QThread 爬坑之旅（三种办法解决QObject: Cannot create children for a parent that is in a different thread）

Cannot create children for a parent that is in a different thread.

在Qt的官方文档，大家知道有两种方式使用QThread。

• You can use worker objects by moving them to the thread using QObject::moveToThread().
• Another way to make code run in a separate thread, is to subclass QThread and reimplement run().

在使用MoveToThread这种方式时，经常会遇到下面类似的问题：

• QObject: Cannot create children for a parent that is in a different thread.

出现这样的问题根本原因就是，调用MoveToThread 之后，在 Worker的槽函数中Worker的私有成员中又进行了new操作，并且将this指针传给了构造函数。看下实例：

mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QThread>
#include <QMainWindow>

#include "worker.h"

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();

signals:
    void doWorkSignal();

private:
    Ui::MainWindow *ui;

    QThread m_thread;

    Worker m_worker;
};

#endif // MAINWINDOW_H

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mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    m_worker.moveToThread(&m_thread);

    connect(this, SIGNAL(doWorkSignal()),
            &m_worker, SLOT(doWork()));

    m_thread.start();

    emit doWorkSignal();

    qDebug() << "MainWin thread: " << QThread::currentThread();
}

MainWindow::~MainWindow()
{
    delete ui;

    m_thread.exit();
    m_thread.wait();
}

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worker.h

#ifndef WORKER_H
#define WORKER_H

#include <QObject>
#include <QThread>
#include <QNetworkAccessManager>

class WorkerA: public QObject
{
    Q_OBJECT
public:
    inline explicit WorkerA(QObject *parent = 0)
    {
        m_net1 = new QNetworkAccessManager(this);

        qDebug() << "Create WorkerA thread: " << QThread::currentThread();
    }

    inline void doWorkA()
    {
        m_net2 = new QNetworkAccessManager(this);

        qDebug() << "DoWorkA thread: " << QThread::currentThread();
        qDebug() << "Net1 Parent: " << m_net1->parent();
        qDebug() << "Net2 Parent: " << m_net2->parent();;
    }

    inline ~WorkerA()
    {
        delete m_net1;
        delete m_net2;
    }

private:
    QNetworkAccessManager *m_net1;
    QNetworkAccessManager *m_net2;
};

class Worker : public QObject
{
    Q_OBJECT
public:
    explicit Worker(QObject *parent = 0);
    ~Worker();

signals:

public slots:
    void doWork();

private:
    WorkerA *m_workerA;

};

#endif // WORKER_H

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worker.cpp

#include <QDebug>

#include "worker.h"

Worker::Worker(QObject *parent) :
    QObject(parent)
{
    m_workerA = new WorkerA(this);

    qDebug() << "Create Worker thread: " << QThread::currentThread();
}

void Worker::doWork()
{
    qDebug() << "doWork thread: " << QThread::currentThread();

    m_workerA->doWorkA();
}

Worker::~Worker()
{
    //delete m_workerTimer;
}

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程序运行输出：

Create WorkerA thread:  QThread(0x4482e8)
Create Worker thread:  QThread(0x4482e8)
MainWin thread:  QThread(0x4482e8)
doWork thread:  QThread(0x28fe1c)
QObject: Cannot create children for a parent that is in a different thread.
(Parent is WorkerA(0x4558a8), parent's thread is QThread(0x4482e8), current thread is QThread(0x28fe1c)
DoWorkA thread:  QThread(0x28fe1c)
Net1 Parent:  WorkerA(0x4558a8)
Net2 Parent:  QObject(0x0)

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在本案例中，Worker在槽函数中调用私有成员WorkerA的doWorkA()，doWorkA()中

m_net2 = new QNetworkAccessManager(this);

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查看官方文档可以知道，doWork槽函数会在另外一个线程被执行。这里 
有new操作，而且传递了this指针，而且我们也可以从打印信息可知道此时this指针和doWorkA()不在同一线程，所以会报出错误：

QObject: Cannot create children for a parent that is in a different thread.
(Parent is WorkerA(0x4558a8), parent's thread is QThread(0x4482e8), current thread is QThread(0x28fe1c)

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解决办法是： 
(1). new时不传递this指针

m_net2 = new QNetworkAccessManager;

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(2). 将new操作放在WorkerA的构造函数中

m_net1 = new QNetworkAccessManager(this);
m_net2 = new QNetworkAccessManager(this);

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(3).使用信号与槽的方法调用doWorkA()

总结

QObject: Cannot create children for a parent that is in a different thread.

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这样的错误，多是由于在槽函数中多层嵌套时new操作出的问题，建议大家尽量避免在槽函数中进行new操作。

测试代码： 
https://github.com/RobinsonSir/QThreadTest1

版权声明：本文为博主原创文章，未经博主允许不得转载。 https://blog.csdn.net/zbc415766331/article/details/52462118

 

其实还是第四种做法，就是把WorkerA再次moveToThread一下，毕竟它是在构造函数里创建的，而那时候Worker所在的线程还是主线程。