Q_OBJECT weirdness

main.cpp

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

int main(int argc, char **argv)
{
    QApplication app(argc, argv);

    Window window;
    window.show();

    return app.exec();
}

output.txt

Undefined symbols for architecture x86_64:
  "vtable for Window", referenced from:
      Window::Window(QWidget*) in window.o
  NOTE: a missing vtable usually means the first non-inline virtual member function has no definition.
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

tutorial.pro

TEMPLATE = app
TARGET = tutorial

QT = core gui

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

SOURCES += \
    main.cpp \
    window.cpp

HEADERS += \
    window.h

window.cpp

#include "window.h"
#include <QPushButton>
#include <QApplication>
#include <QProgressBar>
#include <QSlider>

Window::Window(QWidget *parent) : QWidget(parent)
{
    setFixedSize(800, 600);

    m_button = new QPushButton("Hello, World", this);
    m_button->setGeometry(10, 10, 160, 30);
    m_button->setCheckable(true);

    connect(m_button, &QPushButton::clicked, this, &Window::slotButtonClicked);

    m_slider = new QSlider(this);
    m_slider->setOrientation(Qt::Horizontal);
    m_slider->setRange(0, 100);
    m_slider->setValue(0);
    m_slider->setGeometry(10, 30, 180, 30);

    m_progress_bar = new QProgressBar(this);
    m_progress_bar->setRange(0, 100);
    m_progress_bar->setValue(0);
    m_progress_bar->setGeometry(10, 50, 180, 30);

    connect(m_slider, &QSlider::valueChanged, m_progress_bar, &QProgressBar::setValue);
}

void Window::slotButtonClicked(bool checked)
{
    if (checked) {
        m_button->setText("Checked");
    } else {
        m_button->setText("Hello, World");
    }
}

window.h

#ifndef WINDOW_H
#define WINDOW_H

#include <QWidget>

class QPushButton;
class QProgressBar;
class QSlider;
class Window : public QWidget
{
    //Q_OBJECT
public:
    explicit Window(QWidget *parent = nullptr);

private slots:
    void slotButtonClicked(bool checked);

private:
    QPushButton *m_button;
    QProgressBar *m_progress_bar;
    QSlider *m_slider;
};

#endif // WINDOW_H