无法在QT 4.8中存储从url下载的文件

#include <QCoreApplication>
#include <QNetworkAccessManager>
#include <QNetworkRequest>
#include <QNetworkReply>
#include <QFile>

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    QNetworkAccessManager manager;
    QNetworkRequest request(QUrl("http://example.com/file.txt"));
    QNetworkReply *reply = manager.get(request);

    QEventLoop loop;
    QObject::connect(reply, SIGNAL(finished()), &loop, SLOT(quit()));
    loop.exec();

    if (reply->error() == QNetworkReply::NoError) {
        QFile file("file.txt");
        if (file.open(QIODevice::WriteOnly)) {
            file.write(reply->readAll());
            file.close();
            qDebug() << "File downloaded and saved successfully.";
        }
    } else {
        qDebug() << "Error: " << reply->errorString();
    }

    reply->deleteLater();

    return a.exec();
}