Connect with lambda and capture by reference causes segmentation fault

[VictorLamoine]

Hi,

I'm wondering why capturing by reference in a connect lambda causes a segmentation fault.

CMakeLists.txt

cmake_minimum_required(VERSION 3.5)
project(test_qt LANGUAGES CXX)
add_compile_options(-Wall -Wextra)
set(CMAKE_CXX_STANDARD 14)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

set(CMAKE_AUTOMOC ON)
find_package(Qt5 COMPONENTS Widgets REQUIRED)

add_executable(
  ${PROJECT_NAME}
  mainwindow.hpp
  mainwindow.cpp
  main.cpp
)
target_link_libraries(
  ${PROJECT_NAME}
  Qt5::Widgets
)

main.cpp

#include "mainwindow.hpp"
#include <QApplication>

int main(int argc, char *argv[])
{
  QApplication a(argc, argv);
  MainWindow w;
  w.show();
  return a.exec();
}

mainwindow.hpp

#ifndef MAINWINDOW_HPP
#define MAINWINDOW_HPP

#include <QLabel>
#include <QMainWindow>
#include <QPushButton>
#include <QVBoxLayout>

class MainWindow : public QMainWindow
{
  Q_OBJECT

public:
  MainWindow(QWidget *parent = nullptr);
  ~MainWindow();
};

#endif

mainwindow.cpp

#include "mainwindow.hpp"

MainWindow::MainWindow(QWidget *parent)
  : QMainWindow(parent)
{
  QWidget *widget(new QWidget);
  setCentralWidget(widget);
  QVBoxLayout *layout(new QVBoxLayout(widget));

  QLabel *label(new QLabel("QLabel"));
  QPushButton *change_label(new QPushButton("Change label"));

  layout->addWidget(label);
  layout->addWidget(change_label);

  connect(change_label, &QPushButton::clicked, this, [&]() // FIXME Capture by reference = crash, capture by copy is ok!
  {
    label->setText("Hello");
  });
}

MainWindow::~MainWindow()
{
}

Is it explicitly forbidden in Qt to capture by reference in a connect lambda?

• Kubuntu 18.04
• Qt 5.9.5
• gcc 9.3.0
• CMake 3.19.3

gdb backtrace:

(gdb) run
Starting program: /home/victor/Documents/code/tests/qt/build/test_qt test_qt
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[New Thread 0x7fffebd72700 (LWP 31414)]
[New Thread 0x7fffdc88a700 (LWP 31415)]

