QObject::connect only work with last element when I stored class info into std::vector

HarryC

I have an issue with connect signal and slot. The code looks like this.

class H : QObject {
    Q_OBJECT
...
signals: 
      void stateChanged();
...
}

void H::CheckState()
{
    emit stateChanged();
}

class F : QObject {
    Q_OBJECT
public slots:
    void StateChanged();
private:
    std::vector<H> m_hs;
}

void F::AddH(const H& h) {
    m_hs.push_back(h);

    // connect using loop
    // for(const auto& h : m_hs)
    //    connect(&h, &H:stateChanged, this, &F::StateChanged);
   
    // connect for single h
    connect(&m_hs.back(), &H:stateChanged, this, &F::StateChanged);
}

void F::StateChanged()
{
}

F class has m_hs which is std::vector container contains multiple H class data.
If I use "connect using loop", all signal/slot works for all Hs.
But if I use "connect for single h", it only works for last pushed H class data.

I believe it should work without for loop as it connect signal and slot multiple times for others except last element(H).

Do you have any idea about this?

ChrisW67

Your H class is a QObject subclass I assume. You cannot copy a QObject (by design) but that is what you are attempting to do with your vector. (I am surprised that this compiled.) You should be heap allocating the H instances and storing pointers to them (if you need to), not instances.

HarryC

@ChrisW67 Yes, you are right. Both H and F class are sub class of QObject. I am going to add it to the code.

Does this mean that F and H cannot have copy constructor as well?

Even the connected signals and slots are working when I used "connect using loop" code!

JonB

@HarryC
No class derived from QObject (both your F & H) can have a copy constructor.

As @ChrisW67 said, create your instances via H *h = new H, store in std::vector<H *> m_hs and then connect via

    // connect using loop
    for (const auto h : m_hs)
       connect(h, &H:stateChanged, this, &F::StateChanged);

See how that goes?

HarryC

@JonB Okay Thanks! I will update after checking!

HarryC

@JonB I confirm that it works well when I put pointer of H class to the vector even without using for loop to connect multiple times.

Do you guys usually not recommend using copy constructor/assignment operator for subclass of QObject even we can define them?

I have changed all relevant code to use pointer now based on std::shared_ptr. and, it works well.
But quite curious about your general patterns about using subclass of QObject as all functions that use signals/slots are subclass of QObject.

I am going to set solved once I get answer for this question anyway.

Thanks for your help!

ChrisW67

@HarryC said in QObject::connect only work with last element when I stored class info into std::vector:

Do you guys usually not recommend using copy constructor/assignment operator for subclass of QObject even we can define them?

You cannot define a copy constructor for a QObject derived class because QObject explicitly declares them private and deleted. See No Copy Constructor or Assignment Operator and Qt Objects: Identity vs Value.

So, for example:

#include <QCoreApplication>
#include <QObject>

class Test: public QObject {
public:
        explicit Test(QObject *p = nullptr): QObject(p) { }
        Test(const Test& other): QObject(other) { }
};

int main(int argc, char **argv) {
        QCoreApplication app(argc, argv);
        return 0;
}

results in:

g++ -c -pipe -O2 -Wall -W -D_REENTRANT -fPIC -DQT_DEPRECATED_WARNINGS -DQT_NO_DEBUG -DQT_GUI_LIB -DQT_CORE_LIB -I. -I. -isystem /usr/include/x86_64-linux-gnu/qt5 -isystem /usr/include/x86_64-linux-gnu/qt5/QtGui -isystem /usr/include/x86_64-linux-gnu/qt5/QtCore -I. -I/usr/lib/x86_64-linux-gnu/qt5/mkspecs/linux-g++ -o main.o main.cpp
main.cpp: In copy constructor ‘Test::Test(const Test&)’:
main.cpp:7:40: error: use of deleted function ‘QObject::QObject(const QObject&)’
    7 |  Test(const Test& other): QObject(other) { }
      |                                        ^
In file included from /usr/include/x86_64-linux-gnu/qt5/QtCore/qcoreapplication.h:43,
                 from /usr/include/x86_64-linux-gnu/qt5/QtCore/QCoreApplication:1,
                 from main.cpp:1:
/usr/include/x86_64-linux-gnu/qt5/QtCore/qobject.h:449:5: note: declared here
  449 |     Q_DISABLE_COPY(QObject)
      |     ^~~~~~~~~~~~~~
make: *** [Makefile:358: main.o] Error 1

