Simulating an editable QLabel: cannot clean-up redundant widget.

BwvB

In the application I'm developing it is sometimes desired that the user can modify the text in a QLabel.
The problem of how to do this has been posed several times, but I have seen only suggestions to an answer, not working code. The following approach has been suggested:

create a QLineEdit widget and display it instead of the QLabel..
The user enters a new text in the QLineEdit widget.
when done (on Return), the new text is copied from the QLineEdit to the QLabel widget.
The QLineEdit widget can be eliminated, the QLabel is made visible again.

I have implemented the scheme above, and it works, but the implementation is not 'clean' in that I cannot delete the auxiliary QLineWidget after it has been used. When I delete lineEdit, memory is corrupted. How should I do this instead?

I have trimmed my code to get a minimal example showing the issue. All, I think, is contained in the slot function MyWidget::mousePressEvent(), given below. MyWidget is a Q_OBJECT demo-class, directly derived from QWidget, with only one data member: QLabel* _qlabel;

On a mouse-click, a new QLineEdit instance lineEdit is allocated and initialised with the text of the QLabel. In the layout, the new QLineEdit is swapped with the QLabel. Next, a lambda-expression is defined that is used as a slot when the user presses Return.
The lambda expression copies the text back from QLineEdit to the QLabel, and it swaps the widgets again.

Her is my problem: if I just delete the auxiliary QLineEdit widget, as it is no longer needed, the memory is corrupted (as valgrind clearly shows). If I only hide the lineEdit, memory stays good, everything looks ok, but the memory allocated to lineEdit will only be cleared when the whole widget goes (as QLabel* _qlabel is the parent). Even worse, when I don't set qlable as the parent (i.e. no parent), I still cannot delete lineEdit. This would leave me with a memory leak.
How should I handle such a case? I would like to delete the intermediary lineEdit entirely, leaving no traces that the qlabel text has been edited.

void MyWidget::mousePressEvent( QMouseEvent* )
{
   // Create a lineEdit, initialize with qlabel's text
   auto lineEdit = new QLineEdit( _qlabel );
   lineEdit->setText( _qlabel->text() );
   lineEdit->setFocus();

   // exchange widgets in layout; delete old layout item.
   delete layout()->replaceWidget( _qlabel, lineEdit );

   // lambda for refreshing the label.
   auto doUpdate =
      [this,lineEdit]()
      {
         delete layout()->replaceWidget( lineEdit, _qlabel );

         _qlabel->setText( lineEdit->text() );
#if 0
         lineEdit->hide();        // works, but memory not timely cleaned.
#else
         disconnect( lineEdit );
         lineEdit->setParent(nullptr);
         delete lineEdit;         // crash, or at least memory corruption!
#endif
      };
   connect( lineEdit, &QLineEdit::returnPressed, doUpdate );
}

SGaist

Hi,

You are over-engineering it. Create the QLineEdit once and just show/hide it as required. There's no reason to delete and re-create it all the time.

BwvB

Thanks for responding. But this was precisely my question! Throwing away an object that is no longer needed is not bad engineering, I would say. But throwing it away somehow screws-up the qt-internals,
even when I explicitly disconnect and set the parent to nil. I don't understand. I fear the connect(lineEdit,....) statement somehow puts lineEdit in Qt-s belly where it cannot be undone easily.

Furthermore, I made the QLabel instance the parent of the QLineEdit so that Qt take care of deallocation. Instead, I could also create the QLineEdit without setting its parent. In that case I would have a memory leak if I don't delete the object myself. But the memory corruption is still present.

Below I post a typical output of valgrind after changing the text and pressing the return key,

