery canonicalizes signal and slot names so they "fit". If you use the modern connection approach you do not have to worry about this:
QObject::connect(m_serviceObj, &SomeServiceObjectClass::NotifySomeSlot, this, &ThisObjectClassOnNotifySomeSlot)
Yes, even though you pass the list as a const ref, some other thread that has a non-const ref can change it behind your back.
Yes, passing it by value makes this behave properly. It is also efficient, as QList is an "implicitly shared" datastructure. This means that a copy is only made when a mutation happens, by the thread doing the mutation.