Thread 1 "test_qt" received signal SIGSEGV, Segmentation fault.
0x00007ffff6f67b78 in operator==(QString const&, QString const&) () from /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
(gdb) bt
#0    0x00007ffff6f67b78 in operator==(QString const&, QString const&) + 0x8 () at /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#1    0x00007ffff78183f7 in QLabel::setText(QString const&) + 0x37 () at /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5
#2    0x0000555555556964 in MainWindow::<lambda()>::operator()(void) const (__closure=0x5555558497d0)
                         at /home/victor/code/tests/qt/src/mainwindow.cpp:18
#3    0x0000555555556eee in QtPrivate::FunctorCall<QtPrivate::IndexesList<>, QtPrivate::List<>, void, MainWindow::MainWindow(QWidget*)::<lambda()> >::call(MainWindow::<lambda()> &, void **) (f=@0x5555558497d0, arg=0x7fffffffcd50)
                         at /usr/include/x86_64-linux-gnu/qt5/QtCore/qobjectdefs_impl.h:130
#4    0x0000555555556ec0 in QtPrivate::Functor<MainWindow::MainWindow(QWidget*)::<lambda()>, 0>::call<QtPrivate::List<>, void>(MainWindow::<lambda()> &, void *, void **) (f=@0x5555558497d0, arg=0x7fffffffcd50)
                         at /usr/include/x86_64-linux-gnu/qt5/QtCore/qobjectdefs_impl.h:240
#5    0x0000555555556e8e in QtPrivate::QFunctorSlotObject<MainWindow::MainWindow(QWidget*)::<lambda()>, 0, QtPrivate::List<>, void>::impl(int, QtPrivate::QSlotObjectBase *, QObject *, void **, bool *) (which=1, this_=0x5555558497c0, r=0x7fffffffda30, a=0x7fffffffcd50, ret=0x0)
                         at /usr/include/x86_64-linux-gnu/qt5/QtCore/qobject_impl.h:168
#6    0x00007ffff70fa66f in QMetaObject::activate(QObject*, int, int, void**) + 0x97f () at /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#7    0x00007ffff77d1ba2 in QAbstractButton::clicked(bool) + 0x42 () at /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5
#8    0x00007ffff77d1dba in No symbol matches 0x00007ffff77d1dba. () at /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5
#9    0x00007ffff77d319a in No symbol matches 0x00007ffff77d319a. () at /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5
#10   0x00007ffff77d338d in QAbstractButton::mouseReleaseEvent(QMouseEvent*) + 0xfd () at /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5
#11   0x00007ffff771f048 in QWidget::event(QEvent*) + 0x1f8 () at /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5
#12   0x00007ffff76e083c in QApplicationPrivate::notify_helper(QObject*, QEvent*) + 0x9c () at /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5
#13   0x00007ffff76e865f in QApplication::notify(QObject*, QEvent*) + 0x7ff () at /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5
#14   0x00007ffff70cb8d8 in QCoreApplication::notifyInternal2(QObject*, QEvent*) + 0x118 () at /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#15   0x00007ffff76e7632 in QApplicationPrivate::sendMouseEvent(QWidget*, QMouseEvent*, QWidget*, QWidget*, QWidget**, QPointer<QWidget>&, bool) + 0x1d2 ()
                         at /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5
#16   0x00007ffff773a16b in No symbol matches 0x00007ffff773a16b. () at /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5
#17   0x00007ffff773c7da in No symbol matches 0x00007ffff773c7da. () at /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5
#18   0x00007ffff76e083c in QApplicationPrivate::notify_helper(QObject*, QEvent*) + 0x9c () at /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5
#19   0x00007ffff76e8104 in QApplication::notify(QObject*, QEvent*) + 0x2a4 () at /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5
#20   0x00007ffff70cb8d8 in QCoreApplication::notifyInternal2(QObject*, QEvent*) + 0x118 () at /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#21   0x00007ffff5dfc583 in QGuiApplicationPrivate::processMouseEvent(QWindowSystemInterfacePrivate::MouseEvent*) + 0x6f3 ()
                         at /usr/lib/x86_64-linux-gnu/libQt5Gui.so.5
#22   0x00007ffff5dfe055 in QGuiApplicationPrivate::processWindowSystemEvent(QWindowSystemInterfacePrivate::WindowSystemEvent*) + 0x135 ()
                         at /usr/lib/x86_64-linux-gnu/libQt5Gui.so.5
#23   0x00007ffff5dd52eb in QWindowSystemInterface::sendWindowSystemEvents(QFlags<QEventLoop::ProcessEventsFlag>) + 0xab ()
                         at /usr/lib/x86_64-linux-gnu/libQt5Gui.so.5
#24   0x00007fffefe55260 in No symbol matches 0x00007fffefe55260. () at /usr/lib/x86_64-linux-gnu/libQt5XcbQpa.so.5
#25   0x00007ffff45e8417 in g_main_context_dispatch + 0x2e7 () at /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#26   0x00007ffff45e8650 in No symbol matches 0x00007ffff45e8650. () at /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#27   0x00007ffff45e86dc in g_main_context_iteration + 0x2c () at /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#28   0x00007ffff712488f in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) + 0x5f ()
                         at /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#29   0x00007ffff70c990a in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) + 0x13a () at /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#30   0x00007ffff70d29b4 in QCoreApplication::exec() + 0x94 () at /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#31   0x0000555555557252 in main(int, char**) (argc=2, argv=0x7fffffffdb68) at /home/victor/code/tests/qt/src/main.cpp:9

Bye

[raven-worx]

@VictorLamoine
i guess the reason for this behavior is that your label pointer variable is deleted when it goes out of scope.
i am talking about the pointer variable holding the address, not the actual QLabel instance. If you capture it by value a new pointer variable lives inside the scope of your lambda.
Btw. the default capture type is also by value not by reference.

[VictorLamoine]

That's right!

If instead the widgets are declared private in the class:

class MainWindow : public QMainWindow
{
  Q_OBJECT

public:
  MainWindow(QWidget *parent = nullptr);
  ~MainWindow();

private:
  QLabel *label_;
  QPushButton *change_label_;
};

Then using a connect / lambda with capture by reference works fine:

MainWindow::MainWindow(QWidget *parent)
  : QMainWindow(parent)
{
  QWidget *widget(new QWidget);
  setCentralWidget(widget);
  QVBoxLayout *layout(new QVBoxLayout(widget));

  label_ = new QLabel("QLabel");
  change_label_ = new QPushButton("Change label");

  layout->addWidget(label_);
  layout->addWidget(change_label_);

  connect(change_label_, &QPushButton::clicked, this, [&]() // Capture by reference works!
  {
    label_->setText("Hello");
  });
}

The problem was indeed the scope of the variable.