==9178== Memcheck, a memory error detector
==9178== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==9178== Using Valgrind-3.15.0 and LibVEX; rerun with -h for copyright info
==9178== Command: ./Linux-5.3/bin/editablelabel
==9178== 
==9178== Invalid read of size 8
==9178==    at 0x7E60404: ??? (in /usr/local/qt-5.15.2/lib/libQt5Core.so.5.15.2)
==9178==    by 0x6D05834: QWidgetLineControl::processKeyEvent(QKeyEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x6CFA9DA: QLineEdit::keyPressEvent(QKeyEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x6C07C86: QWidget::event(QEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x6BCA5BE: QApplicationPrivate::notify_helper(QObject*, QEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x6BD1126: QApplication::notify(QObject*, QEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x7E2C3E9: QCoreApplication::notifyInternal2(QObject*, QEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Core.so.5.15.2)
==9178==    by 0x6C213CC: ??? (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x6BCA5BE: QApplicationPrivate::notify_helper(QObject*, QEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x7E2C3E9: QCoreApplication::notifyInternal2(QObject*, QEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Core.so.5.15.2)
==9178==    by 0x7402D31: QGuiApplicationPrivate::processKeyEvent(QWindowSystemInterfacePrivate::KeyEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Gui.so.5.15.2)
==9178==    by 0x73DE33B: QWindowSystemInterface::sendWindowSystemEvents(QFlags<QEventLoop::ProcessEventsFlag>) (in /usr/local/qt-5.15.2/lib/libQt5Gui.so.5.15.2)
==9178==  Address 0x18a11418 is 8 bytes inside a block of size 248 free'd
==9178==    at 0x4C2FBEB: operator delete(void*, unsigned long) (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==9178==    by 0x7E5470D: QObjectPrivate::deleteChildren() (in /usr/local/qt-5.15.2/lib/libQt5Core.so.5.15.2)
==9178==    by 0x6C02FF5: QWidget::~QWidget() (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x6CFA7E8: QLineEdit::~QLineEdit() (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x4037D8: MyWidget::mousePressEvent(QMouseEvent*)::{lambda()#1}::operator()() const (MyWidget.cpp:38)
==9178==    by 0x403BB3: QtPrivate::FunctorCall<QtPrivate::IndexesList<>, QtPrivate::List<>, void, MyWidget::mousePressEvent(QMouseEvent*)::{lambda()#1}>::call({lambda()#1}&, void**) (qobjectdefs_impl.h:146)
==9178==    by 0x403B94: void QtPrivate::Functor<MyWidget::mousePressEvent(QMouseEvent*)::{lambda()#1}, 0>::call<QtPrivate::List<>, void>({lambda()#1}&, void*, {lambda()#1}&*) (qobjectdefs_impl.h:256)
==9178==    by 0x403B63: QtPrivate::QFunctorSlotObject<MyWidget::mousePressEvent(QMouseEvent*)::{lambda()#1}, 0, QtPrivate::List<>, void>::impl(int, QtPrivate::QSlotObjectBase*, QObject*, void**, bool*) (qobjectdefs_impl.h:443)
==9178==    by 0x7E608B2: ??? (in /usr/local/qt-5.15.2/lib/libQt5Core.so.5.15.2)
==9178==    by 0x7E608E4: ??? (in /usr/local/qt-5.15.2/lib/libQt5Core.so.5.15.2)
==9178==    by 0x6D0582C: QWidgetLineControl::processKeyEvent(QKeyEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x6CFA9DA: QLineEdit::keyPressEvent(QKeyEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==  Block was alloc'd at
==9178==    at 0x4C2E94F: operator new(unsigned long) (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==9178==    by 0x6CF840F: ??? (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x6CFC130: QLineEdit::QLineEdit(QWidget*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x403830: MyWidget::mousePressEvent(QMouseEvent*) (MyWidget.cpp:19)
==9178==    by 0x6C06E1D: QWidget::event(QEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x6BCA5BE: QApplicationPrivate::notify_helper(QObject*, QEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x6BD0D75: QApplication::notify(QObject*, QEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x7E2C3E9: QCoreApplication::notifyInternal2(QObject*, QEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Core.so.5.15.2)
==9178==    by 0x6BCFE42: QApplicationPrivate::sendMouseEvent(QWidget*, QMouseEvent*, QWidget*, QWidget*, QWidget**, QPointer<QWidget>&, bool, bool) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x6C1E286: ??? (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x6C2135D: ??? (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x6BCA5BE: QApplicationPrivate::notify_helper(QObject*, QEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178== 
