Qt5 Tutorial QNetworkAccessManager - Downloading Files - 2020

In this tutorial, we will learn how to download a file using QNetworkAccessManager.

Note: Qt5 document

The QFtp, QUrlInfo, QHttp classes are not public anymore (QHttp has been discouraged since Qt 4.7). Use QNetworkAccessManager instead to avoid binary breaks in the future. Programs that require raw FTP/HTTP streams, can use the compatibility add-ons QtFtp and QtHttp which provides the QFtp and QHttp classes as they existed in Qt 4.

We'll start with Qt Console Application.

First, we need to add network module to our project file, QHpptDownload.pro:

QT       += core
QT       += network
QT       -= gui

TARGET = QHttpDownload
CONFIG   += console
CONFIG   -= app_bundle

TEMPLATE = app


SOURCES += main.cpp

Then, we want to create a new class called Downloader.

In general, this tutorial is almost the same with the one of my C++ tutorials Sockets - Server and Client using Qt : Http Download. However, this tutorial is using NetworkAccessManager instead of QHttp as recommended by Qt5.

Now, let's work on our header file, downloader.h.

We want to include <QNetworkAccessManager>, <QNetworkRequest>, <QNetworkReply>, <QUrl>, <QDateTime>, <QFile>, and <QDebug>. Then, we need to put a reference to QNetworkAccessManager as a private member.

Also, we need to emit a signal, replyFinished().

We want to declare a public function doDownload().

#ifndef DOWNLOADER_H
#define DOWNLOADER_H

#include <QObject>
#include <QNetworkAccessManager>
#include <QNetworkRequest>
#include <QNetworkReply>
#include <QUrl>
#include <QDateTime>
#include <QFile>
#include <QDebug>


class Downloader : public QObject
{
    Q_OBJECT
public:
    explicit Downloader(QObject *parent = 0);

    void doDownload();

signals:

public slots:
    void replyFinished (QNetworkReply *reply);

private:
   QNetworkAccessManager *manager;

};

Lets move on to the implementation:

The whole process will be kick off by doDownload():

void Downloader::doDownload()
{
    manager = new QNetworkAccessManager(this);

    connect(manager, SIGNAL(finished(QNetworkReply*)),
            this, SLOT(replyFinished(QNetworkReply*)));

    manager->get(QNetworkRequest(QUrl("http://bogotobogo.com")));
}

Here is the description for QNetworkAccessManager from Qt 5 document.

The QNetworkAccessManager class allows the application to send network requests and receive replies.

The Network Access API is constructed around one QNetworkAccessManager object, which holds the common configuration and settings for the requests it sends. It contains the proxy and cache configuration, as well as the signals related to such issues, and reply signals that can be used to monitor the progress of a network operation. One QNetworkAccessManager should be enough for the whole Qt application.

Once a QNetworkAccessManager object has been created, the application can use it to send requests over the network. A group of standard functions are supplied that take a request and optional data, and each return a QNetworkReply object. The returned object is used to obtain any data returned in response to the corresponding request.

The doDownload() function is called from main(), and the slot replyFinished() is the place we do whatever we want with the QNetworkReply *:

void Downloader::replyFinished (QNetworkReply *reply)
{
    if(reply->error())
    {
        qDebug() << "ERROR!";
        qDebug() << reply->errorString();
    }
    else
    {
        qDebug() << reply->header(QNetworkRequest::ContentTypeHeader).toString();
        qDebug() << reply->header(QNetworkRequest::LastModifiedHeader).toDateTime().toString();;
        qDebug() << reply->header(QNetworkRequest::ContentLengthHeader).toULongLong();
        qDebug() << reply->attribute(QNetworkRequest::HttpStatusCodeAttribute).toInt();
        qDebug() << reply->attribute(QNetworkRequest::HttpReasonPhraseAttribute).toString();

        QFile *file = new QFile("C:/Qt/Dummy/downloaded.txt");
        if(file->open(QFile::Append))
        {
            file->write(reply->readAll());
            file->flush();
            file->close();
        }
        delete file;
    }

    reply->deleteLater();
}

For more information on enums such as
QNetworkRequest::Attribute,
QNetworkRequest::CacheLoadControl,
and QNetworkRequest::KnownHeaders,
visit http://qt-project.org/doc/qt-5.0/qtnetwork/qnetworkrequest.html.

