How to finish Qt programm from any place?

Question

My example:

main.cpp:
    QApplication a(argc, argv);
    MainWindow w;
    w.start();
    return a.exec();

w.start():
    if (cf.exec()){
        this->show();
    } else {
        qDebug()<<"Need exit";
        //here should be exit
    }

At comment place, I tried to do: qApp->exit() and qApp->quit() and this->close() (but 'this' not shown and of cource close() is not working). How can I finish app from any place of code?

Whole code:
main.cpp

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

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.start();

    return a.exec();
}

mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include "CadrForm.h"
#include "TeacherForm.h"
#include "DepartmentForm.h"
#include "CategoriesForm.h"
#include "PostForm.h"
#include "RanksAndDegreesForm.h"
#include "TeachersRankAndDegreeForm.h"
#include "ConnectionForm.h"

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    void start();
    ~MainWindow();
signals:
    void widgetChanged(QWidget*);
private slots:
    void on_actionCadr_triggered();
    void resetMainLayout(QWidget* w);
    void on_actionTeacher_triggered();

    void on_actionDepartment_triggered();

    void on_actionPost_triggered();

    void on_actionCategories_triggered();

    void on_actionRanksAndDegrees_triggered();

    void on_actionTeachersRD_triggered();

private:
    ConnectionForm cf;
    CadrForm *cadrForm;
    TeacherForm *teacherForm;
    DepartmentForm *departmentForm;
    CategoriesForm *categoriesForm;
    PostForm *postForm;
    RanksAndDegreesForm *ranksAndDegreesForm;
    TeachersRankAndDegreeForm *teachersRankAndDegreeForm;
    QWidget *current;
    Ui::MainWindow *ui;

    void addWidgetToMainLayout(QWidget *w);
};

#endif // MAINWINDOW_H

mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QDebug>

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    connect(this, SIGNAL(widgetChanged(QWidget*)), this, SLOT(resetMainLayout(QWidget*)));

    ui->setupUi(this);
    cadrForm = new CadrForm(this);
    teacherForm = new TeacherForm(this);
    departmentForm = new DepartmentForm(this);
    categoriesForm = new CategoriesForm(this);
    postForm = new PostForm(this);
    ranksAndDegreesForm = new RanksAndDegreesForm(this);
    teachersRankAndDegreeForm = new TeachersRankAndDegreeForm(this);

    addWidgetToMainLayout(cadrForm);
    addWidgetToMainLayout(teacherForm);
    addWidgetToMainLayout(departmentForm);
    addWidgetToMainLayout(categoriesForm);
    addWidgetToMainLayout(postForm);
    addWidgetToMainLayout(ranksAndDegreesForm);
    addWidgetToMainLayout(teachersRankAndDegreeForm);
}

void MainWindow::start()
{
    if (cf.exec()){
        this->show();
    } else {
        qDebug()<<"Need exit";
        qApp->quit();
        qDebug()<<"still working";
    }
}

void MainWindow::addWidgetToMainLayout(QWidget *w)
{
    ui->mainLayout->insertWidget(0, w);
    ui->mainLayout->itemAt(0)->widget()->hide();
}

MainWindow::~MainWindow()
{
    delete ui;
}

void MainWindow::resetMainLayout(QWidget *w)
{
    int index;
    if (current)
    {
        index = ui->mainLayout->indexOf(current);
        ui->mainLayout->itemAt(index)->widget()->hide();
    }
    index = ui->mainLayout->indexOf(w);
    if (index != -1) ui->mainLayout->itemAt(index)->widget()->show();
    else {
        addWidgetToMainLayout(w);
        resetMainLayout(w);
    }
    current = w;
    setWindowTitle(current->windowTitle());
}

void MainWindow::on_actionCadr_triggered()
{
    emit widgetChanged(cadrForm);
}

void MainWindow::on_actionTeacher_triggered()
{
    emit widgetChanged(teacherForm);
}

void MainWindow::on_actionDepartment_triggered()
{
    emit widgetChanged(departmentForm);
}

void MainWindow::on_actionPost_triggered()
{
    emit widgetChanged(postForm);
}

void MainWindow::on_actionCategories_triggered()
{
    emit widgetChanged(categoriesForm);
}

void MainWindow::on_actionRanksAndDegrees_triggered()
{
    emit widgetChanged(ranksAndDegreesForm);
}

void MainWindow::on_actionTeachersRD_triggered()
{
    emit widgetChanged(teachersRankAndDegreeForm);
}

And ConnectionForm - it's just a QDialog with some GUI and without any additional code.

Answer 1

It looks like the problem is you're not allowed to call QApplication::quit() or QApplication::exit() until the QApplication event loop has actually started. The event loop gets started by QApplication::exec(), so you're calling quit()/exit() too soon for it to have an effect.

You can fix this either by moving the quit()/exit() call so that it's in the event loop (e.g. by moving your MainWindow::start() code to the QMainWindow::showEvent() slot), or by changing your MainWindow::start() to return a value, inspect that value in main, and exit (without calling QApplication::exec()`) if it's a value that indicates your process should exit early.