Question

I have created a very simple GUI project in Qt as follows:

main:

#include <QApplication>
#include "dialog.h"
int main(int c, char* v[])
{
    QApplication app(c,v);
    Dialog* d = new Dialog;
    d->show();
    app.exec();
}

dialog.h:

#ifndef DIALOG_H
#define DIALOG_H

#include <QDialog>
#include "ui_dialog.h"
#include "progress_dialog.h"

class Dialog : public QDialog, private Ui::Dialog
{
    Q_OBJECT

public:
    explicit Dialog(QWidget *parent = 0);
    ~Dialog();

private:
    progress_dialog* progress_dialog_;
public slots:
    void create_and_show_progress_dialog();
    void delete_progress_dialog();
};

#endif // DIALOG_H

dialog.cpp:

#include "dialog.h"
#include "ui_dialog.h"
#include <QMessageBox>
Dialog::Dialog(QWidget *parent) :
    QDialog(parent),progress_dialog_(new progress_dialog)
{
    setupUi(this);
    connect(pushButton,SIGNAL(clicked()),this,SLOT(create_and_show_progress_dialog()));
    connect(progress_dialog_,SIGNAL(rejected()),this,SLOT(delete_progress_dialog()));
}

void Dialog::create_and_show_progress_dialog()
{
    if (!progress_dialog_)
        progress_dialog_ = new progress_dialog;
    progress_dialog_->exec();
}

void Dialog::delete_progress_dialog()
{
    delete progress_dialog_;
    progress_dialog_ = nullptr;
    QMessageBox::information(this,"Progress destroyed","I've just destroyed progress");
}

Dialog::~Dialog()
{
}

progress_dialog:

#ifndef PROGRESS_DIALOG_H
#define PROGRESS_DIALOG_H

#include "ui_progress_dialog.h"

class progress_dialog : public QDialog, private Ui::progress_dialog
{
    Q_OBJECT

public:
    explicit progress_dialog(QWidget *parent = 0);
public slots:
    void exec_me();
};

#endif // PROGRESS_DIALOG_H

progress_dialog.cpp:

#include "progress_dialog.h"

progress_dialog::progress_dialog(QWidget *parent) :
    QDialog(parent)
{
    setupUi(this);
}

So the problem is that after running this app and pressing push_button on main dialog, progress_dialog is being displayed. After clicking on push_buttong on progress_dialog which is connected to a reject slot this dialog is closing and message is being displayed: QMessageBox::information(this,”Progress destroyed”,”I’ve just destroyed progress”);

But when I do this second time (press button on main dialog and then close the progress_dialog) no message is being displayed. Tried to debug this and set breakpoint on:

void Dialog::delete_progress_dialog()
{
    delete progress_dialog_;//HERE BREAKPOINT WAS PLACED
    progress_dialog_ = nullptr;
    QMessageBox::information(this,"Progress destroyed","I've just destroyed progress");
}

but this breakpoint is being hit just first time, and after that no hits on this breakpoint are performed.
What’s going on?

Answer 1

You need to reconnect the signal each time you create a new progress_dialog – when you destroy the old one, the connection is lost (how could that be otherwise, you’ve just zapped the source of the signal).

So do the connection in create_and_show_progress_dialog when you create a new object.