==9178== Invalid read of size 1
==9178==    at 0x7E6040C: ??? (in /usr/local/qt-5.15.2/lib/libQt5Core.so.5.15.2)
==9178==    by 0x6D05834: QWidgetLineControl::processKeyEvent(QKeyEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x6CFA9DA: QLineEdit::keyPressEvent(QKeyEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x6C07C86: QWidget::event(QEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x6BCA5BE: QApplicationPrivate::notify_helper(QObject*, QEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x6BD1126: QApplication::notify(QObject*, QEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x7E2C3E9: QCoreApplication::notifyInternal2(QObject*, QEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Core.so.5.15.2)
==9178==    by 0x6C213CC: ??? (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x6BCA5BE: QApplicationPrivate::notify_helper(QObject*, QEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x7E2C3E9: QCoreApplication::notifyInternal2(QObject*, QEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Core.so.5.15.2)
==9178==    by 0x7402D31: QGuiApplicationPrivate::processKeyEvent(QWindowSystemInterfacePrivate::KeyEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Gui.so.5.15.2)
==9178==    by 0x73DE33B: QWindowSystemInterface::sendWindowSystemEvents(QFlags<QEventLoop::ProcessEventsFlag>) (in /usr/local/qt-5.15.2/lib/libQt5Gui.so.5.15.2)
==9178==  Address 0x147cb380 is 32 bytes inside a block of size 88 free'd
==9178==    at 0x4C2FBEB: operator delete(void*, unsigned long) (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==9178==    by 0x6D071D4: ??? (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x7E5470D: QObjectPrivate::deleteChildren() (in /usr/local/qt-5.15.2/lib/libQt5Core.so.5.15.2)
==9178==    by 0x6C02FF5: QWidget::~QWidget() (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x6CFA7E8: QLineEdit::~QLineEdit() (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x4037D8: MyWidget::mousePressEvent(QMouseEvent*)::{lambda()#1}::operator()() const (MyWidget.cpp:38)
==9178==    by 0x403BB3: QtPrivate::FunctorCall<QtPrivate::IndexesList<>, QtPrivate::List<>, void, MyWidget::mousePressEvent(QMouseEvent*)::{lambda()#1}>::call({lambda()#1}&, void**) (qobjectdefs_impl.h:146)
==9178==    by 0x403B94: void QtPrivate::Functor<MyWidget::mousePressEvent(QMouseEvent*)::{lambda()#1}, 0>::call<QtPrivate::List<>, void>({lambda()#1}&, void*, {lambda()#1}&*) (qobjectdefs_impl.h:256)
==9178==    by 0x403B63: QtPrivate::QFunctorSlotObject<MyWidget::mousePressEvent(QMouseEvent*)::{lambda()#1}, 0, QtPrivate::List<>, void>::impl(int, QtPrivate::QSlotObjectBase*, QObject*, void**, bool*) (qobjectdefs_impl.h:443)
==9178==    by 0x7E608B2: ??? (in /usr/local/qt-5.15.2/lib/libQt5Core.so.5.15.2)
==9178==    by 0x7E608E4: ??? (in /usr/local/qt-5.15.2/lib/libQt5Core.so.5.15.2)
==9178==    by 0x6D0582C: QWidgetLineControl::processKeyEvent(QKeyEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==  Block was alloc'd at
==9178==    at 0x4C2E94F: operator new(unsigned long) (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==9178==    by 0x7E5F174: QObject::QObject(QObject*) (in /usr/local/qt-5.15.2/lib/libQt5Core.so.5.15.2)
==9178==    by 0x7583182: QInputControl::QInputControl(QInputControl::Type, QObject*) (in /usr/local/qt-5.15.2/lib/libQt5Gui.so.5.15.2)
==9178==    by 0x6CF841E: ??? (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x6CFC130: QLineEdit::QLineEdit(QWidget*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x403830: MyWidget::mousePressEvent(QMouseEvent*) (MyWidget.cpp:19)
==9178==    by 0x6C06E1D: QWidget::event(QEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x6BCA5BE: QApplicationPrivate::notify_helper(QObject*, QEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x6BD0D75: QApplication::notify(QObject*, QEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x7E2C3E9: QCoreApplication::notifyInternal2(QObject*, QEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Core.so.5.15.2)
==9178==    by 0x6BCFE42: QApplicationPrivate::sendMouseEvent(QWidget*, QMouseEvent*, QWidget*, QWidget*, QWidget**, QPointer<QWidget>&, bool, bool) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x6C1E286: ??? (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178== 
