QThread 爬坑之旅（三種辦法解決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的官方文檔，你們知道有兩種方式使用 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這種方式時，常常會遇到下面相似的問題：

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

出現這樣的問題根本緣由就是，調用MoveToThread 以後，在 Worker的槽函數中Worker的私有成員中又進行了new操做，而且將this指針傳給了構造函數。看下實例：

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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程序運行輸出：

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在槽函數中調用私有成員WorkerA的doWorkA()，doWorkA()中

m_net2 = new QNetworkAccessManager(this);

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查看官方文檔能夠知道，doWork槽函數會在另一個線程被執行。這裏 
有new操做，並且傳遞了this指針，並且咱們也能夠從打印信息可知道此時this指針和doWorkA()不在同一線程，因此會報出錯誤：

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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解決辦法是： 
(1). new時不傳遞this指針

m_net2 = new QNetworkAccessManager;

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(2). 將new操做放在WorkerA的構造函數中

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

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(3).使用信號與槽的方法調用doWorkA()

總結

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

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這樣的錯誤，可能是因爲在槽函數中多層嵌套時new操做出的問題，建議你們儘可能避免在槽函數中進行new操做。

測試代碼： 
https://github.com/RobinsonSir/QThreadTest1

版權聲明：本文爲博主原創文章，未經博主容許不得轉載。 https://blog.csdn.net/zbc415766331/article/details/52462118

 

其實仍是第四種作法，就是把WorkerA再次moveToThread一下，畢竟它是在構造函數裏建立的，而那時候Worker所在的線程仍是主線程。