Qt：处理后台线程处理程序抛出的异常答案

【问题标题】：Qt: handling exceptions thrown from handlers of background threadsQt：处理后台线程处理程序抛出的异常
【发布时间】：2014-08-07 20:17:58
【问题描述】：

我知道您需要重新实现 QApplication::notify() 方法以正确捕获从主线程的事件处理程序抛出的异常。但是其他线程呢？比如说，我有一个带槽的对象，这个对象存在于一个 QThread 中（使用默认的 run() 方法，它只调用 exec()），即这个对象的线程亲和性是后台 QThread。那么，我应该在哪里捕获从这个对象的插槽抛出的异常？

IOW，如何重新实现后台线程的 notify() 方法？

【问题讨论】：

可能重载 exec 并放一个 try() { QThread::exec()} catch(...) { }
@drescherjm 你的意思是从插槽抛出的异常将“飞出” exec() 方法？它似乎对我不起作用。
是的，我希望它可以工作。对不起。我对此一无所知，但订阅了。
@drescherjm : 你不能重载 QThread::exec，这个方法不是虚拟的
谢谢。这就解释了为什么这不起作用..

标签： c++ multithreading qt exception-handling qt4

【解决方案1】：

当您使用重写的通知方法创建自定义应用程序时；您创建的 QThread 也使用此覆盖方法（作为主线程）一旦它开始了自己的事件循环

这意味着在实践中，如果您将任何插槽连接到 QThread::started 信号；然后这个槽在线程的事件循环之外执行，因此不在被覆盖的通知方法中。

这是一个有助于理解发生了什么的代码示例：

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

#include <QDebug>
#include <QThread>
#include <QTimer>
#include <exception>

class ThrowingObject : public QObject
{
public:
    void doThrowInNotify()
    {
        qDebug() << "I am execution on thread id" << QThread::currentThreadId() << " and I am throwing";
        throw std::exception("KBOOOM");
    }

    void scheduleThrow()
    {
        QTimer* singleShot = new QTimer(this);
        singleShot->setSingleShot(true);

        connect(singleShot, &QTimer::timeout, this, &ThrowingObject::doThrow);

        singleShot->start();

        qDebug() << "I am execution on thread id" << QThread::currentThreadId() << " and I will throw in run";
    }

    void doThrow()
    {
        qDebug() << "I am execution on thread id" << QThread::currentThreadId() << " and I am throwing right now";

        //This exception is not catched, and definitly crash the process
        throw std::exception("KBOOOM");
    }

    void doThrowOutsideNotify()
    {
        //wait 5s for demo purpose, this slot is called by Object2, after Object1 throw in thread1 event loop
        QThread::sleep(5);
        qDebug() << "I am execution on thread id" << QThread::currentThreadId() << " and I am throwing right now";

        //This exception is not catched, and definitly crash the process
        throw std::exception("FATAL KBOOOM");
    }

};

class ApplicationWithExceptionCatchedInNotify : public QApplication
{
public:
    ApplicationWithExceptionCatchedInNotify(int argc, char *argv[]) :
        QApplication(argc,argv)
    {}

    bool notify(QObject* receiver, QEvent *e) override
    {
        try {
            return QApplication::notify(receiver, e);
        }
        catch(std::runtime_error e)
        {
            qDebug() << "std::runtime_error in thread : " << QThread::currentThreadId();
            qDebug() << e.what();
        }
        catch(std::exception e)
        {
            qDebug() << "std::exception in thread : " << QThread::currentThreadId();
            qDebug() << e.what();
        }
        catch(...)
        {
            qDebug() << "exception thread : " << QThread::currentThreadId();
        }

        qDebug() << "catch in notify ";
        return false;
    }

};



int main(int argc, char *argv[])
{
ApplicationWithExceptionCatchedInNotify app(argc, argv);

qDebug() << "Main QThread id" << QThread::currentThreadId();

//Object o1 will throw in its event loop (in notify)
QThread thread1;
ThrowingObject o1;
o1.moveToThread(&thread1);

QObject::connect(&thread1, &QThread::started, &o1, &ThrowingObject::scheduleThrow);

thread1.start();

//Object o2 will throw before the event loop is installed
QThread thread2;
ThrowingObject o2;
o2.moveToThread(&thread2);
//Connect to started signal.
QObject::connect(&thread2, &QThread::started, &o2, &ThrowingObject::doThrowOutsideNotify);
thread2.start();

app.exec();
}

在 windows 上运行此代码示例，带有 Qt creator 的 Qt 5.9 给出了示例：

输出：

Main QThread id 0x11e4
I am execution on thread id 0x180c  and I will throw in run
I am execution on thread id 0x180c  and I am throwing right now
std::exception in thread :  0x180c
KBOOOM
catch in notify 
I am execution on thread id 0x27b8  and I am throwing right now

然后弹出一个 Microsoft Visual Studio 运行时库：

Microsoft Visual C++ Runtime Library

Debug Error!

