passing signal between threads with Qvector as parameter

Daddedebad

Hey, I have an issue where I can pass a signal and get it right on the other end between threads, but this no longer works when I try to pass a Qvector with the signal as a parameter. Other variable is fine though, I can pass a Qstring without issue.

Here is my code:

h-files:

// Main thread class
class QtWidgetsApplication : public QWidget
{
    Q_OBJECT
    QThread secondThread;
public:
    secondThreadClass* secondClass = nullptr;
    QtWidgetsApplication(QWidget *parent = nullptr);
    ~QtWidgetsApplication();

private:
    Ui::QtWidgetsApplicationClass ui;

signals:
    void doWorkInSecondThread(const QString&, QVector<int>&);
};

// Second thread class
class secondThreadClass :public inheritedParent
{
public:
    secondThreadClass();
public slots:
    void sayHello(const QString& text, QVector<int>& dataSet);
};

// Inherited class
class inheritedParent : public QObject
{
    Q_OBJECT
};

c-files:

//Main thread class
QtWidgetsApplication::QtWidgetsApplication(QWidget* parent)
    : QWidget(parent)
{
    QVector<int> dataSet = { 0,3,1,2 };
    ui.setupUi(this);
    secondClass = new secondThreadClass();

    secondClass->moveToThread(&secondThread);
    connect(&secondThread, &QThread::finished, secondClass, &QObject::deleteLater);
    connect(this, &QtWidgetsApplication::doWorkInSecondThread, secondClass, &secondThreadClass::sayHello);

    secondThread.start();

    emit doWorkInSecondThread(QString("Hello"), dataSet);
}

QtWidgetsApplication::~QtWidgetsApplication()
{}

// Second thread class
secondThreadClass::secondThreadClass(){}

void secondThreadClass::sayHello(const QString& text, QVector<int>& dataSet)
{
    print(text);
    return;
}

InheritedParent is empty. It is in this example just to show exactly what I did. This is from debugging another code where I made my structure like this.

If I remove the QVector parameter, the code works and text "Hello" is passed between the threads and executed in secondThreadClass.

I am thankful for all the help I can get!

PS. I am using QT 5.15.2

jsulm

@Daddedebad said in passing signal between threads with Qvector as parameter:

but this no longer works when I try to pass a Qvector with the signal as a parameter

What exactly does not work? What happens?
Does the slot have same parameters?

Bonnie

@Daddedebad Take a look at your application output, it should print some hint when it tries to connect the signal but fails.
I think QVector<int> needs to be registered by qRegisterMetaType or something like that.
For your reference: https://doc.qt.io/qt-6/custom-types.html#creating-and-destroying-custom-objects

JonB

@Daddedebad
As @Bonnie says qRegisterMetaType.

A separate point for your own benefit. I think you should consider making the signal/slot parameter be const QVector<int>& dataSet. Passing across threads (queued connection) will copy the QVector in either case, but the const will remind you that you cannot alter the original QVector in the other thread. [Expert correct me if I am wrong here, but seems useful to me.]

Daddedebad

@Bonnie I think you are right, this is what the console says:

QObject::connect: Cannot queue arguments of type 'QVector<int>&'
(Make sure 'QVector<int>&' is registered using qRegisterMetaType().)

I tried registering using qRegisterMetaType(QVector<int>) but got the same output. It needs the reference specifically. However, I cannot register QVector<int>& because then it doesn't compile.

my main.cpp code now:

#include "QtWidgetsApplication2.h"
#include <QtWidgets/QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    qRegisterMetaType<QVector<int>&>();

    QtWidgetsApplication2 w;
    w.show();
    return a.exec();
}

Gives errors:

Severity	Code	Description	Project	File	Line	Suppression State
Error	C2338	Type is not registered, please use the Q_DECLARE_METATYPE macro to make it known to Qt's meta-object system	QtWidgetsApplication	X:\X\X\X\QtCore\qmetatype.h	1916	

Severity	Code	Description	Project	File	Line	Suppression State
Error	C2039	'qt_metatype_id': is not a member of 'QMetaTypeId2<T>'	QtWidgetsApplication	X:\X\X\X\X\QtCore\qmetatype.h	1917	

Severity	Code	Description	Project	File	Line	Suppression State
Error	C3861	'qt_metatype_id': identifier not found	QtWidgetsApplication	X:\X\X\X\X\QtCore\qmetatype.h	1917

I'm trying to find how to do it, but it seems that references are tricky to register?

Also I'm using QT 5.15.2, so far I don't think it matters but I will add it to the main post.

jsulm

@Daddedebad Don't register references! Just register QVector<int>.

Daddedebad

@jsulm registering without the reference like this:

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    qRegisterMetaType<QVector<int>>();

    QtWidgetsApplication2 w;
    w.show();
    return a.exec();
}

Gives the console output:

QObject::connect: Cannot queue arguments of type 'QVector<int>&'
(Make sure 'QVector<int>&' is registered using qRegisterMetaType().)

So if I register without the reference, what should the next step be?

JonB

@Daddedebad said in passing signal between threads with Qvector as parameter:

QObject::connect: Cannot queue arguments of type 'QVector<int>&'

I suggested you prepend const. If you do so does that affect this error? I'm not sure that you can pass a non-const reference as an argument in signals and slots when using a queued connection. You might also not bother with the & reference at all, since as I said it gets copied for queued anyway.

Daddedebad

@JonB Yes that seemed to solve it! Thank you! I added const.

@JonB said in passing signal between threads with Qvector as parameter:

You might also not bother with the & reference at all, since as I said it gets copied for queued anyway.

Aha ok, I don't think I need it for my program, but that means I cannot pass references between threads doing this?

jsulm

@Daddedebad said in passing signal between threads with Qvector as parameter:

I cannot pass references between threads doing this?

No, parameters are copied across different threads (to avoid problems with musltithreading).

Christian Ehrlicher

@JonB said in passing signal between threads with Qvector as parameter:

I'm not sure that you can pass a non-const reference as an argument in signals and slots when using a queued connection.

You can't - how should it work if you would be able to? And this was the initial problem here.

JonB

@Christian-Ehrlicher
Yes, I said from the start that was the case, it would not work as you would expect. I was suggesting that the error message QObject::connect: Cannot queue arguments of type 'QVector<int>&' might be because connect() refuses to accept them for queueing, but would allow if const.

Christian Ehrlicher

@JonB said in passing signal between threads with Qvector as parameter:

might be because connect() refuses to accept them for queueing.

correct