Qt之Cannot create children for a parent that is in a different thread问题分析

问题

在多线程场景中，使用QSerialPort,QTcpSocket等QIODevice设备时出现报Cannot create children for a parent that is in a different thread

分析

QObject构造函数中会检查父对象的线程数据与当前对象的线程数据是否一致

static bool check_parent_thread(QObject *parent,
                                QThreadData *parentThreadData,
                                QThreadData *currentThreadData)
{
    if (parent && parentThreadData != currentThreadData) {
        QThread *parentThread = parentThreadData->thread.loadAcquire();
        QThread *currentThread = currentThreadData->thread.loadAcquire();
        qWarning("QObject: Cannot create children for a parent that is in a different thread.\n"
                 "(Parent is %s(%p), parent's thread is %s(%p), current thread is %s(%p)",
                 parent->metaObject()->className(),
                 parent,
                 parentThread ? parentThread->metaObject()->className() : "QThread",
                 parentThread,
                 currentThread ? currentThread->metaObject()->className() : "QThread",
                 currentThread);
        return false;
    }
    return true;
}

在公有构造函数explicit QObject(QObject *parent=nullptr);和保护构造函数QObject(QObjectPrivate &dd, QObject *parent = nullptr)

QObject::QObject(QObject *parent)
    : d_ptr(new QObjectPrivate)
{
    Q_ASSERT_X(this != parent, Q_FUNC_INFO, "Cannot parent a QObject to itself");

    Q_D(QObject);
    d_ptr->q_ptr = this;
    d->threadData = (parent && !parent->thread()) ? parent->d_func()->threadData : QThreadData::current();
    d->threadData->ref();
    if (parent) {
        QT_TRY {
            if (!check_parent_thread(parent, parent ? parent->d_func()->threadData : 0, d->threadData))
                parent = 0;
            setParent(parent);
        } QT_CATCH(...) {
            d->threadData->deref();
            QT_RETHROW;
        }
    }
#if QT_VERSION < 0x60000
    qt_addObject(this);
#endif
    if (Q_UNLIKELY(qtHookData[QHooks::AddQObject]))
        reinterpret_cast<QHooks::AddQObjectCallback>(qtHookData[QHooks::AddQObject])(this);
    Q_TRACE(QObject_ctor, this);
}

QObject::QObject(QObjectPrivate &dd, QObject *parent)
    : d_ptr(&dd)
{
    Q_ASSERT_X(this != parent, Q_FUNC_INFO, "Cannot parent a QObject to itself");

    Q_D(QObject);
    d_ptr->q_ptr = this;
    d->threadData = (parent && !parent->thread()) ? parent->d_func()->threadData : QThreadData::current();
    d->threadData->ref();
    if (parent) {
        QT_TRY {
            if (!check_parent_thread(parent, parent ? parent->d_func()->threadData : 0, d->threadData))
                parent = 0;
            if (d->isWidget) {
                if (parent) {
                    d->parent = parent;
                    d->parent->d_func()->children.append(this);
                }
                // no events sent here, this is done at the end of the QWidget constructor
            } else {
                setParent(parent);
            }
        } QT_CATCH(...) {
            d->threadData->deref();
            QT_RETHROW;
        }
    }
#if QT_VERSION < 0x60000
    qt_addObject(this);
#endif
    if (Q_UNLIKELY(qtHookData[QHooks::AddQObject]))
        reinterpret_cast<QHooks::AddQObjectCallback>(qtHookData[QHooks::AddQObject])(this);
    Q_TRACE(QObject_ctor, this);
}