Qt Network

Introduction#

Qt Network provide tools to easily use many network protocols in your application.

TCP Client

To create a TCP connection in Qt, we will use QTcpSocket. First, we need to connect with connectToHost.

So for example, to connect to a local tcp serveur: _socket.connectToHost(QHostAddress("127.0.0.1"), 4242);

Then, if we need to read datas from the server, we need to connect the signal readyRead with a slot. Like that:

connect(&_socket, SIGNAL(readyRead()), this, SLOT(onReadyRead()));

and finally, we can read the datas like that:

void MainWindow::onReadyRead()
{
    QByteArray datas = _socket.readAll();
    qDebug() << datas;
}

To write datas, you can use the write(QByteArray) method:

_socket.write(QByteArray("ok !\n"));

So a basic TCP Client can look like that:

main.cpp:

#include "mainwindow.h"
#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();

    return a.exec();
}

mainwindow.h:

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QTcpSocket>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();

public slots:
    void onReadyRead();

private:
    Ui::MainWindow *ui;
    QTcpSocket  _socket;
};

#endif // MAINWINDOW_H

mainwindow.cpp:

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

#include <QDebug>
#include <QHostAddress>

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow),
    _socket(this)
{
    ui->setupUi(this);
    _socket.connectToHost(QHostAddress("127.0.0.1"), 4242);
    connect(&_socket, SIGNAL(readyRead()), this, SLOT(onReadyRead()));
}

MainWindow::~MainWindow()
{
    delete ui;
}

void MainWindow::onReadyRead()
{
    QByteArray datas = _socket.readAll();
    qDebug() << datas;
    _socket.write(QByteArray("ok !\n"));
}

mainwindow.ui: (empty here)

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>MainWindow</class>
 <widget class="QMainWindow" name="MainWindow">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>400</width>
    <height>300</height>
   </rect>
  </property>
  <property name="windowTitle">
   <string>MainWindow</string>
  </property>
  <widget class="QWidget" name="centralWidget"/>
  <widget class="QMenuBar" name="menuBar">
   <property name="geometry">
    <rect>
     <x>0</x>
     <y>0</y>
     <width>400</width>
     <height>25</height>
    </rect>
   </property>
  </widget>
  <widget class="QToolBar" name="mainToolBar">
   <attribute name="toolBarArea">
    <enum>TopToolBarArea</enum>
   </attribute>
   <attribute name="toolBarBreak">
    <bool>false</bool>
   </attribute>
  </widget>
  <widget class="QStatusBar" name="statusBar"/>
 </widget>
 <layoutdefault spacing="6" margin="11"/>
 <resources/>
 <connections/>
</ui>

TCP Server

Create a TCP server in Qt is also very easy, indeed, the class QTcpServer already provide all we need to do the server.

First, we need to listen to any ip, a random port and do something when a client is connected. like that:

 _server.listen(QHostAddress::Any, 4242);
 connect(&_server, SIGNAL(newConnection()), this, SLOT(onNewConnection()));

Then, When here is a new connection, we can add it to the client list and prepare to read/write on the socket. Like that:

   QTcpSocket *clientSocket = _server.nextPendingConnection();
   connect(clientSocket, SIGNAL(readyRead()), this, SLOT(onReadyRead()));
   connect(clientSocket, SIGNAL(stateChanged(QAbstractSocket::SocketState)), this, SLOT(onSocketStateChanged(QAbstractSocket::SocketState)));
   _sockets.push_back(clientSocket);

The stateChanged(QAbstractSocket::SocketState) allow us to remove the socket to our list when the client is disconnected.

So here a basic chat server:

main.cpp:

#include "mainwindow.h"
#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();

    return a.exec();
}

mainwindow.h:

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QTcpServer>
#include <QTcpSocket>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();

public slots:
    void onNewConnection();
    void onSocketStateChanged(QAbstractSocket::SocketState socketState);
    void onReadyRead();
private:
    Ui::MainWindow *ui;
    QTcpServer  _server;
    QList<QTcpSocket*>  _sockets;
};

#endif // MAINWINDOW_H

mainwindow.cpp:

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

#include <QDebug>
#include <QHostAddress>
#include <QAbstractSocket>

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow),
    _server(this)
{
    ui->setupUi(this);
    _server.listen(QHostAddress::Any, 4242);
    connect(&_server, SIGNAL(newConnection()), this, SLOT(onNewConnection()));
}

MainWindow::~MainWindow()
{
    delete ui;
}

void MainWindow::onNewConnection()
{
   QTcpSocket *clientSocket = _server.nextPendingConnection();
   connect(clientSocket, SIGNAL(readyRead()), this, SLOT(onReadyRead()));
   connect(clientSocket, SIGNAL(stateChanged(QAbstractSocket::SocketState)), this, SLOT(onSocketStateChanged(QAbstractSocket::SocketState)));

    _sockets.push_back(clientSocket);
    for (QTcpSocket* socket : _sockets) {
        socket->write(QByteArray::fromStdString(clientSocket->peerAddress().toString().toStdString() + " connected to server !\n"));
    }
}

void MainWindow::onSocketStateChanged(QAbstractSocket::SocketState socketState)
{
    if (socketState == QAbstractSocket::UnconnectedState)
    {
        QTcpSocket* sender = static_cast<QTcpSocket*>(QObject::sender());
        _sockets.removeOne(sender);
    }
}

void MainWindow::onReadyRead()
{
    QTcpSocket* sender = static_cast<QTcpSocket*>(QObject::sender());
    QByteArray datas = sender->readAll();
    for (QTcpSocket* socket : _sockets) {
        if (socket != sender)
            socket->write(QByteArray::fromStdString(sender->peerAddress().toString().toStdString() + ": " + datas.toStdString()));
    }
}

(use the same mainwindow.ui that the previous example)