Failed to start QThread when using QNetworkAccessManager

QNetworkAccessManager::finished signal is documented to be emitted whenever a pending network reply is finished.

This means that if the QNetworkAccessManager is used to run multiple requests at a time (and this is perfectly normal usage). finished signal will be emitted once for every request. Since you have a shared instance of QNetworkAccessManager between your FileDownloader objects, the finished signal gets emitted for every get call you have made. So, all the FileDownloader objects get a finished signal as soon as the first FileDownloader finishes downloading.

Instead of using QNetworkAccessManager::finished, you can use QNetworkReply::finished to avoid mixing up signals. Here is an example implementation:

#include <QtNetwork>
#include <QtWidgets>

class FileDownloader : public QObject
{
    Q_OBJECT
    //using constructor injection instead of a static QNetworkAccessManager pointer
    //This allows to share the same QNetworkAccessManager
    //object with other classes utilizing network access
    QNetworkAccessManager* m_nam;
    QNetworkReply* m_reply;
    QByteArray m_downloadedData;

public:
    explicit FileDownloader(QUrl imageUrl, QNetworkAccessManager* nam,
                            QObject* parent= nullptr)
        :QObject(parent), m_nam(nam)
    {
        QNetworkRequest request(imageUrl);
        m_reply = m_nam->get(request);
        connect(m_reply, &QNetworkReply::finished, this, &FileDownloader::fileDownloaded);
    }
    ~FileDownloader() = default;

    QByteArray downloadedData()const{return m_downloadedData;}

signals:
    void downloaded();
private slots:
    void fileDownloaded(){
        m_downloadedData= m_reply->readAll();
        m_reply->deleteLater();
        emit downloaded();
    }
};

//sample usage
int main(int argc, char* argv[]){
    QApplication a(argc, argv);

    QNetworkAccessManager nam;
    FileDownloader fileDownloader(QUrl("http://i.imgur.com/Rt8fqpt.png"), &nam);
    QLabel label;
    label.setAlignment(Qt::AlignCenter);
    label.setText("Downloading. . .");
    label.setMinimumSize(640, 480);
    label.show();
    QObject::connect(&fileDownloader, &FileDownloader::downloaded, [&]{
        QPixmap pixmap;
        pixmap.loadFromData(fileDownloader.downloadedData());
        label.setPixmap(pixmap);
    });

    return a.exec();
}

#include "main.moc"

^{If you are using this method to download large files, consider having a look at this question.}