Question

The title more or less says it all. I create a widget, add it to QTabWidget and when I switch to the new tab the app crashes.

preferencestab.h (The widget I want to add)

#ifndef PREFERENCESTAB_H
#define PREFERENCESTAB_H

#include <QWidget>

#include "tab.h"

class PreferencesTab : public QWidget
{
    Q_OBJECT
public:
    explicit PreferencesTab(QWidget *parent = 0);
    ~PreferencesTab();
    int num;

private:


private slots:
};

#endif // PREFERENCESTAB_H

preferencestab.cpp

#include "preferencestab.h"

#include <QDebug>

PreferencesTab::PreferencesTab(QWidget *parent) : QWidget(parent)
{
}

PreferencesTab::~PreferencesTab()
{

}

tabmanager.cpp (subclass of QTabWidget and where I add the new tab)

...
void TabManager::openPreferences()
{
    PreferencesTab *pref = new PreferencesTab();
    int index = this->addTab(pref, "Edit");

    this->setCurrentIndex(index); // It crashes on this line
}

If I remove the line where it crashed it succeeds at creating the new tab but crashes when I manually switch to it.

It must be something stupid but I just can’t find the error. Help Please

Answer 1

Try to debug your application.

In QtCreator, you start the debug mode by pressing F5 (⌘-Y on macs) instead of Ctrl-R. This will launch a debugger (e.g. GDB) which tells QtCreator (which then tells you) what kind of error occurred. You also can set breakpoints, step through your code and inspect the variable values (before the error occures of course).

Another option is to print out the variables using qDebug. Candidates are the this pointer, pref and the index variable; check their values.

That said, the problem which causes the crash is most likely outside the PreferencesTab class, maybe even outside the TabManager class. That the code within a method of a class is executed doesn’t mean that is was a “valid” call (= the this pointer was valid). So checking the this pointer is always a good idea to track calls to null or invalid pointers.

If TabManager is a QObject, even debugging the this pointer will fail (if it is invalid), because qDebug() << this will ask for the metaObject(), which will be (most likely) an invalid read if the this pointer is invalid.