void TripServer::incomingConnection(int socketId)

qintptr is different from int. Here's the documentiation on incommingConnection().

You also need to adjust incomingConnection method adding the line addPendingConnection(socket); to comply with http://doc.qt.io/qt-5/qtcpserver.html#incomingConnection

Note: If another socket is created in the reimplementation of this method, it needs to be added to the Pending Connections mechanism by calling addPendingConnection().

Make sure you start the server first and then the client

#ifndef TRIPPLANNER_H
#define TRIPPLANNER_H

#include <QDialog>
#include <QTcpSocket>

#include "ui_tripplanner.h"

class QPushButton;

class TripPlanner : public QDialog, private Ui::TripPlanner
{
    Q_OBJECT

public:
    TripPlanner(QWidget *parent = 0);

private slots:
    void connectToServer();
    void sendRequest();
    void updateTableWidget();
    void stopSearch();
    void connectionClosedByServer();
    void error();

private:
    void closeConnection();

    QPushButton *searchButton;
    QPushButton *stopButton;
    QTcpSocket tcpSocket;
    quint16 nextBlockSize;
};

#endif

tripplanner.cpp

#include <QtWidgets>
#include <QtNetwork>

#include "tripplanner.h"

TripPlanner::TripPlanner(QWidget *parent)
    : QDialog(parent)
{
    setupUi(this);

    searchButton = buttonBox->addButton(tr("&Search"),
                                        QDialogButtonBox::ActionRole);
    stopButton = buttonBox->addButton(tr("S&top"),
                                      QDialogButtonBox::ActionRole);
    stopButton->setEnabled(false);
    buttonBox->button(QDialogButtonBox::Close)->setText(tr("&Quit"));

    QDateTime dateTime = QDateTime::currentDateTime();
    dateEdit->setDate(dateTime.date());
    timeEdit->setTime(QTime(dateTime.time().hour(), 0));

    progressBar->hide();
    progressBar->setSizePolicy(QSizePolicy::Preferred,
                               QSizePolicy::Ignored);

    tableWidget->verticalHeader()->hide();
    tableWidget->setEditTriggers(QAbstractItemView::NoEditTriggers);

    connect(searchButton, SIGNAL(clicked()),
            this, SLOT(connectToServer()));
    connect(stopButton, SIGNAL(clicked()), this, SLOT(stopSearch()));
    connect(buttonBox, SIGNAL(rejected()), this, SLOT(reject()));

    connect(&tcpSocket, SIGNAL(connected()), this, SLOT(sendRequest()));
    connect(&tcpSocket, SIGNAL(disconnected()),
            this, SLOT(connectionClosedByServer()));
    connect(&tcpSocket, SIGNAL(readyRead()),
            this, SLOT(updateTableWidget()));
    connect(&tcpSocket, SIGNAL(error(QAbstractSocket::SocketError)),
            this, SLOT(error()));
}

void TripPlanner::connectToServer()
{
#if 1
    tcpSocket.connectToHost(QHostAddress::LocalHost, 6178);
#else
    tcpSocket.connectToHost("tripserver.zugbahn.de", 6178);
#endif

    tableWidget->setRowCount(0);
    searchButton->setEnabled(false);
    stopButton->setEnabled(true);
    statusLabel->setText(tr("Connecting to server..."));
    progressBar->show();

    nextBlockSize = 0;
}

void TripPlanner::sendRequest()
{
    QByteArray block;
    QDataStream out(&block, QIODevice::WriteOnly);
    out.setVersion(QDataStream::Qt_4_3);
    out << quint16(0) << quint8('S') << fromComboBox->currentText()
        << toComboBox->currentText() << dateEdit->date()
        << timeEdit->time();

    if (departureRadioButton->isChecked()) {
        out << quint8('D');
    } else {
        out << quint8('A');
    }
    out.device()->seek(0);
    out << quint16(block.size() - sizeof(quint16));
    tcpSocket.write(block);

    statusLabel->setText(tr("Sending request..."));
}

void TripPlanner::updateTableWidget()
{
    QDataStream in(&tcpSocket);
    in.setVersion(QDataStream::Qt_4_3);

    forever {
        int row = tableWidget->rowCount();

        if (nextBlockSize == 0) {
            if (tcpSocket.bytesAvailable() < sizeof(quint16))
                break;
            in >> nextBlockSize;
        }

        if (nextBlockSize == 0xFFFF) {
            closeConnection();
            statusLabel->setText(tr("Found %1 trip(s)").arg(row));
            break;
        }

        if (tcpSocket.bytesAvailable() < nextBlockSize)
            break;

