(C++) Exercise: Qt hide and show #1

 

Difficulty: 2/10

Date added: 14th of October 2012

 

In this exercise I will show an error I have wrestled with for some weeks, with the goal to prevent you having the same trouble. I will assume some basic Qt knowledge.

 

I desired to have the following program flow:

• On startup, the first dialog shows:
  
• On the first dialog, pressing the button with the text 'Show second', results in hiding the first and showing the second dialog:
  
• On the second dialog, pressing the button with the text 'Back to first', results in closing the second dialog and showing the first dialog

 

I want to hide the first dialog when showing the second dialog. This prevent the user from needlessly seeing the older dialog.

 

Also note that the program returns an exit code of zero, which denotes that the program ended without an error.

 

 

 

 

Question

 

The code shown below, however, ends the program when pressing 'Back to first'. Why?

 

The solution can really be found in the code shown.

 

Note that the -from my point of- unexpected behavior is actually the expected behavior, according to this bug report.

 

• Download the Qt Creator project 'CppExerciseQtHideAndShow1' (zip)
• View the answer

 

 

 

 

 

Code

 

Qt project file: CppExerciseQtHideAndShow1.pro

 

---

QT       += core gui TARGET = CppExercise: Qt hide and show #1 TEMPLATE = app SOURCES += \     main.cpp\     firstdialog.cpp \     seconddialog.cpp HEADERS  +=  \     firstdialog.h \     seconddialog.h FORMS    += \     firstdialog.ui \     seconddialog.ui

 

 

 

 

 

firstdialog.h

 

---

#ifndef FIRSTDIALOG_H #define FIRSTDIALOG_H #include <QDialog> namespace Ui { class FirstDialog; } class FirstDialog : public QDialog {     Q_OBJECT      public:     explicit FirstDialog(QWidget *parent = 0);     ~FirstDialog();      private slots:     void on_pushButton_clicked(); private:     Ui::FirstDialog *ui; }; #endif // FIRSTDIALOG_H

 

 

 

 

 

firstdialog.cpp

 

---

#include "seconddialog.h" #include "firstdialog.h" #include "ui_firstdialog.h" FirstDialog::FirstDialog(QWidget *parent) :     QDialog(parent),     ui(new Ui::FirstDialog) {   ui->setupUi(this); } FirstDialog::~FirstDialog() {     delete ui; } void FirstDialog::on_pushButton_clicked() {   SecondDialog d;   this->hide();   d.exec();   this->show(); }

 

 

 

 

 

main.cpp

 

---

#include <QtGui/QApplication> #include "firstdialog.h" int main(int argc, char *argv[]) {   QApplication a(argc, argv);   FirstDialog w;   w.exec(); }

 

 

 

 

 

seconddialog.h

 

---

#ifndef SECONDDIALOG_H #define SECONDDIALOG_H #include <QDialog> namespace Ui { class SecondDialog; } class SecondDialog : public QDialog {     Q_OBJECT      public:     explicit SecondDialog(QWidget *parent = 0);     ~SecondDialog();      private slots:     void on_pushButton_clicked(); private:     Ui::SecondDialog *ui; }; #endif // SECONDDIALOG_H

 

 

 

 

 

seconddialog.cpp

 

---

#include "seconddialog.h" #include "ui_seconddialog.h" SecondDialog::SecondDialog(QWidget *parent) :     QDialog(parent),     ui(new Ui::SecondDialog) {   ui->setupUi(this); } SecondDialog::~SecondDialog() {   delete ui; } void SecondDialog::on_pushButton_clicked() {   close(); }