==9178== Invalid read of size 8
==9178==    at 0x7E60416: ??? (in /usr/local/qt-5.15.2/lib/libQt5Core.so.5.15.2)
==9178==    by 0x6D05834: QWidgetLineControl::processKeyEvent(QKeyEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x6CFA9DA: QLineEdit::keyPressEvent(QKeyEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x6C07C86: QWidget::event(QEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x6BCA5BE: QApplicationPrivate::notify_helper(QObject*, QEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x6BD1126: QApplication::notify(QObject*, QEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x7E2C3E9: QCoreApplication::notifyInternal2(QObject*, QEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Core.so.5.15.2)
==9178==    by 0x6C213CC: ??? (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x6BCA5BE: QApplicationPrivate::notify_helper(QObject*, QEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x7E2C3E9: QCoreApplication::notifyInternal2(QObject*, QEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Core.so.5.15.2)
==9178==    by 0x7402D31: QGuiApplicationPrivate::processKeyEvent(QWindowSystemInterfacePrivate::KeyEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Gui.so.5.15.2)
==9178==    by 0x73DE33B: QWindowSystemInterface::sendWindowSystemEvents(QFlags<QEventLoop::ProcessEventsFlag>) (in /usr/local/qt-5.15.2/lib/libQt5Gui.so.5.15.2)
==9178==  Address 0x147cb3a8 is 72 bytes inside a block of size 88 free'd
==9178==    at 0x4C2FBEB: operator delete(void*, unsigned long) (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==9178==    by 0x6D071D4: ??? (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x7E5470D: QObjectPrivate::deleteChildren() (in /usr/local/qt-5.15.2/lib/libQt5Core.so.5.15.2)
==9178==    by 0x6C02FF5: QWidget::~QWidget() (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x6CFA7E8: QLineEdit::~QLineEdit() (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x4037D8: MyWidget::mousePressEvent(QMouseEvent*)::{lambda()#1}::operator()() const (MyWidget.cpp:38)
==9178==    by 0x403BB3: QtPrivate::FunctorCall<QtPrivate::IndexesList<>, QtPrivate::List<>, void, MyWidget::mousePressEvent(QMouseEvent*)::{lambda()#1}>::call({lambda()#1}&, void**) (qobjectdefs_impl.h:146)
==9178==    by 0x403B94: void QtPrivate::Functor<MyWidget::mousePressEvent(QMouseEvent*)::{lambda()#1}, 0>::call<QtPrivate::List<>, void>({lambda()#1}&, void*, {lambda()#1}&*) (qobjectdefs_impl.h:256)
==9178==    by 0x403B63: QtPrivate::QFunctorSlotObject<MyWidget::mousePressEvent(QMouseEvent*)::{lambda()#1}, 0, QtPrivate::List<>, void>::impl(int, QtPrivate::QSlotObjectBase*, QObject*, void**, bool*) (qobjectdefs_impl.h:443)
==9178==    by 0x7E608B2: ??? (in /usr/local/qt-5.15.2/lib/libQt5Core.so.5.15.2)
==9178==    by 0x7E608E4: ??? (in /usr/local/qt-5.15.2/lib/libQt5Core.so.5.15.2)
==9178==    by 0x6D0582C: QWidgetLineControl::processKeyEvent(QKeyEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==  Block was alloc'd at
==9178==    at 0x4C2E94F: operator new(unsigned long) (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==9178==    by 0x7E5F174: QObject::QObject(QObject*) (in /usr/local/qt-5.15.2/lib/libQt5Core.so.5.15.2)
==9178==    by 0x7583182: QInputControl::QInputControl(QInputControl::Type, QObject*) (in /usr/local/qt-5.15.2/lib/libQt5Gui.so.5.15.2)
==9178==    by 0x6CF841E: ??? (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x6CFC130: QLineEdit::QLineEdit(QWidget*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x403830: MyWidget::mousePressEvent(QMouseEvent*) (MyWidget.cpp:19)
==9178==    by 0x6C06E1D: QWidget::event(QEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x6BCA5BE: QApplicationPrivate::notify_helper(QObject*, QEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x6BD0D75: QApplication::notify(QObject*, QEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x7E2C3E9: QCoreApplication::notifyInternal2(QObject*, QEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Core.so.5.15.2)
==9178==    by 0x6BCFE42: QApplicationPrivate::sendMouseEvent(QWidget*, QMouseEvent*, QWidget*, QWidget*, QWidget**, QPointer<QWidget>&, bool, bool) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x6C1E286: ??? (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178== 
