(C++) Exercise: Qt hide and show #2

 

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 going back to the first dialog\
• On the second dialog, pressing the button with the text 'Go to third', results in hiding the second and showing the third dialog:
  
• On the third dialog, pressing the button with the text 'Back to first', results in going back to the first dialog.
• On the third dialog, pressing the button with the text 'Back to second', results in going back to the second dialog.

 

I want to hide a dialog when showing another. 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, has an incorrect flow: from the third dialog, the user is always taken back to the first dialog. Why?

 

It is checked that the if statement in SecondDialog really takes the correct branch.

If the if-statement in SecondDialog is replaced by 'this->show()', the incorrect behavior persists.

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 'CppExerciseQtHideAndShow2' (zip)
• View the answer
• The forum this question is posted on

 

 

 

 

 

Code

 

Qt project file: CppExerciseQtHideAndShow2.pro

 

---

QT       += core gui TARGET = CppExerciseQtHideAndShow2 TEMPLATE = app SOURCES += \     main.cpp\     firstdialog.cpp \     seconddialog.cpp \     thirddialog.cpp HEADERS  +=  \     firstdialog.h \     seconddialog.h \     thirddialog.h FORMS    += \     firstdialog.ui \     seconddialog.ui \     thirddialog.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.show();   a.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_button_goto_third_clicked();     void on_button_back_to_first_clicked(); private:     Ui::SecondDialog *ui; }; #endif // SECONDDIALOG_H

 

 

 

 

 

seconddialog.cpp

 

---

#include <cassert> #include "seconddialog.h" #include "ui_seconddialog.h" #include "thirddialog.h" SecondDialog::SecondDialog(QWidget *parent) :     QDialog(parent),     ui(new Ui::SecondDialog) {   ui->setupUi(this); } SecondDialog::~SecondDialog() {   delete ui; } void SecondDialog::on_button_back_to_first_clicked() {   close(); } void SecondDialog::on_button_goto_third_clicked() {   ThirdDialog d;   this->hide();   d.exec();   if (d.m_back_to_which_dialog == 1)   {     this->close();   }   else   {     this->show();   } }

 

 

 

 

 

thirddialog.h

 

---

#ifndef THIRDDIALOG_H #define THIRDDIALOG_H #include <QDialog> namespace Ui { class ThirdDialog; } class ThirdDialog : public QDialog {     Q_OBJECT      public:     explicit ThirdDialog(QWidget *parent = 0);     ~ThirdDialog();     int m_back_to_which_dialog; private slots:     void on_button_back_to_first_clicked();     void on_button_back_to_second_clicked(); private:     Ui::ThirdDialog *ui; }; #endif // THIRDDIALOG_H

 

 

 

 

 

thirddialog.cpp

 

---

#include "thirddialog.h" #include "ui_thirddialog.h" ThirdDialog::ThirdDialog(QWidget *parent) :     QDialog(parent),     m_back_to_which_dialog(2), //When user closes the dialog, go back to the previous/second dialog     ui(new Ui::ThirdDialog) {   ui->setupUi(this); } ThirdDialog::~ThirdDialog() {   delete ui; } void ThirdDialog::on_button_back_to_first_clicked() {   m_back_to_which_dialog = 1;   close(); } void ThirdDialog::on_button_back_to_second_clicked() {   m_back_to_which_dialog = 2;   close(); }