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.