QNetworkAccessManager::get() cause pure virtual method called

I'm trying download web page with QNetworkAccessManager::get() with a separate thread, When I running the program it will crash random (may be one hour or two hours). The code example:

class SpiderThread : public QThread
    void    run();
    bool    _stopped;

void SpiderThread::run()
    while (!_stopped) {
        DownloadManager downloadManager();
        QUrl u = getOneUrl();
        QString content = downloadManager.getContent(u);
        // some other code

QString DownloadManager::getContent(const QUrl &url)
    QEventLoop  loop;
    QNetworkAccessManager manager;

    QNetworkRequest request(url);
    request.setRawHeader("User-Agent", _userAgent.toAscii());
    QNetworkReply *reply = manager.get(request);
    connect(reply, SIGNAL(finished()),
    connect(reply, SIGNAL(finished()),
            &loop, SLOT(quit()));

    if (reply->error() != QNetworkReply::NoError) {
        if (reply->isRunning())
        return QString();

    int httpCode = reply->attribute(QNetworkRequest::HttpStatusCodeAttribute).toInt();

    if (httpCode != 200)
        return QString();
    QByteArray data = reply->readAll();
    return QString(data);

I'm using QEventloop to block the http request until it finshed, When I run these code, It always crash and show error message:

    pure virtual method called
    terminate called without an active exception

the gdb debug info:

    (gdb) bt
    #0  0x0000003c0dc30285 in raise () from /lib64/libc.so.6
    #1  0x0000003c0dc31d30 in abort () from /lib64/libc.so.6
    #2  0x0000003d992bed94 in __gnu_cxx::__verbose_terminate_handler() () from /usr/lib64/libstdc++.so.6
    #3  0x0000003d992bce46 in ?? () from /usr/lib64/libstdc++.so.6
    #4  0x0000003d992bce73 in std::terminate() () from /usr/lib64/libstdc++.so.6
    #5  0x0000003d992bd3cf in __cxa_pure_virtual () from /usr/lib64/libstdc++.so.6
    #6  0x00002aaaabc3cc35 in QCoreApplication::postEvent(QObject*, QEvent*, int) () 
           from /usr/local/Trolltech/Qt-4.8.2/lib/libQtCore.so.4
    #7  0x00002aaaabc5301d in QMetaObject::activate(QObject*, QMetaObject const*, int, void**) () 
           from /usr/local/Trolltech/Qt-4.8.2/lib/libQtCore.so.4
    #8  0x00002aaaab7b81a1 in ?? () from /usr/local/Trolltech/Qt-4.8.2/lib/libQtNetwork.so.4
    #9  0x00002aaaab7ac3ad in ?? () from /usr/local/Trolltech/Qt-4.8.2/lib/libQtNetwork.so.4
    #10 0x00002aaaab7c8d1d in ?? () from /usr/local/Trolltech/Qt-4.8.2/lib/libQtNetwork.so.4
    #11 0x00002aaaab7cadb0 in ?? () from /usr/local/Trolltech/Qt-4.8.2/lib/libQtNetwork.so.4
    #12 0x00002aaaab7a89ce in QNetworkAccessManager::createRequest(QNetworkAccessManager::Operation, QNetworkRequest const&, QIODevice*)               () 
          from /usr/local/Trolltech/Qt-4.8.2/lib/libQtNetwork.so.4
    #13 0x00002aaaab7a5ea4 in QNetworkAccessManager::get(QNetworkRequest const&) ()
                     from /usr/local/Trolltech/Qt-4.8.2/lib/libQtNetwork.so.4
    #14 0x00000000004088fe in DownloadManager::getContent (this=0x409fff40, url=..., toCodec=...)
                         at src/downloadmanager.cpp:55
    #15 0x0000000000415248 in SpiderThread::run (this=0x7fffffffe2b0) at src/spiderthread.cpp:27
    #16 0x00002aaaabb2c3b9 in ?? () from /usr/local/Trolltech/Qt-4.8.2/lib/libQtCore.so.4
    #17 0x0000003c0e40677d in start_thread () from /lib64/libpthread.so.0
    #18 0x0000003c0dcd325d in clone () from /lib64/libc.so.6

Please help me, I spent so much time to find what's wrong,but it still not work.I made some mistake?


You've needlessly complicated things. The default implementation of QThread::run() already creates and runs an event loop. Don't derive from QThread. Put your functionality into a QObject, and call moveToThread(thread) on that object once you have started the thread, if necessary! Use signals and slots to get things done. QNetworkAccessManager already uses a worker thread IIRC, so it's probably pointless to create it in a yet another dedicated thread.

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