        QDate date;
        QTime departureTime;
        QTime arrivalTime;
        quint16 duration;
        quint8 changes;
        QString trainType;

        in >> date >> departureTime >> duration >> changes >> trainType;
        arrivalTime = departureTime.addSecs(duration * 60);

        tableWidget->setRowCount(row + 1);

        QStringList fields;
        fields << date.toString(Qt::LocalDate)
               << departureTime.toString(tr("hh:mm"))
               << arrivalTime.toString(tr("hh:mm"))
               << tr("%1 hr %2 min").arg(duration / 60)
                                    .arg(duration % 60)
               << QString::number(changes)
               << trainType;
        for (int i = 0; i < fields.count(); ++i)
            tableWidget->setItem(row, i,
                                 new QTableWidgetItem(fields[i]));
        nextBlockSize = 0;
    }
}

void TripPlanner::stopSearch()
{
    statusLabel->setText(tr("Search stopped"));
    closeConnection();
}

void TripPlanner::connectionClosedByServer()
{
    if (nextBlockSize != 0xFFFF)
        statusLabel->setText(tr("Error: Connection closed by server"));
    closeConnection();
}

void TripPlanner::error()
{
    statusLabel->setText(tcpSocket.errorString());
    closeConnection();
}

void TripPlanner::closeConnection()
{
    tcpSocket.close();
    searchButton->setEnabled(true);
    stopButton->setEnabled(false);
    progressBar->hide();
}

tripplanner.pro

TEMPLATE      = app
QT           += network widgets
HEADERS       = tripplanner.h
SOURCES       = main.cpp \
                tripplanner.cpp
FORMS         = tripplanner.ui

main.cpp

#include <QApplication>

#include "tripplanner.h"

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    TripPlanner tripPlanner;
    tripPlanner.show();
    return app.exec();
}