==9178== Invalid read of size 8
==9178==    at 0x7E53B70: QObjectPrivate::maybeSignalConnected(unsigned int) const (in /usr/local/qt-5.15.2/lib/libQt5Core.so.5.15.2)
==9178==    by 0x7E60485: ??? (in /usr/local/qt-5.15.2/lib/libQt5Core.so.5.15.2)
==9178==    by 0x6D05834: QWidgetLineControl::processKeyEvent(QKeyEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x6CFA9DA: QLineEdit::keyPressEvent(QKeyEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x6C07C86: QWidget::event(QEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x6BCA5BE: QApplicationPrivate::notify_helper(QObject*, QEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x6BD1126: QApplication::notify(QObject*, QEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x7E2C3E9: QCoreApplication::notifyInternal2(QObject*, QEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Core.so.5.15.2)
==9178==    by 0x6C213CC: ??? (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x6BCA5BE: QApplicationPrivate::notify_helper(QObject*, QEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x7E2C3E9: QCoreApplication::notifyInternal2(QObject*, QEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Core.so.5.15.2)
==9178==    by 0x7402D31: QGuiApplicationPrivate::processKeyEvent(QWindowSystemInterfacePrivate::KeyEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Gui.so.5.15.2)
==9178==  Address 0x147cb3a0 is 64 bytes inside a block of size 88 free'd
==9178==    at 0x4C2FBEB: operator delete(void*, unsigned long) (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==9178==    by 0x6D071D4: ??? (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x7E5470D: QObjectPrivate::deleteChildren() (in /usr/local/qt-5.15.2/lib/libQt5Core.so.5.15.2)
==9178==    by 0x6C02FF5: QWidget::~QWidget() (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x6CFA7E8: QLineEdit::~QLineEdit() (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x4037D8: MyWidget::mousePressEvent(QMouseEvent*)::{lambda()#1}::operator()() const (MyWidget.cpp:38)
==9178==    by 0x403BB3: QtPrivate::FunctorCall<QtPrivate::IndexesList<>, QtPrivate::List<>, void, MyWidget::mousePressEvent(QMouseEvent*)::{lambda()#1}>::call({lambda()#1}&, void**) (qobjectdefs_impl.h:146)
==9178==    by 0x403B94: void QtPrivate::Functor<MyWidget::mousePressEvent(QMouseEvent*)::{lambda()#1}, 0>::call<QtPrivate::List<>, void>({lambda()#1}&, void*, {lambda()#1}&*) (qobjectdefs_impl.h:256)
==9178==    by 0x403B63: QtPrivate::QFunctorSlotObject<MyWidget::mousePressEvent(QMouseEvent*)::{lambda()#1}, 0, QtPrivate::List<>, void>::impl(int, QtPrivate::QSlotObjectBase*, QObject*, void**, bool*) (qobjectdefs_impl.h:443)
==9178==    by 0x7E608B2: ??? (in /usr/local/qt-5.15.2/lib/libQt5Core.so.5.15.2)
==9178==    by 0x7E608E4: ??? (in /usr/local/qt-5.15.2/lib/libQt5Core.so.5.15.2)
==9178==    by 0x6D0582C: QWidgetLineControl::processKeyEvent(QKeyEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==  Block was alloc'd at
==9178==    at 0x4C2E94F: operator new(unsigned long) (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==9178==    by 0x7E5F174: QObject::QObject(QObject*) (in /usr/local/qt-5.15.2/lib/libQt5Core.so.5.15.2)
==9178==    by 0x7583182: QInputControl::QInputControl(QInputControl::Type, QObject*) (in /usr/local/qt-5.15.2/lib/libQt5Gui.so.5.15.2)
==9178==    by 0x6CF841E: ??? (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x6CFC130: QLineEdit::QLineEdit(QWidget*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x403830: MyWidget::mousePressEvent(QMouseEvent*) (MyWidget.cpp:19)
==9178==    by 0x6C06E1D: QWidget::event(QEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x6BCA5BE: QApplicationPrivate::notify_helper(QObject*, QEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x6BD0D75: QApplication::notify(QObject*, QEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x7E2C3E9: QCoreApplication::notifyInternal2(QObject*, QEvent*) (in /usr/local/qt-5.15.2/lib/libQt5Core.so.5.15.2)
==9178==    by 0x6BCFE42: QApplicationPrivate::sendMouseEvent(QWidget*, QMouseEvent*, QWidget*, QWidget*, QWidget**, QPointer<QWidget>&, bool, bool) (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==    by 0x6C1E286: ??? (in /usr/local/qt-5.15.2/lib/libQt5Widgets.so.5.15.2)
==9178==

