Get undefined reference error but don't know why

[casisto]

Hello there,

I have code a experimental minimal QApp to test how to open new Windows after pushing a button. But i get the error
"undefined reference to 'vtable for MainWindow'".

Here my SourceCode:
mainwindow.h

#pragma once
#include <QWidget>
#include <QBoxLayout>
#include <QPushButton>
#include "newwindow.h"

class MainWindow : public QWidget
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = 0);

public slots:
    void openNewWindow();
};

newwindow.h

#pragma once

#include <QWidget>
#include <QBoxLayout>
#include <QLabel>

class NewWindow : public QWidget
{
public:
    NewWindow(QWidget *parent = 0);
};

mainwindow.cpp

#include "mainwindow.h"

MainWindow::MainWindow(QWidget *parent) : QWidget(parent)
{
    QVBoxLayout *vbox = new QVBoxLayout;
        vbox->setMargin(20);
    QPushButton *btn= new QPushButton("New window", this);
        btn->setMinimumSize(QSize(200,50));

    vbox->addWidget(btn, 2, Qt::AlignVCenter);
    setLayout(vbox);

    connect(btn, SIGNAL(clicked()), this, SLOT(openNewWindow()));
}
void MainWindow::openNewWindow()
{
    NewWindow *newwindow = new NewWindow();
    newwindow->show();
}

newwindow.cpp

#include "newwindow.h"

NewWindow::NewWindow(QWidget *parent) : QWidget(parent)
{
    QVBoxLayout *vbox = new QVBoxLayout;
        vbox->setMargin(20);
    QLabel *label = new QLabel("New Window", this);

    vbox->addWidget(label, 2, Qt::AlignVCenter);
    setLayout(vbox);
}

main.cpp

#include "mainwindow.h"
#include <QApplication>

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

Thanks for all they want help
-Casisto

Edited: Put code after 3 backticks(```) and end with the same - p3c0

[casisto]

ok,
i had can solve it by myself.

For all they have they problem too:

The error is sourced from the use of "Q_OBJECT".
To solve the error, right-cklick on the Project and choose "Run qmake" and after this: "Rebuild".

Then the error should be disappeared ;-)

-casisto

[JKSH]

Hi @casisto, I'm glad to hear that you've solved your problem. Thanks for sharing your solution!

[panoramix]

I've also had the same problem that I solved the same way.
I happened on Windows and ever on Linux