Undefined reference to vtable... Q_OBJECT macro

Question

When I uncomment the Q_OBJECT macro that I need for signal-slot I get a undefined reference to vtable for MyApp error, but without the macro it compiles perfectly but I can't use signals and slots without it. I think I may be doing something stupid wrong, but please try helping because I realy can't find the problem. O and I know my code is untidy and am working on it.

myapp.h:

#ifndef MYAPP_H
#define MYAPP_H

#include <QApplication>
#include <QEvent>
#include <QObject>
#include <QDebug>

class MyApp : public QApplication
{

public:
    MyApp( int argc, char** argv );

protected:
    bool eventFilter(QObject *object, QEvent *event);

signals:
    void focusG();
    void focusL();
};

#endif // MYAPP_H

myapp.cpp:

#include "myapp.h"

MyApp::MyApp(int argc, char **argv): QApplication(argc, argv)
{
    installEventFilter(this);
}

bool MyApp::eventFilter(QObject *object, QEvent *event)
{
    if (event->type() == QEvent::ApplicationDeactivate)
    {
        qDebug() << "Focus lost";
        //focusL();
    }
    if (event->type() == QEvent::ApplicationActivate)
    {
        qDebug() << "Focus gained";
        //focusG();
    }

    return false;
}

main.cpp:

#include <QtGui/QApplication>
#include "qmlapplicationviewer.h"
#include <QObject>
#include <QGraphicsObject>
#include <QTimer>
#include <QVariant>
#include "timecontrol.h"
#include "scorecontrol.h"
#include "Retry.h"
#include <QEvent>
#include "myapp.h"

int main(int argc, char *argv[])
{
    //QApplication app(argc, argv);
    MyApp app(argc, argv);

    QmlApplicationViewer viewer;
    viewer.setOrientation(QmlApplicationViewer::ScreenOrientationLockLandscape);
    viewer.setMainQmlFile(QLatin1String("qml/Raker/main.qml"));
    viewer.showExpanded();

    QObject *rootObject = viewer.rootObject();

    QTimer *timmer = new QTimer;
    timmer->setInterval(1000);

    TimeControl *timcon = new TimeControl;

    scorecontrol *scorer = new scorecontrol;

    Retry *probeer = new Retry;

    QObject::connect(timmer, SIGNAL(timeout()), timcon, SLOT(updateTime()));
    QObject::connect(timcon, SIGNAL(setTime(QVariant)), rootObject, SLOT(setTime(QVariant)));
    QObject::connect(rootObject, SIGNAL(blockClicked(int, int)), scorer, SLOT(checkRight(int, int)));
    QObject::connect(scorer, SIGNAL(setScore(QVariant)), rootObject, SLOT(setScore(QVariant)));
    QObject::connect(scorer, SIGNAL(setState(QVariant)), rootObject, SLOT(setState(QVariant)));

    QObject::connect(rootObject, SIGNAL(start()), probeer, SLOT(Reetry()));
    QObject::connect(probeer, SIGNAL(start()), timmer, SLOT(start()));
    QObject::connect(probeer, SIGNAL(stop()), timmer, SLOT(stop()));
    QObject::connect(probeer, SIGNAL(start(int)), scorer, SLOT(randomNum(int)));
    QObject::connect(probeer, SIGNAL(sReset()), timcon, SLOT(reset()));
    QObject::connect(probeer, SIGNAL(tReset()), scorer, SLOT(reset()));
    QObject::connect(timcon, SIGNAL(timeOut()), scorer, SLOT(reset()));

    QObject::connect(timcon, SIGNAL(setState(QVariant)), rootObject, SLOT(setState(QVariant)));
    QObject::connect(timcon, SIGNAL(changeFinal()), scorer, SLOT(changeFinal()));
    QObject::connect(scorer, SIGNAL(setFinal(QVariant)), rootObject, SLOT(setFinal(QVariant)));