Here are the files used in this tutorial.

main.cpp:

#include <QCoreApplication>
#include "downloader.h"

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    
    Downloader d;
    d.doDownload();

    return a.exec();
}

downloader.h:

#ifndef DOWNLOADER_H
#define DOWNLOADER_H

#include <QObject>
#include <QNetworkAccessManager>
#include <QNetworkRequest>
#include <QNetworkReply>
#include <QUrl>
#include <QDateTime>
#include <QFile>
#include <QDebug>

class Downloader : public QObject
{
    Q_OBJECT
public:
    explicit Downloader(QObject *parent = 0);

    void doDownload();

signals:
    
public slots:
    void replyFinished (QNetworkReply *reply);

private:
   QNetworkAccessManager *manager;
    
};

#endif // DOWNLOADER_H

downloader.cpp:

#include "downloader.h"

Downloader::Downloader(QObject *parent) :
    QObject(parent)
{
}

void Downloader::doDownload()
{
    manager = new QNetworkAccessManager(this);

    connect(manager, SIGNAL(finished(QNetworkReply*)),
            this, SLOT(replyFinished(QNetworkReply*)));

    manager->get(QNetworkRequest(QUrl("http://bogotobogo.com")));
}

void Downloader::replyFinished (QNetworkReply *reply)
{
    if(reply->error())
    {
        qDebug() << "ERROR!";
        qDebug() << reply->errorString();
    }
    else
    {
        qDebug() << reply->header(QNetworkRequest::ContentTypeHeader).toString();
        qDebug() << reply->header(QNetworkRequest::LastModifiedHeader).toDateTime().toString();
        qDebug() << reply->header(QNetworkRequest::ContentLengthHeader).toULongLong();
        qDebug() << reply->attribute(QNetworkRequest::HttpStatusCodeAttribute).toInt();
        qDebug() << reply->attribute(QNetworkRequest::HttpReasonPhraseAttribute).toString();

        QFile *file = new QFile("C:/Qt/Dummy/downloaded.txt");
        if(file->open(QFile::Append))
        {
            file->write(reply->readAll());
            file->flush();
            file->close();
        }
        delete file;
    }

    reply->deleteLater();
}

If we run the code:

"text/html"
"Sun Dec 23 00:11:02 2012"
0
200
"OK"

If not successful. Otherwise, it will print our error message from reply->error().

For extended version using UI and QProgressDialog, please visit Http File Download with UI and QProgressDialog.

Qt 5 Tutorial

1. Hello World
2. Signals and Slots
3. Q_OBJECT Macro
4. MainWindow and Action
5. MainWindow and ImageViewer using Designer A
6. MainWindow and ImageViewer using Designer B
7. Layouts
8. Layouts without Designer
9. Grid Layouts
10. Splitter
11. QDir
12. QFile (Basic)
13. Resource Files (.qrc)
14. QComboBox
15. QListWidget
16. QTreeWidget
17. QAction and Icon Resources
18. QStatusBar
19. QMessageBox
20. QTimer
21. QList
22. QListIterator
23. QMutableListIterator
24. QLinkedList
25. QMap
26. QHash
27. QStringList
28. QTextStream
29. QMimeType and QMimeDatabase
30. QFile (Serialization I)
31. QFile (Serialization II - Class)
32. Tool Tips in HTML Style and with Resource Images
33. QPainter
34. QBrush and QRect
35. QPainterPath and QPolygon
36. QPen and Cap Style
37. QBrush and QGradient
38. QPainter and Transformations
39. QGraphicsView and QGraphicsScene
40. Customizing Items by inheriting QGraphicsItem
41. QGraphicsView Animation
42. FFmpeg Converter using QProcess
43. QProgress Dialog - Modal and Modeless
44. QVariant and QMetaType
45. QtXML - Writing to a file
46. QtXML - QtXML DOM Reading
47. QThreads - Introduction
48. QThreads - Creating Threads
49. Creating QThreads using QtConcurrent
50. QThreads - Priority
51. QThreads - QMutex
52. QThreads - GuiThread
53. QtConcurrent QProgressDialog with QFutureWatcher
54. QSemaphores - Producer and Consumer
55. QThreads - wait()
56. MVC - ModelView with QListView and QStringListModel
57. MVC - ModelView with QTreeView and QDirModel
58. MVC - ModelView with QTreeView and QFileSystemModel
59. MVC - ModelView with QTableView and QItemDelegate
60. QHttp - Downloading Files
61. QNetworkAccessManager and QNetworkRequest - Downloading Files
62. Qt's Network Download Example - Reconstructed
63. QNetworkAccessManager - Downloading Files with UI and QProgressDialog
64. QUdpSocket
65. QTcpSocket
66. QTcpSocket with Signals and Slots
67. QTcpServer - Client and Server
68. QTcpServer - Loopback Dialog
69. QTcpServer - Client and Server using MultiThreading
70. QTcpServer - Client and Server using QThreadPool
71. Asynchronous QTcpServer - Client and Server using QThreadPool
72. Qt Quick2 QML Animation - A
73. Qt Quick2 QML Animation - B
74. Short note on Ubuntu Install
75. OpenGL with QT5
76. Qt5 Webkit : Web Browser with QtCreator using QWebView Part A
77. Qt5 Webkit : Web Browser with QtCreator using QWebView Part B
78. Video Player with HTML5 QWebView and FFmpeg Converter
79. Qt5 Add-in and Visual Studio 2012
80. Qt5.3 Installation on Ubuntu 14.04
81. Qt5.5 Installation on Ubuntu 14.04
82. Short note on deploying to Windows