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)- 1
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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);- 1
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)- 1
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解决办法是：
(1). new时不传递this指针
m_net2 = new QNetworkAccessManager;- 1
(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.- 1
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。
