Implement function to QT Button - c++

Question

I cant find any solution for the problem. I created an ui with qt and wanted to implement a function for the button. It works if i delete the buttons in the header file.
Error
/usr/bin/ld: CMakeFiles/QT3.dir/QT3_autogen/mocs_compilation.cpp.o: in function MainWindow::qt_metacall(QMetaObject::Call, int, void**)': mocs_compilation.cpp:(.text+0x10a): undefined reference to MainWindow::on_nextStep_clicked()'
/usr/bin/ld: CMakeFiles/QT3.dir/QT3_autogen/mocs_compilation.cpp.o: in function MainWindow::qt_static_metacall(QObject*, QMetaObject::Call, int, void**)': mocs_compilation.cpp:(.text+0x41): undefined reference to MainWindow::on_nextStep_clicked()'
Header File
#ifndef MAINWINDOW3_H_
#define MAINWINDOW3_H_
#include <QMainWindow>
#include <opencv2/highgui/highgui.hpp>
#include <QGraphicsScene>
//QT_BEGIN_NAMESPACE
//namespace Ui { class MainWindow; }
//QT_END_NAMESPACE
namespace Ui {
class MainWindow;
}
class MainWindow: public QMainWindow {
Q_OBJECT
public:
explicit MainWindow(QWidget *parent = nullptr);
~MainWindow();
signals:
private slots:
// void on_pushButton_5_clicked(); // add Wall
// void on_pushButton_clicked(); // oben
// void on_pushButton_2_clicked(); // links
// void on_pushButton_3_clicked(); // rechts
// void on_pushButton_4_clicked(); // unten
void on_pushButton_clicked();
void on_nextStep_clicked();
private:
Ui::MainWindow *ui;
QGraphicsScene *scene;
protected:
// bool eventFilter(QObject *, QEvent *);
};
#endif /* MAINWINDOW3_H_ */
Cpp File
#include "mainwindow3.h"
#include "ui_mainwindow.h"
#include <QGraphicsScene>
#include <QGraphicsView>
#include <QGraphicsItem>
#include <QDebug>
#include <QMouseEvent>
#include <QObject>
#include <iostream>
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent), ui(new Ui::MainWindow) {
ui->setupUi(this);
scene = new QGraphicsScene(this);
ui->graphicsView->setScene(scene);
// ui->graphicsView->installEventFilter(this);
}
MainWindow::~MainWindow() {
delete ui;
}
void MainWindow::on_pushButton_clicked() {
std::cout << "add btn clicked()" << std::endl;
}

Answer 1

a slot is just another method in Qt for asyn events
if you define in the header:
void on_nextStep_clicked();
then you need a implementation in the cpp:
void MainWindow::on_nextStep_clicked()
{
qDebug() << "Hallo Welt!";
qDebug() << "on_nextStep_clicked()";
}