Christian Ehrlicher

Please provide a minimal, compilable example so we can reproduce your issue. Deleting a widget will not crash Qt - you must be doing something wrong. For instance you must not delete a widget inside a slot which is called due to a signal of this widget but use deleteLater() as you do in your example.

/moved to General Qt since it's a general Qt usage question.

mrjj

Hi
Just as a note:
You can have an editable Qlabel

ui->label->setTextInteractionFlags(Qt::TextSelectableByMouse | Qt::TextEditable);
    QTextDocument *td = ui->label->findChild<QTextDocument *>();
    if (td) {
        connect( td, &QTextDocument::contentsChanged, this, [td]() {
            qDebug() << td->toRawText();
        });
    }

BwvB

@mrjj Thanks! I was not aware of this possibility. Maybe I should follow this route.
I will still post my original code, as suggested earlier, as something must be wrong there, and I don't see the culprit
(meaning I still don't fully understand Qt)..
.

BwvB

@Christian-Ehrlicher

Here is a small example.
In succession I post a header file and two .cpp files + qmake .pro.

header file: MyWidget.h:

#include <QtWidgets/QWidget>
#include <QtWidgets/QLabel>

class MyWidget : public QWidget
{
   Q_OBJECT
private:
   QLabel* _qlabel;
public:
   MyWidget();
protected:
   void mousePressEvent(QMouseEvent*) override;
};

Implementation file MyWidget.cpp:

#include "MyWidget.h"

#include <QtWidgets/QHBoxLayout>
#include <QtWidgets/QLineEdit>

MyWidget::MyWidget()
{
   _qlabel = new QLabel( "Click to edit text.");

   QHBoxLayout *layout = new QHBoxLayout(this);
   layout->addWidget( _qlabel );

   show();
}
void MyWidget::mousePressEvent( QMouseEvent* )
{
   // Create a lineEdit, initialize with qlabel's text
   auto lineEdit = new QLineEdit( _qlabel );
   lineEdit->setText( _qlabel->text() );
   lineEdit->setFocus();

   // exchange widgets in layout; delete old layout item.
   delete layout()->replaceWidget( _qlabel, lineEdit );

   // lambda for refreshing the label.
   auto doUpdate =
      [this,lineEdit]()
      {
         delete layout()->replaceWidget( lineEdit, _qlabel );
         _qlabel->setText( lineEdit->text() );
#if 0
         lineEdit->hide();        // works, but memory leaked or not timely cleaned.
#else
         disconnect( lineEdit );
         lineEdit->setParent(nullptr);
         delete lineEdit;         // crash, or at least memory corruption!
#endif
      };  
   connect( lineEdit, &QLineEdit::returnPressed, doUpdate );
}

main.cpp:

#include "MyWidget.h"
#include <QtWidgets/QApplication>
int main(int argc, char *argv[])
{
    QApplication app( argc, argv );
    auto widget = std::unique_ptr<MyWidget>(new MyWidget);
    return app.exec();
}

and finally a qmake .pro file: qmake -o Makefile MyWidget.pro

TEMPLATE = app
QT += widgets
CONFIG += qt
SOURCES += MyWidget.cpp main.cpp
HEADERS += MyWidget.h
TARGET = MyWidget

mrjj

Hi
did you try

delete lineEdit;

to

lineEdit-deleteLater();

Christian Ehrlicher

@BwvB You created an example but did not read my post:

For instance you must not delete a widget inside a slot which is called due to a signal of this widget but use deleteLater() as you do in your example.

BwvB

@Christian-Ehrlicher
Christian, you were quit right. In my eagerness to post a minimal example, I forgot to follow-up on your suggestion...

Indeed, changing

delete lineEdit

into

lineEdit->deleteLater()

resolved the issue. valgrind is happy and so am I.
Thanks for your help!

Regards,
Bertwim