QThread finished() 连接到 QObject 的删除器

Question

在这里问这个问题之前，我想了很多，阅读了很多文章。没有一篇文章给了我正确的答案。

http://mayaposch.wordpress.com/2011/11/01/how-to-really-truly-use-qthreads-the-full-explanation/

 QThread* thread = new QThread;
 Worker* worker = new Worker();
 worker->moveToThread(thread);
 connect(worker, SIGNAL(error(QString)), this, SLOT(errorString(QString)));
 connect(thread, SIGNAL(started()), worker, SLOT(process()));
 connect(worker, SIGNAL(finished()), thread, SLOT(quit()));
 connect(worker, SIGNAL(finished()), worker, SLOT(deleteLater()));
 connect(thread, SIGNAL(finished()), thread, SLOT(deleteLater()));
 thread->start();

Worker 对象具有新线程的亲和性。

1. Worker 完成信号将在线程上调用 quit()。这将结束线程的事件循环并启动线程完成信号。

2. Worker finished 信号连接到worker deleteLater()。根据deleteLater()文档

计划删除此对象。 当控制返回事件循环时，该对象将被删除。 如果事件循环没有运行 调用此函数时（例如，deleteLater() 在 QCoreApplication::exec()) 之前的对象，该对象将被删除 一旦事件循环开始。

注意进入和离开一个新的 事件循环（例如，通过打开模式对话框）不会执行 延迟删除；对于要删除的对象，控件必须 返回到调用 deleteLater() 的事件循环。

注意：它 多次调用此函数是安全的；当第一次延期 删除事件被传递，对象的任何未决事件都是 从事件队列中移除。**

所以当没有事件循环时，由于线程已经退出并且它已经发出了完成信号，我们将不再再次启动同一个线程。在这种情况下，deleteLater() 将永远不会被处理，因为事件循环不存在并且工作对象根本不会被删除。这不会造成内存泄漏吗？

 connect(worker, SIGNAL(finished()), worker, SLOT(deleteLater()));
 connect(worker, SIGNAL(finished()), thread, SLOT(quit()));

如果我们认为交换两条线可以解决问题，那么我还有另一个问题。 QT 明确指出发出信号时调用槽的顺序是不确定的

上面提到的文章链接中有一堆cmets。连作者都无法完整回答这个问题

Answer 1

QThread 将在发送完成信号后执行一个事件类型为 QEvent::DeferredDelete 的QCoreApplication::sendPostedEvents

也就是说QThread会收集所有待处理的deleteLater，并在run返回后执行

来源：https://qt.gitorious.org/qt/qtbase/source/c657bb7b51115d6e1719166fb502bb0ca1dcf4e8:src/corelib/thread/qthread_win.cpp#L363-462

Answer 2

//! put the following code in constructor
QThread *thread = new QThread;
//! type of m_weakThread is QWeakPointer<QThread>
m_weakThread = thread;
Worker *worker = new Worker;
//! type of m_weakWorker is QWeakPointer<Worker>
m_weakWorker = worker;
worker->moveToThread(thread);
connect(worker, SIGNAL(error(QString)), this, SLOT(errorString(QString)));
connect(thread, SIGNAL(started()), worker, SLOT(process()));
connect(worker, SIGNAL(finished()), worker, SLOT(deleteLater()));
//! instead of finished() signal, connect destroyed() signal to thread's quit() slot
connect(worker, SIGNAL(destroyed()), thread, SLOT(quit()));
connect(thread, SIGNAL(finished()), thread, SLOT(deleteLater()));
thread->start();

//! put the following code in destructor
if (!m_weakThread.isNull()) {
    QThread *thread = m_weakThread.data();
    if (thread->isRunning()) {
        thread->quit();
        thread->wait();
    }
}
if (!m_weakWorker.isNull()) {
    Worker *worker = m_weakWorker.data();
    m_weakWorker.clear();   //! optional, a little optimization
    //! it's safe to release worker since the secondary thread exits
    delete worker;
}
if (!m_weakThread.isNull()) {
    QThread *thread = m_weakThread.data();
    m_weakThread.clear();
    //! it's safe to release thread since it exits and all objects in it has released
    delete thread;
}