Error: undefined reference to 'vtable for Widget'

[8Observer8]

Hi,

How to solve the error error: undefined reference to 'vtable for Widget' in the example:

#include <QApplication>
#include <QWidget>
#include <QTimer>
#include <QDebug>

class Widget : public QWidget
{
    Q_OBJECT
public:
    Widget(QWidget *parent = nullptr) : QWidget(parent) {
        connect(&m_timer, &QTimer::timeout, this, &Widget::printMessage);
    }
private slots:
    void printMessage() {
        qDebug() << "timer";
    }
private:
    QTimer m_timer;
};

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

[JonB]

@8Observer8
If you added the Q_OBJECT macro at some point, I would advise clearing out the debug/release directory and trying a complete rebuild. Does that solve it?

The other possibility is that you need a virtual ~Widget() { }, at one point I thought that was required for QObjects but I don't actually think you have to have it now....

[J.Hilk]

@8Observer8

you're trying to create a QObject based class inside main.cpp

that's possible, but not intended.

Like @JonB said, definitely add the Q_OBJECT macro,

then explicitly include the moc file inside your main

#include "main.moc"

AFTER the class declaration

[8Observer8]

Clear/Rebuild - does not work.

Clear/Run qmake/Rebuild/ -does not wok.

Closing QtCreater and deleting the "build..." folder - does not work.

This error accrues on these lines:

class Widget : public QWidget

    Widget(QWidget *parent = nullptr) : QWidget(parent) {

[8Observer8]

@J-Hilk said in Error: undefined reference to 'vtable for Widget':

then explicitly include the moc file inside your main

Error: main.cpp:22: error: main.moc: No such file or directory #include "main.moc" ^~~~~~~~~~

[J.Hilk]

@8Observer8

class Widget : public QWidget
{
    Q_OBJECT
public:
    Widget(QWidget *parent = nullptr) : QWidget(parent) {
        connect(&m_timer, &QTimer::timeout, this, &Widget::printMessage);
        m_timer.start(100);
    }
private slots:
    void printMessage() {
        qDebug() << "timer";
    }
private:
    QTimer m_timer;
};

#include "main.moc"

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

explicitly execute qmake, as the moc file does not yet exists,
than run normal build

[8Observer8]

RMB on the project > "Run qmake" > but I see error: main.moc: No such file or directory #include "main.moc"

[J.Hilk]

@8Observer8 QtCreator is not updated (like I said its not the normal way to do this),
if you executed qmake, simply build the project

[8Observer8]

Yes, it works! Thank you very much!

#include <QApplication>
#include <QWidget>
#include <QTimer>
#include <QDebug>

class Widget : public QWidget
{
    Q_OBJECT
public:
    Widget(QWidget *parent = nullptr) : QWidget(parent) {
        connect(&m_timer, &QTimer::timeout, this, &Widget::printMessage);
        m_timer.start(1000);
    }
private slots:
    void printMessage() {
        qDebug() << "timer";
    }
private:
    QTimer m_timer;
};

#include "main.moc"

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