Qt segfault trying to show 2 dialogs in test application - qt

Question

I'm just learning Qt. I reproduced my problem in a simple test application. I created a new project. Added 2 dialogs Dialog1 and Dialog2. I added 2 menu entries to call dialog1 and dialog2.
The main window's header file looks like this:
#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <QMainWindow>
#include "dialog1.h"
#include "dialog2.h"
QT_BEGIN_NAMESPACE
namespace Ui { class MainWindow; }
QT_END_NAMESPACE
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
MainWindow(QWidget *parent = nullptr);
~MainWindow();
private slots:
void on_actiondialog1_triggered();
void on_actiondialog2_triggered();
private:
Ui::MainWindow *ui;
Dialog1 *mDialog1;
Dialog2 *mDialog2;
};
#endif // MAINWINDOW_H
The main window's cpp looks like this:
#include "mainwindow.h"
#include "ui_mainwindow.h"
MainWindow::MainWindow(QWidget *parent)
: QMainWindow(parent)
, ui(new Ui::MainWindow)
{
ui->setupUi(this);
}
MainWindow::~MainWindow()
{
delete ui;
}
void MainWindow::on_actiondialog1_triggered()
{
if (!mDialog1) {
mDialog1 = new Dialog1(this);
mDialog1->setModal(false);
}
mDialog1->show();
mDialog1->activateWindow();
}
void MainWindow::on_actiondialog2_triggered()
{
if (!mDialog2) {
mDialog2 = new Dialog2(this);
mDialog2->setModal(false);
}
mDialog2->show();
mDialog2->activateWindow();
}
So it is basically as empty as it gets... If I only show one dialog by commenting out the relevant sections it works but as soon as I want to use 2 or more is segfaults. The given dialogs constructor isn't even being called anymore.
I am using:
Qt Creator 4.12.3
Based on Qt 5.14.2 (GCC 5.3.1 20160406 (Red Hat 5.3.1-6), 64 bit)
Built on Jun 16 2020 04:15:35
I don't understand what I'm doing wrong. Can someone please clarify?

Answer 1

First, it is not about Qt. It is about your implementation:In your constructor, I don't see code, which initializes mDialog1 and mDialog2.
So, when go to the slot what is the value of mDialog1 ? It is undefined, especially in Release build.
Here is one of the approaches:
In your constructor, just create the objects:
MainWindow::MainWindow(QWidget *parent)
: QMainWindow(parent)
, ui(new Ui::MainWindow)
{
ui->setupUi(this);
mDialog1 = new Dialog1(this);
mDialog2 = new Dialog2(this);
}
Then in slot (you don't need to check the pointer):
void MainWindow::on_actiondialog1_triggered()
{
mDialog1->setModal(false); //Not required, because of show()
mDialog1->show();
mDialog1->activateWindow();
}
Another approach, which creates objects on demand:
MainWindow::MainWindow(QWidget *parent)
: QMainWindow(parent)
, ui(new Ui::MainWindow)
{
ui->setupUi(this);
//Initalize these members
m_dialog1 = nullptr;
m_dialog2 = nullptr;
}
And then:
void MainWindow::on_actiondialog2_triggered()
{
if (!mDialog2)
{
mDialog2 = new Dialog2(this);
mDialog2->setModal(false);
}
mDialog2->show();
mDialog2->activateWindow();
}