我怎么能从互联网上下载文件在Qt

我不知道是什么问题…编译器(Qt)运行程序没有错误，但文件没有下载…你能告诉我出什么事了吗?

我以"下载"为例，它位于qt文件夹中。唯一的区别是他们有一个控制台，而我有一个windows应用程序。

mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <QtNetwork>
#include <QMainWindow>
namespace Ui {
    class MainWindow;
}
class MainWindow : public QMainWindow
{
    Q_OBJECT
public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();
private slots:
    void on_pushButton_clicked();
private:
    Ui::MainWindow *ui;
};
class DownloadManager: public QObject
{
    Q_OBJECT
    QNetworkAccessManager manager;
    QList<QNetworkReply *> currentDownloads;
public:
    DownloadManager();
    void doDownload(const QUrl &url);
    QString saveFileName(const QUrl &url);
    bool saveToDisk(const QString &filename, QIODevice *data);
public slots:
    void execute();
    void downloadFinished(QNetworkReply *reply);
};

#endif // MAINWINDOW_H

main.cpp

#include <QtGui/QApplication>
#include "mainwindow.h"
#include <QtNetwork>
#include <QNetworkAccessManager>
#include <QStringList>
#include <QCoreApplication>
#include <QFile>
#include <QFileInfo>
#include <QList>
#include <QNetworkAccessManager>
#include <QNetworkRequest>
#include <QNetworkReply>
#include <QStringList>
#include <QTimer>
#include <QUrl>
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
#if defined(Q_WS_S60)
    w.showMaximized();
#else
    w.show();
#endif
    return a.exec();
}
DownloadManager::DownloadManager()
{
    connect(&manager, SIGNAL(finished(QNetworkReply*)),
            SLOT(downloadFinished(QNetworkReply*)));
}
void DownloadManager::doDownload(const QUrl &url)
{
    QNetworkRequest request(url);
    QNetworkReply *reply = manager.get(request);
    currentDownloads.append(reply);
}
QString DownloadManager::saveFileName(const QUrl &url)
{
    QString path = url.path();
    QString basename = QFileInfo(path).fileName();
    if (basename.isEmpty())
        basename = "download";
    if (QFile::exists(basename)) {
        // already exists, don't overwrite
        int i = 0;
        basename += '.';
        while (QFile::exists(basename + QString::number(i)))
            ++i;
        basename += QString::number(i);
    }
    return basename;
}
bool DownloadManager::saveToDisk(const QString &filename, QIODevice *data)
{
    QFile file(filename);
    if (!file.open(QIODevice::WriteOnly)) {
        fprintf(stderr, "Could not open %s for writing: %sn",
                qPrintable(filename),
                qPrintable(file.errorString()));
        return false;
    }
    file.write(data->readAll());
    file.close();
    return true;
}
void DownloadManager::execute()
{
    QStringList args = QCoreApplication::instance()->arguments();
    args[0]="http://www.google.ru/images/srpr/logo3w.png";
QString arg=args[0];
        QUrl url = QUrl::fromEncoded(arg.toLocal8Bit());
        doDownload(url);
}
void DownloadManager::downloadFinished(QNetworkReply *reply)
{
    QUrl url = reply->url();
    if (reply->error()) {
        fprintf(stderr, "Download of %s failed: %sn",
                url.toEncoded().constData(),
                qPrintable(reply->errorString()));
    } else {
        QString filename = saveFileName(url);
        if (saveToDisk(filename, reply))
            printf("Download of %s succeded (saved to %s)n",
                   url.toEncoded().constData(), qPrintable(filename));
    }
    currentDownloads.removeAll(reply);
    reply->deleteLater();
    if (currentDownloads.isEmpty())
      //   all downloads finished
        QCoreApplication::instance()->quit();
}

mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
}
MainWindow::~MainWindow()
{
    delete ui;
}

void MainWindow::on_pushButton_clicked()
{
    DownloadManager manager;
    manager.execute();
    QTimer::singleShot(0, &manager, SLOT(execute()));
}