tripplanner.ui

<ui version="4.0" >
 <class>TripPlanner</class>
 <widget class="QDialog" name="TripPlanner" >
  <property name="geometry" >
   <rect>
    <x>0</x>
    <y>0</y>
    <width>382</width>
    <height>369</height>
   </rect>
  </property>
  <property name="windowTitle" >
   <string>Trip Planner</string>
  </property>
  <layout class="QGridLayout" >
   <item row="0" column="0" >
    <widget class="QGroupBox" name="tripInfoGroupBox" >
     <property name="title" >
      <string>Trip Information</string>
     </property>
     <layout class="QGridLayout" >
      <item row="3" column="0" >
       <widget class="QLabel" name="timeLabel" >
        <property name="text" >
         <string>Appro&amp;ximate Time:</string>
        </property>
        <property name="buddy" >
         <cstring>timeEdit</cstring>
        </property>
       </widget>
      </item>
      <item row="2" column="1" >
       <widget class="QDateTimeEdit" name="dateEdit" >
        <property name="displayFormat" >
         <string>yyyy-MM-dd</string>
        </property>
       </widget>
      </item>
      <item row="3" column="1" >
       <widget class="QDateTimeEdit" name="timeEdit" >
        <property name="displayFormat" >
         <string>hh:mm:ss</string>
        </property>
       </widget>
      </item>
      <item row="2" column="0" >
       <widget class="QLabel" name="dateLabel" >
        <property name="text" >
         <string>&amp;Date:</string>
        </property>
        <property name="buddy" >
         <cstring>dateEdit</cstring>
        </property>
       </widget>
      </item>
      <item row="1" column="0" >
       <widget class="QLabel" name="toLabel" >
        <property name="text" >
         <string>&amp;To:</string>
        </property>
        <property name="buddy" >
         <cstring>toComboBox</cstring>
        </property>
       </widget>
      </item>
      <item row="4" column="0" colspan="2" >
       <layout class="QHBoxLayout" >
        <item>
         <widget class="QRadioButton" name="departureRadioButton" >
          <property name="text" >
           <string>D&amp;eparture</string>
          </property>
          <property name="shortcut" >
           <string>Alt+E</string>
          </property>
          <property name="checked" >
           <bool>true</bool>
          </property>
         </widget>
        </item>
        <item>
         <widget class="QRadioButton" name="arrivalRadioButton" >
          <property name="text" >
           <string>&amp;Arrival</string>
          </property>
          <property name="shortcut" >
           <string>Alt+A</string>
          </property>
         </widget>
        </item>
       </layout>
      </item>
      <item row="0" column="1" >
       <widget class="QComboBox" name="fromComboBox" >
        <property name="currentIndex" >
         <number>0</number>
        </property>
        <item>
         <property name="text" >
          <string>Berlin</string>
         </property>
        </item>
        <item>
         <property name="text" >
          <string>Bonn</string>
         </property>
        </item>
        <item>
         <property name="text" >
          <string>Bremen</string>
         </property>
        </item>
        <item>
         <property name="text" >
          <string>Dresden</string>
         </property>
        </item>
        <item>
         <property name="text" >
          <string>Düsseldorf</string>
         </property>
        </item>
        <item>
         <property name="text" >
          <string>Dortmund</string>
         </property>
        </item>
        <item>
         <property name="text" >
          <string>Frankfurt am Main</string>
         </property>
        </item>
        <item>
         <property name="text" >
          <string>Hannover</string>
         </property>
        </item>
        <item>
         <property name="text" >
          <string>Hamburg</string>
         </property>
        </item>
        <item>
         <property name="text" >
          <string>München</string>
         </property>
        </item>
        <item>
         <property name="text" >
          <string>Nürnberg</string>
         </property>
        </item>
        <item>
         <property name="text" >
          <string>Rostock</string>
         </property>
        </item>
        <item>
         <property name="text" >
          <string>Stuttgart</string>
         </property>
        </item>
       </widget>
      </item>
      <item row="1" column="1" >
       <widget class="QComboBox" name="toComboBox" >
        <property name="currentIndex" >
         <number>0</number>
        </property>
        <item>
         <property name="text" >
          <string>Berlin</string>
         </property>
        </item>
        <item>
         <property name="text" >
          <string>Bonn</string>
         </property>
        </item>
        <item>
         <property name="text" >
          <string>Bremen</string>
         </property>
        </item>
        <item>
         <property name="text" >
          <string>Dresden</string>
         </property>
        </item>
        <item>
         <property name="text" >
          <string>Düsseldorf</string>
         </property>
        </item>
        <item>
         <property name="text" >
          <string>Dortmund</string>
         </property>
        </item>
        <item>
         <property name="text" >
          <string>Frankfurt am Main</string>
         </property>
        </item>
        <item>
         <property name="text" >
          <string>Hannover</string>
         </property>
        </item>
        <item>
         <property name="text" >
          <string>Hamburg</string>
         </property>
        </item>
        <item>
         <property name="text" >
          <string>München</string>
         </property>
        </item>
        <item>
         <property name="text" >
          <string>Nürnberg</string>
         </property>
        </item>
        <item>
         <property name="text" >
          <string>Rostock</string>
         </property>
        </item>
        <item>
         <property name="text" >
          <string>Stuttgart</string>
         </property>
        </item>
       </widget>
      </item>
      <item row="0" column="0" >
       <widget class="QLabel" name="fromLabel" >
        <property name="text" >
         <string>&amp;From:</string>
        </property>
        <property name="buddy" >
         <cstring>fromComboBox</cstring>
        </property>
       </widget>
      </item>
     </layout>
    </widget>
   </item>
   <item rowspan="3" row="0" column="1" >
    <widget class="QDialogButtonBox" name="buttonBox" >
     <property name="orientation" >
      <enum>Qt::Vertical</enum>
     </property>
     <property name="standardButtons" >
      <set>QDialogButtonBox::Close</set>
     </property>
    </widget>
   </item>
   <item row="1" column="0" >
    <widget class="QTableWidget" name="tableWidget" >
     <property name="columnCount" >
      <number>6</number>
     </property>
     <column>
      <property name="text" >
       <string>Date</string>
      </property>
     </column>
     <column>
      <property name="text" >
       <string>Departure</string>
      </property>
     </column>
     <column>
      <property name="text" >
       <string>Arrival</string>
      </property>
     </column>
     <column>
      <property name="text" >
       <string>Duration</string>
      </property>
     </column>
     <column>
      <property name="text" >
       <string>Changes</string>
      </property>
     </column>
     <column>
      <property name="text" >
       <string>Train type</string>
      </property>
     </column>
    </widget>
   </item>
   <item row="2" column="0" >
    <layout class="QHBoxLayout" >
     <item>
      <widget class="QLabel" name="statusLabel" >
       <property name="sizePolicy" >
        <sizepolicy vsizetype="Preferred" hsizetype="Expanding" >
         <horstretch>0</horstretch>
         <verstretch>0</verstretch>
        </sizepolicy>
       </property>
       <property name="text" >
        <string>Ready</string>
       </property>
      </widget>
     </item>
     <item>
      <widget class="QProgressBar" name="progressBar" >
       <property name="minimum" >
        <number>0</number>
       </property>
       <property name="maximum" >
        <number>100</number>
       </property>
       <property name="orientation" >
        <enum>Qt::Horizontal</enum>
       </property>
      </widget>
     </item>
    </layout>
   </item>
  </layout>
 </widget>
 <resources/>
 <connections/>
</ui>

