Qt won't connect to a slot in a subclass

Question

I have subclassed QTabWidget and added a new public slot. When I try to connect to that slot, Qt tells me that my slot doesn't exist. But seems to be looking in the parent class. What am I missing?

Here is a minimal program that reproduces the problem. I am running it in Qt 5.13 with a MinGW 32-bit package.

#include <QMainWindow>
#include <QApplication>
#include <QTabWidget>
#include <QTabBar>
#include <QDebug>

class MyTabWidget : public QTabWidget
{
public:
    MyTabWidget(QWidget *parent) : QTabWidget(parent)
    {
        qDebug() << "connect() returns " <<
        connect(this->tabBar(),SIGNAL(tabBarDoubleClicked(int)),this,SLOT(changeTabName(int)));
    }

public slots:
    void changeTabName(int index)
    {
        tabBar()->setTabText(index,"New Name");
    }
};

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    QMainWindow *mw = new QMainWindow();
    QWidget *tw;

    MyTabWidget *t = new MyTabWidget(mw);
    tw = new QWidget(mw);
    t->addTab(tw,"Double-click This Tab");

    mw->setCentralWidget(t);
    mw->show();
    return a.exec();
}

Here is the output. When connect() is called, "this" is a pointer to the subclass, right? So why is it looking for the slot in the superclass?

QObject::connect: No such slot QTabWidget::changeTabName(int) in main.cpp:14
QObject::connect:  (sender name:   'qt_tabwidget_tabbar')
connect() returns  false

Answer 1

You have the following errors:

• If a class that inherits from QObject like all widgets is going to have signals, slots, etc. then you must use the Q_OBJECT macro to create the necessary implementations associated with those elements. Therefore, if you are using Q_OBJECT in the main file (filename.cpp) then you must include "filename.moc" to include the above.

• You must use the new connection syntax, although it is not an error but can be a source of silent bugs.

Considering the above, the solution is:

main.cpp

#include <QMainWindow>
#include <QApplication>
#include <QTabWidget>
#include <QTabBar>
#include <QDebug>

class MyTabWidget : public QTabWidget
{
    Q_OBJECT
public:
    MyTabWidget(QWidget *parent) : QTabWidget(parent)
    {
        qDebug() << "connect() returns " <<
        connect(tabBar(), &QTabBar::tabBarDoubleClicked, this, &MyTabWidget::changeTabName);
    }

public slots:
    void changeTabName(int index)
    {
        tabBar()->setTabText(index,"New Name");
    }
};

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    QMainWindow *mw = new QMainWindow();
    QWidget *tw;

    MyTabWidget *t = new MyTabWidget(mw);
    tw = new QWidget(mw);
    t->addTab(tw,"Double-click This Tab");

    mw->setCentralWidget(t);
    mw->show();
    return a.exec();
}

#include "main.moc"