Using "QWidget *QLayoutItem::widget()" throws a segment-fault (into qobjectdefs_impl.h) - qt

Question

I'm using QTabWidget(s) inside QTabWidget tabs and when I try to get a "child" QTabWidget which is stored in the layout of a tab of the "parent" QTabWidget, I have an segment-fault error.
PS : I'm using Qt Creator v4.8.1, Qtv5.6.3 and Windows 7 64-bits OS (VM under VBox v6.0.4)
Initially, the following line throws the error
dbTablesTabWidget = static_cast<QTabWidget*>(ui->tabWidget->currentWidget()->layout()->takeAt(i)->widget());
I have tried without the "static_cast" with the following line but the error is always here
QWidget *widget = ui->tabWidget->currentWidget()->layout()->takeAt(i)->widget();
My last try (without return the QWidget pointer) with always the same "segment-fault" error.
ui->tabWidget->currentWidget()->layout()->takeAt(i)->widget();
That I don't understand is that the following line (first line in the first for loop)
QString itemClass = ui->tabWidget->currentWidget()->layout()->takeAt(i)->widget()->metaObject()->className();
works fine althought it includes the same "base code".
The segment-fault error appears in the "qobjectdefs_impl.h" file :
1 MainWindow::addRow mainwindow.cpp 298 0x403de6
2 QtPrivate::FunctorCall<QtPrivate::IndexesList<>, QtPrivate::List<>, void, void (MainWindow:: *)()>::call(void (MainWindow:: *)(), MainWindow *, void * *) qobjectdefs_impl.h 501 0x410175
3 QtPrivate::FunctionPointer<void (MainWindow:: *)()>::call<QtPrivate::List<>, void>(void (MainWindow:: *)(), MainWindow *, void * *) qobjectdefs_impl.h 520 0x4103e2
4 QtPrivate::QSlotObject<void (MainWindow:: *)(), QtPrivate::List<>, void>::impl(int, QtPrivate::QSlotObjectBase *, QObject *, void * *, bool *) qobject_impl.h 143 0x410243
5 QtPrivate::QSlotObjectBase::call qobject_impl.h 124 0x6ba5d241
6 QMetaObject::activate qobject.cpp 3715 0x6b8fc213
7 QMetaObject::activate qobject.cpp 3595 0x6b8fbc3e
8 QAbstractButton::clicked moc_qabstractbutton.cpp 307 0x9c0685
9 QAbstractButtonPrivate::emitClicked qabstractbutton.cpp 404 0x9be459
10 QAbstractButtonPrivate::click qabstractbutton.cpp 397 0x9be3f4
11 QAbstractButton::mouseReleaseEvent qabstractbutton.cpp 1002 0x9bf6f7
12 QWidget::event qwidget.cpp 8757 0x8f6881
13 QAbstractButton::event qabstractbutton.cpp 959 0x9bf56c
14 QPushButton::event qpushbutton.cpp 673 0xa55168
15 QApplicationPrivate::notify_helper qapplication.cpp 3804 0x8bf880
16 QApplication::notify qapplication.cpp 3277 0x8bd5f1
17 QCoreApplication::notifyInternal2 qcoreapplication.cpp 1015 0x6b8d37dd
18 QCoreApplication::sendSpontaneousEvent qcoreapplication.h 228 0xc2fb5d
19 QApplicationPrivate::sendMouseEvent qapplication.cpp 2773 0x8bc2cb
20 QWidgetWindow::handleMouseEvent qwidgetwindow.cpp 607 0x90f314
... <plus>
Below, an extract of the addRow Method ("custom" private slot called after the click on a QPushButton)
// var
int dbIndex = ui->tabWidget->currentIndex();
int tableIndex = 0;
QTabWidget *dbTablesTabWidget;
// Détection de l'index de l'item du layout de l'onglet de la BDD actuellement sélectionné
// -> Détection de la position du widget (jeu d'onglets des tables) dans le layout pour récupérer l'index de la table actuelle.
for (int i=0; i<ui->tabWidget->currentWidget()->layout()->count(); i++)
{
QString itemClass = ui->tabWidget->currentWidget()->layout()->takeAt(i)->widget()->metaObject()->className();
qDebug(itemClass.toLatin1());
if (itemClass == "QTabWidget")
{
qDebug("Cet élément est un QTabWidget !!!");
//*** The following line throws a segment fault ***
//dbTablesTabWidget = static_cast<QTabWidget*>(ui->tabWidget->currentWidget()->layout()->takeAt(i)->widget());
//QWidget *widget = ui->tabWidget->currentWidget()->layout()->takeAt(i)->widget();
ui->tabWidget->currentWidget()->layout()->takeAt(i)->widget();
tableIndex = dbTablesTabWidget->currentIndex();
}
}
I'm a newbie with Qt and I don't see where the segment-fault error could come from.
The compilation process is OK, please see below (french language version)
15:53:12: Configuration inchangée, étape qmake sautée.
15:53:12: Débute : "C:\Qt\Tools\mingw492_32\bin\mingw32-make.exe" -j1
C:/Qt/Tools/mingw492_32/bin/mingw32-make -f Makefile.Debug
mingw32-make[1]: Entering directory 'E:/Projects_Info_Slink/_Training/C++/QTCreatorProject/DataBaseViewerGUI/build-DataBaseViewerGUI-Desktop_Qt_5_6_3_MinGW_32bit-Debug'
mingw32-make[1]: Nothing to be done for 'first'.
mingw32-make[1]: Leaving directory 'E:/Projects_Info_Slink/_Training/C++/QTCreatorProject/DataBaseViewerGUI/build-DataBaseViewerGUI-Desktop_Qt_5_6_3_MinGW_32bit-Debug'
15:53:13: Le processus "C:\Qt\Tools\mingw492_32\bin\mingw32-make.exe" s'est terminé normalement.
15:53:13: Temps écoulé : 00:01.

Answer 1

Your page in tabwidget can have any widget along with a child tab widget.
To keep it simple, probably you can try the below way:
//GET YOUR CURRENT PAGE IN FIRST TAB WIDGET
QWidget *widget = ui->tabWidget->currentWidget();
//FROM THE CURRENT PAGE FIND THE CHILD TAB WIDGETS
QList<QTabWidget*> tabWidgets = widget->findChildren<QTabWidget*>();
//USE THE FOUND WIDGET FOR YOUR PURPOSE. DO SOME SAFETY CHECKS FOR LIST.
QTabWidget* requiredTabWidget = tabWidgets[0];

Answer 2

I have finally found the error :
I should use layout()->itemAt(i) instead layout()->takeAt(i).
It explains why the error occurs at the second time.
takeAt remove the item from the list, so during the next call, the item doesn't exist in the list.
PS : for QList object, this is at instead itemAt to get the element from the list of element.