TEMPLATE      = app
QT           += network widgets
HEADERS       = clientsocket.h \
                tripserver.h
SOURCES       = clientsocket.cpp \
                main.cpp \
                tripserver.cpp

tripserver.h

#ifndef TRIPSERVER_H
#define TRIPSERVER_H

#include <QTcpServer>

class TripServer : public QTcpServer
{
    Q_OBJECT

public:
    TripServer(QObject *parent = 0);

private:
    void incomingConnection(int socketId);
};

#endif

tripserver.cpp

#include <QtCore>

#include "clientsocket.h"
#include "tripserver.h"

TripServer::TripServer(QObject *parent)
    : QTcpServer(parent)
{
}

void TripServer::incomingConnection(int socketId)
{
    ClientSocket *socket = new ClientSocket(this);
    socket->setSocketDescriptor(socketId);
}

clientsocket.h

#ifndef CLIENTSOCKET_H
#define CLIENTSOCKET_H

#include <QTcpSocket>

class QDate;
class QTime;

class ClientSocket : public QTcpSocket
{
    Q_OBJECT

public:
    ClientSocket(QObject *parent = 0);

private slots:
    void readClient();

private:
    void generateRandomTrip(const QString &from, const QString &to,
                            const QDate &date, const QTime &time);

    quint16 nextBlockSize;
};

#endif

clientsocket.cpp

#include <QtNetwork>

#include "clientsocket.h"

ClientSocket::ClientSocket(QObject *parent)
    : QTcpSocket(parent)
{
    connect(this, SIGNAL(readyRead()), this, SLOT(readClient()));
    connect(this, SIGNAL(disconnected()), this, SLOT(deleteLater()));

    nextBlockSize = 0;
}

void ClientSocket::readClient()
{
    QDataStream in(this);
    in.setVersion(QDataStream::Qt_4_3);

    if (nextBlockSize == 0) {
        if (bytesAvailable() < sizeof(quint16))
            return;
        in >> nextBlockSize;
    }

    if (bytesAvailable() < nextBlockSize)
        return;

    quint8 requestType;
    QString from;
    QString to;
    QDate date;
    QTime time;
    quint8 flag;

    in >> requestType;
    if (requestType == 'S') {
        in >> from >> to >> date >> time >> flag;

        std::srand(from.length() * 3600 + to.length() * 60
                   + time.hour());
        int numTrips = std::rand() % 8;
        for (int i = 0; i < numTrips; ++i)
            generateRandomTrip(from, to, date, time);

        QDataStream out(this);
        out << quint16(0xFFFF);
    }

    close();
}

void ClientSocket::generateRandomTrip(const QString & /* from */,
        const QString & /* to */, const QDate &date, const QTime &time)
{
    QByteArray block;
    QDataStream out(&block, QIODevice::WriteOnly);
    out.setVersion(QDataStream::Qt_4_3);
    quint16 duration = std::rand() % 200;
    out << quint16(0) << date << time << duration << quint8(1)
        << QString("InterCity");
    out.device()->seek(0);
    out << quint16(block.size() - sizeof(quint16));
    write(block);
}

main.cpp

#include <QtWidgets>
#include <iostream>

#include "tripserver.h"

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    TripServer server;
    if (!server.listen(QHostAddress::Any, 6178)) {
        std::cerr << "Failed to bind to port" << std::endl;
        return 1;
    }

    QPushButton quitButton(QObject::tr("&Quit"));
    quitButton.setWindowTitle(QObject::tr("Trip Server"));
    QObject::connect(&quitButton, SIGNAL(clicked()),
                     &app, SLOT(quit()));
    quitButton.show();
    return app.exec();
}

Also from your clip I gathered you don't use a debugger, install and use a debugger, that's what it was invented for - to track down programming errors.

It is a very good book to learn Qt. I thought somebody would have been interested.

Does QTcpSocket work at all?

Yes, since I've started using Qt - about 10 years ago. My suspicion it was working long before that too ...

Did somebody use for something usefull so far?

Thousands upon thousands of people I'd assume.

@jalomann said in Is QTcpSocket example working:

I didn't want to paste the whole code here because it is in nice package.

Paste only the relevant part - where the connection is supposed to be established, then we can ask for more information if it's needed.

As @jsulm said, can u provide the code, so the users in the forum can come to know.
Regarding the above mentioned Topic.

Thanks,

I see a lot of questions on forum about QTcpSocket.
So the question is: Does QTcpSocket work at all? Did somebody
use for something usefull so far?