技術記事：QThread の started() と finished() シグナルの送信についての考察

#include <QCoreApplication>
#include <QThread>
#include <QTimer>
#include <QDebug>

using namespace std::chrono_literals;

void print(QAnyStringView message)
{
    qDebug().noquote().nospace() << message << ": isMainThread() = " << QThread::isMainThread();
}

int main(int argc, char** argv)
{
    QCoreApplication app(argc, argv);

    QThread thread;

    QObject::connect(&thread, &QThread::started, [] { print("started"); });
    QObject::connect(&thread, &QThread::finished, [] { print("finished"); });

    thread.start();

    QTimer::singleShot(1ms, [&thread] {
                                thread.quit();
                                thread.wait();
                                QCoreApplication::instance()->quit();
                            });

    return app.exec();
}

started: isMainThread() = false
finished: isMainThread() = false

$ lldb startedandfinished
(lldb) b QThread::started
(lldb) b QThread::run
(lldb) r

* thread #5, name = 'QThread', stop reason = breakpoint 1.1
    frame #0: 0x0000000100dde7e6 QtCore`QThread::started(this=0x00007ff7bfefedf0, _t1=QPrivateSignal @ 0x000070000a7bff38) at moc_qthread.cpp:193:18 [opt]

(lldb) bt
* thread #5, name = 'QThread', stop reason = breakpoint 1.1
  * frame #0: 0x0000000100dde7e6 QtCore`QThread::started(this=0x00007ff7bfefedf0, _t1=QPrivateSignal @ 0x0000700008a64f38) at moc_qthread.cpp:193:18 [opt]
    frame #1: 0x0000000100e8573c QtCore`QThreadPrivate::start(void*) [inlined] QThreadPrivate::start(void*)::$_1::operator()(this=<unavailable>) const at qthread_unix.cpp:331:19 [opt]
    frame #2: 0x0000000100e85649 QtCore`QThreadPrivate::start(void*) [inlined] void (anonymous namespace)::terminate_on_exception<QThreadPrivate::start(void*)::$_1>(t=<unavailable>) at qthread_unix.cpp:264:9 [opt]
    frame #3: 0x0000000100e85649 QtCore`QThreadPrivate::start(arg=0x00007ff7bfefedf0) at qthread_unix.cpp:295:5 [opt]
    frame #4: 0x00007ff80f62c1d3 libsystem_pthread.dylib`_pthread_start + 125
    frame #5: 0x00007ff80f627bd3 libsystem_pthread.dylib`thread_start + 15

(lldb) thread list
Process 39119 stopped
  thread #1: tid = 0x7518ce, 0x0000000100cea1a0 QtCore`QArrayDataPointer<QEventLoop*>::reallocateAndGrow(this=0x0000600003308010, where=GrowsAtEnd, n=1, old=0x0000000000000000) at qarraydatapointer.h:220, queue = 'com.apple.main-thread'
  thread #2: tid = 0x7518ea, 0x00007ff80f5eec3e libsystem_kernel.dylib`__workq_kernreturn + 10
  thread #3: tid = 0x7518eb, 0x00007ff80f5eec3e libsystem_kernel.dylib`__workq_kernreturn + 10
  thread #4: tid = 0x7518ec, 0x00007ff80f3217e8 dyld`dyld3::MachOLoaded::findClosestSymbol(unsigned long long, char const**, unsigned long long*) const + 528
* thread #5: tid = 0x7518ed, 0x0000000100dde7e6 QtCore`QThread::started(this=0x00007ff7bfefedf0, _t1=QPrivateSignal @ 0x0000700008a64f38) at moc_qthread.cpp:193:18, name = 'QThread', stop reason = breakpoint 1.1

(lldb) c
* thread #5, name = 'QThread', stop reason = breakpoint 2.1
    frame #0: 0x0000000100dde174 QtCore`QThread::run(this=0x00007ff7bfefedf0) at qthread.cpp:742:12 [opt]
(lldb)

#include <QCoreApplication>
#include <QThread>
#include <QTimer>
#include <QDebug>

using namespace std::chrono_literals;

void print(QAnyStringView message)
{
    qDebug().noquote().nospace() << message << ": isMainThread() = " << QThread::isMainThread();
}

class Worker : public QObject
{
    Q_OBJECT

public:
    explicit Worker(QObject* parent = nullptr) : QObject(parent) {
        print(Q_FUNC_INFO);
    }

    ~Worker() {
        print(Q_FUNC_INFO);
    }

public slots:
    void doWork(const QString& parameter) {
        print(Q_FUNC_INFO);
        qDebug().nospace() << "    parameter = " << parameter;

        QString result;
        result = parameter + ": result";
        emit resultReady(result);
    }

signals:
    void resultReady(const QString& result);
};

class Controller : public QObject
{
    Q_OBJECT

public:
    explicit Controller(QObject* parent = nullptr) : QObject(parent) {
        print(Q_FUNC_INFO);

        Worker* const worker = new Worker;
        worker->moveToThread(&workerThread);

        connect(&workerThread, &QThread::finished, worker, &QObject::deleteLater);
        connect(this, &Controller::operate, worker, &Worker::doWork);
        connect(worker, &Worker::resultReady, this, &Controller::handleResults);

        workerThread.start();
    }

    ~Controller() {
        print(Q_FUNC_INFO);

        workerThread.quit();
        workerThread.wait();  // Since workerThread is destroyed upon exiting this destructor.
    }

public slots:
    void handleResults(const QString& result) {
        print(Q_FUNC_INFO);
        qDebug().nospace() << "    result = " << result;
    };

signals:
    void operate(const QString&);

private:
    QThread workerThread;
};

int main(int argc, char** argv)
{
    print(Q_FUNC_INFO);

    QCoreApplication app(argc, argv);

    const auto controller = new Controller{};

    QTimer::singleShot(1ms, [controller] { emit controller->operate("Operate");});
    QTimer::singleShot(2ms, [controller] { delete controller;});
    QTimer::singleShot(20ms, [] { QCoreApplication::instance()->quit();});

    return app.exec();
}

#include "main.moc"

$ ./deletecontrollerandworker
int main(int, char **): isMainThread() = true
Controller::Controller(QObject *): isMainThread() = true
Worker::Worker(QObject *): isMainThread() = true
void Worker::doWork(const QString &): isMainThread() = false
    parameter = "Operate"
void Controller::handleResults(const QString &): isMainThread() = true
    result = "Operate: result"
virtual Controller::~Controller(): isMainThread() = true
virtual Worker::~Worker(): isMainThread() = false
$