Program: ...ad-Desktop_Qt_5_9_2_MSVC2017_64bit-Debug\debug\DemoThread.exe

abort() has been called

(Press Retry to debug the application)

如果你放断点；曾经可以意识到：

对象 o1 在 QThread::exec 方法中抛出，沿着调用堆栈我们可以看到我们在 ApplicationWithExceptionCatchedInNotify::Notify

1 ThrowingObject::doThrow main.cpp 35 0x7ff66615352b 2 QtPrivate::FunctorCall,QtPrivate::List,void,void (__cdecl ThrowingObject:: *)(void) __ptr64>::call qobjectdefs_impl.h 136 0x7ff66615358c 3 QtPrivate::FunctionPointer::call,void> qobjectdefs_impl.h 170 0x7ff666152ce7 4 QtPrivate::QSlotObject,void>::impl qobject_impl.h 121 0x7ff66615363e 5 QtPrivate::QSlotObjectBase::call qobject_impl.h 101 0x54a82428
6 QMetaObject::activate qobject.cpp 3754 0x54a70ee0
7 QMetaObject::激活 qobject.cpp 3629 0x54a707a8
8 QTimer::timeout moc_qtimer.cpp 202 0x54a8f739
9 QTimer::timerEvent qtimer.cpp 257 0x54a8f79a
10 QObject::event qobject.cpp 1228 0x54a72b73
11 QApplicationPrivate::notify_helper qapplication.cpp 3722 0x53aeb8ee
12 QApplication::notify qapplication.cpp 3094 0x53ae6323
13 ApplicationWithExceptionCatchedInNotify::notify main.cpp 60 0x7ff666158730 14 QCoreApplication::notifyInternal2 qcoreapplication.cpp 1018 0x54a1b0c6
15 QCoreApplication::sendEvent qcoreapplication.h 233 0x54a26062
16 QEventDispatcherWin32::事件 qeventdispatcher_win.cpp 1041 0x54ad8cab
17 QApplicationPrivate::notify_helper qapplication.cpp 3722 0x53aeb8ee
18 QApplication::notify qapplication.cpp 3094 0x53ae6323
19 ApplicationWithExceptionCatchedInNotify::notify main.cpp 60 0x7ff666158730 20 QCoreApplication::notifyInternal2 qcoreapplication.cpp 1018 0x54a1b0c6
21 QCoreApplication::sendEvent qcoreapplication.h 233 0x54a26062
22 QCoreApplicationPrivate::sendPostedEvents qcoreapplication.cpp 1678 0x54a1c982
23 QEventDispatcherWin32::sendPostedEvents qeventdispatcher_win.cpp 1064 0x54ad8e6a
24 qt_internal_proc qeventdispatcher_win.cpp 237 0x54ad6b47
25 CallWindowProcW USER32 0x7ffba1571c24 26 DispatchMessageW USER32 0x7ffba157156c 27 QEventDispatcherWin32::processEvents qeventdispatcher_win.cpp 628 0x54ad755b
28 QEventLoop::processEvents qeventloop.cpp 135 0x54a15498
29 QEventLoop::exec qeventloop.cpp 212 0x54a156de
30 QThread::exec qthread.cpp 515 0x5465028f
31 QThread::run qthread.cpp 583 0x546501c3
32 QThreadPrivate::start qthread_win.cpp 380 0x5465caed
33 BaseThreadInitThunk 内核32 0x7ffb9f5b8364 34 RtlUserThreadStart ntdll 0x7ffba1bb7091

Object o2 在 QThread::start 方法中抛出；在 thread2 事件循环之外

1 ThrowingObject::doThrowOutsideNotify main.cpp 45 0x7ff666152ba6 2 QtPrivate::FunctorCall,QtPrivate::List,void,void (__cdecl ThrowingObject:: *)(void) __ptr64>::call qobjectdefs_impl.h 136 0x7ff66615358c 3 QtPrivate::FunctionPointer::call,void> qobjectdefs_impl.h 170 0x7ff666152ce7 4 QtPrivate::QSlotObject,void>::impl qobject_impl.h 121 0x7ff66615363e 5 QtPrivate::QSlotObjectBase::call qobject_impl.h 101 0x54a82428
6 QMetaObject::activate qobject.cpp 3754 0x54a70ee0
7 QMetaObject::activate qobject.cpp 3629 0x54a707a8
8 QThread::started moc_qthread.cpp 160 0x54650149
9 QThreadPrivate::start qthread_win.cpp 377 0x5465cad6
10 BaseThreadInitThunk 内核32 0x7ffb9f5b8364 11 RtlUserThreadStart ntdll 0x7ffba1bb7091

【讨论】：

【解决方案2】：

由于某些版本的 Qt5，QCoreApplication::notify 手册说：

这个函数被所有发送到任意对象的事件调用线程。

所以这意味着我最初的问题的正确答案是“做你一直在做的事情”。

有趣的是，当我发现这一点时，这些新发现的知识立即被同一手册中的这句话所过时：

这个函数不会被存在于 Qt 6 中的主线程。需要该功能的应用程序应该同时为他们的事件检查需求寻找其他解决方案。更改可能会扩展到主线程，导致此功能将被弃用。

哈哈。我还没有迁移到 Qt6，所以我不知道那里的情况，但是当我这样做时，我可能会重新打开这个确切的问题。

【讨论】：