    QObject::connect(&app, SIGNAL(focusL()), probeer, SLOT(focusL()));
    QObject::connect(&app, SIGNAL(focusG()), probeer, SLOT(focusG()));

    return app.exec();
}

BlockToucher.pro:

# Add more folders to ship with the application, here
folder_01.source = qml/Raker
folder_01.target = qml
DEPLOYMENTFOLDERS = folder_01

# Additional import path used to resolve QML modules in Creator's code model
QML_IMPORT_PATH =

symbian:TARGET.UID3 = 0x2004b49f

# Smart Installer package's UID
# This UID is from the protected range and therefore the package will
# fail to install if self-signed. By default qmake uses the unprotected
# range value if unprotected UID is defined for the application and
# 0x2002CCCF value if protected UID is given to the application
symbian:DEPLOYMENT.installer_header = 0x2002CCCF

# Allow network access on Symbian
symbian {
    #TARGET.CAPABILITY += NetworkServices

    vendorinfo = "%{\"Gerhard de Clercq\"}" ":\"Gerhard de Clercq\""
    ICON = BlockToucher.svg
}

# If your application uses the Qt Mobility libraries, uncomment the following
# lines and add the respective components to the MOBILITY variable.
# CONFIG += mobility
# MOBILITY +=

# The .cpp file which was generated for your project. Feel free to hack it.
SOURCES += main.cpp \
    timecontrol.cpp \
    scorecontrol.cpp \
    Retry.cpp \
    myapp.cpp \
    myapplication.cpp

# Please do not modify the following two lines. Required for deployment.
include(qmlapplicationviewer/qmlapplicationviewer.pri)
qtcAddDeployment()

HEADERS += \
    timecontrol.h \
    scorecontrol.h \
    Retry.h \
    myapp.h \
    myapplication.h

OTHER_FILES += \
    qtc_packaging/debian_fremantle/rules \
    qtc_packaging/debian_fremantle/README \
    qtc_packaging/debian_fremantle/copyright \
    qtc_packaging/debian_fremantle/control \
    qtc_packaging/debian_fremantle/compat \
    qtc_packaging/debian_fremantle/changelog
compat \
    qtc_packaging/debian_fremantle/changelog

score 6 · Answer 1 · answered Oct 26 '11 at 15:23

6

Q_OBJECT is used by the MOC system to produce the code you need for the signals. My best guess would be that you are not included the MOC generated files in your project (the CPP files which it generates).

How you do this greatly depends on your build system (CMake, QMake, AUtomake, MSVC) but you should refer to the tutorial with QMake first.

That is, the undefined reference to vtable is the error I sometimes get when I forget to update my Cmake files and have a Q_OBJECT.

Also, your ctor for MyApp is wrong, the signature must be:

MyApp( int & argc, char** argv );

The & is important should you ever use command-line parameters.

answered Oct 26 '11 at 15:23

edA-qa mort-ora-y

30,295
39
137
267

I use Qt Creator which appears to run Qmake before it compiles – Gerharddc Oct 26 '11 at 15:56
2

@Gerhman It does, but only if the .pro file changes. If non-mocable files becomes mocable (= contain the `Q_OBJECT` macro), qmake has to be run manually, so that the Makefile is updated to execute the moc on them. – alexisdm Oct 26 '11 at 16:42
Why is the `&` important? – Adrian Maire Jul 24 '23 at 09:21

S.M.Mousavi · Answer 2 · 2020-07-03T21:18:23.683

I had same issue when i decided to add Q_OBJECT into my header file.

First try run qmake manually.
If not resolved, so delete build folder and build project again
If building all of the project is a heavy task, so try following steps:
1. Add a new temp class with QObject as its base class
2. Double check your header and source file for any mistake using new temp class
3. Build your project. It should built successfully
4. Remove your temp class completely (.h and .cpp file and relative lines on .pro file)

Good luck :)

Undefined reference to vtable... Q_OBJECT macro

2 Answers2

Linked