Signal & Slot weard error (Qt 5.7.1)

michalos

Hi,
can someone tell my why when I use this:

    connect(window,&MainWindow::sendHistoryRequestWithMarker,pbxConneciton,&PBXConnect::getHistoryCallUpdates);

I get an compiler error:
C2664: 'QMetaObject::Connection QObject::connect(const QObject *,const char *,const char ,Qt::ConnectionType) const': cannot convert argument 2 from 'void (__cdecl MainWindow:: )(QString &)' to 'const char *'

, and this:

    connect(window,SIGNAL(sendHistoryRequestWithMarker(QString&)),pbxConneciton,SLOT(getHistoryCallUpdates(QString&)));

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

one line below I have :

    connect(window,&MainWindow::mainwindowSendUpdate_REQ,pbxConneciton,&PBXConnect::sendUpdate_REQ);

that works perfectly?

VRonin

could you show us the signature of MainWindow::sendHistoryRequestWithMarker and PBXConnect::getHistoryCallUpdates? are you overloading any of these functions?

michalos

Yes, I have :

    void getHistoryCallUpdates();
///and
    void getHistoryCallUpdates(QString &);

michalos

I've changed the functions name.

now, when I send the signal I get:

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

VRonin

ok, 2 more things:

what's the signature of MainWindow::sendHistoryRequestWithMarker
what version of Qt are you using?

mrjj

@michalos
Hi
Using the new syntax , you must use cast when you have a overloaded signal.
( same name, different paramters)

lIke in

connect(mySpinBox, static_cast<void (QSpinBox::*)(int)>(&QSpinBox::valueChanged), mySlider, &QSlider::setValue);

https://wiki.qt.io/New_Signal_Slot_Syntax

VRonin

@mrjj said in Signal & Slot weard error (Qt 5.7.1):

you must use cast when you have a overloaded signal

or for Qt>=5.7 qOverload

Edit:
Are you using Qt::QueuedConnection (i.e. sender and receiver are in 2 different threads)?
Do you really mean to pass a non-const QString reference?

michalos

@VRonin @mrjj
That's helpful to know.

But renaming the Signal and the Slot does not solve the problem.
I still get the error :
QObject::connect: Cannot queue arguments of type 'QString&'
(Make sure 'QString&' is registered using qRegisterMetaType().)

VRonin

Could you answer all pending questions?

Are you using Qt::QueuedConnection (i.e. sender and receiver are in 2 different threads)?
Do you really mean to pass a non-const QString reference?
what's the signature of MainWindow::sendHistoryRequestWithMarker
what version of Qt are you using?

michalos

@VRonin

Yes, I am using Qt::QueuedConnection
Can You tell me why passing a non-const QString reference is a problem?
signature (if I understand correctly) is:

void sendHistoryRequestWithMarkerMain(QString &);

I am using Qt 5.7.1 (MSVC 2015 64 bit)

Changing the QString & to const QString & did help, but I would appreciate If You could explain me why (or point me in the right direction).

VRonin

@michalos said in Signal & Slot weard error (Qt 5.7.1):

Can You tell me why passing a non-const QString reference is a problem?

Because that string could be modified on the other side so you cannot store "a copy of it" in a queue