HarryC

@ChrisW67

Hi Chris, thanks for your example. But when I tested with example on vs2019 with qt vs tools. Following code compiled and ran very well. Originally it didn't have copy constructor but I have added empty copy constructor for test purpose.

When I checked qobject.h file, I couldn't find deleted or private copy constructor or assignment operator. I am using Qt 5.15.2

update: Oh.. I found this
Q_DISABLE_COPY(QObject)
Which Q_DISABLE_COPY is

#define Q_DISABLE_COPY(Class) \
    Class(const Class &) = delete;\
    Class &operator=(const Class &) = delete;

// main.cpp

#include "QtWidgetsApplication1.h"
#include <QtWidgets/QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    QtWidgetsApplication1 w;
    w.show();
    return a.exec();
}

// QtWidgetsApplication1.cpp

#include "QtWidgetsApplication1.h"

QtWidgetsApplication1::QtWidgetsApplication1(QWidget *parent)
    : QMainWindow(parent)
{
    ui.setupUi(this);
}

// QtWidgetsApplication1.h

#pragma once

#include <QtWidgets/QMainWindow>
#include "ui_QtWidgetsApplication1.h"

class QtWidgetsApplication1 : public QMainWindow
{
    Q_OBJECT

public:
    QtWidgetsApplication1(QWidget *parent = Q_NULLPTR);
    
    // empty copy constructor here
    QtWidgetsApplication1(const QtWidgetsApplication1& other) {}

private:
    Ui::QtWidgetsApplication1Class ui;
};

HarryC

@ChrisW67 I realised I was wrong. It should be like this :

QtWidgetsApplication1(const QtWidgetsApplication1& other): : QMainWindow(other) {}

This this I can see compile error successfully.

Thank you all!

ChrisW67

This:

   // empty copy constructor here
    QtWidgetsApplication1(const QtWidgetsApplication1& other) {}

does not call the parent copy constructor (The sub-class copy's QObject instance will be default constructed). The equivalent in my code looks like:

Test(const Test& other): { }

and generates this warning:

g++ -c -pipe -O2 -Wall -W -D_REENTRANT -fPIC -DQT_DEPRECATED_WARNINGS -DQT_NO_DEBUG -DQT_GUI_LIB -DQT_CORE_LIB -I. -I. -isystem /usr/include/x86_64-linux-gnu/qt5 -isystem /usr/include/x86_64-linux-gnu/qt5/QtGui -isystem /usr/include/x86_64-linux-gnu/qt5/QtCore -I. -I/usr/lib/x86_64-linux-gnu/qt5/mkspecs/linux-g++ -o main.o main.cpp
main.cpp: In copy constructor ‘Test::Test(const Test&)’:
main.cpp:8:2: warning: base class ‘class QObject’ should be explicitly initialized in the copy constructor [-Wextra]
    8 |  Test(const Test& other) { }
      |  ^~~~
main.cpp:8:19: warning: unused parameter ‘other’ [-Wunused-parameter]
    8 |  Test(const Test& other) { }
      |       ~~~~~~~~~~~~^~~~~
g++ -Wl,-O1 -o test main.o   /usr/lib/x86_64-linux-gnu/libQt5Gui.so /usr/lib/x86_64-linux-gnu/libQt5Core.so /usr/lib/x86_64-linux-gnu/libGL.so -lpthread

If these were two value classes then failing to call the parent class copy constructor would lead to an incomplete copy.