Qt C++ unable to connect to QTcpServer using Telnet

Question

I have watched VoidRealm's Youtube video, C++ Qt 67 - QTCPServer - a basic TCP server application and followed his instructions, however, I can't connect to the QTcpServer I created using Telnet.

My Code:

//myserver.h

#ifndef MYSERVER_H
#define MYSERVER_H

#include <QObject>
#include <QDebug>
#include <QTcpServer>
#include <QTcpSocket>

class MyServer : public QObject
{
    Q_OBJECT
public:
    explicit MyServer(QObject *parent = nullptr);

    void newConnection();
signals:

private:
    QTcpServer *server;
};
#endif // MYSERVER_H

//myserver.cpp

#include "myserver.h"

MyServer::MyServer(QObject *parent)
    : QObject{parent}
{
    server = new QTcpServer(this);

    connect(server,SIGNAL(newConnection()),this,SLOT(newConnection()));

    if(!server->listen(QHostAddress::Any, 1234))
    {
        qDebug() << "Server could not start!";
    }else{
        qDebug() << "Server started!";
    }
}

void MyServer::newConnection()
{
    QTcpSocket *socket = server->nextPendingConnection();

    socket->write("hello client\r\n");
    socket->flush();

    socket->waitForBytesWritten(3000);

    socket->close();
}

Running this code, in the Console screen I get

qt.core.qobject.connect: QObject::connect: No such slot MyServer::newConnection() in ..\First_Server\myserver.cpp:8
Server started!

but when I open a command prompt and do the following steps:

...>telnet

Welcome to Microsoft Telnet Client

Escape Character is 'CTRL+]'

Microsoft Telnet> open 127.0.0.1 1234
Connecting To 127.0.0.1...

It is not connecting. Can anyone tell me what I have done wrong? I am using Qt 6.3.0.

Your console msg told you: "no such slot". Looks like you have a typo and forgot `public slots:` section. That's because you use old connection syntax, with the new one, it is no longer a requirement, but still good to have for clarity. https://stackoverflow.com/questions/47196736/are-public-slots-sections-still-necessary-in-qt5 — pptaszni, Jul 29 '22 at 07:21
As for telnet connection, technically should work, but you are on windows, so ... I assume that in `main` you have `QApplication::exec`? — pptaszni, Jul 29 '22 at 07:24

score 1 · Accepted Answer · answered Jul 29 '22 at 07:50

You error message indicates the issue:

qt.core.qobject.connect: QObject::connect: No such slot MyServer::newConnection() in ..\First_Server\myserver.cpp:8 Server started!

You have created the connection between the signal and slot here.

connect(server,SIGNAL(newConnection()),this,SLOT(newConnection()));

First of all, it is not a good idea to have the same slot name as the signal as it becomes confusing. I would call your slot as "handleNewConnection" for example. Secondly, you ought to use the "new" signal-slot syntax. Thirdly, and most importantly, you have not declared your newConnection() method to be a slot. You would need to declare it so in your header file:

private Q_SLOTS:
    void newConnection();

You can also make the slots public if it has to be accessed from outside, it looks like that you would only use it privately here.

Excellent idea to suggest the "new" (well... it was introduced in Qt 5? or late Qt 4?) Signal-slot syntax as it helps avoid all kind of headaches ("Why the hell does it not work...? *5 hours later* Ah, typo in the name for the SLOT() macro.... ARGH) - maybe you could include an example to help get the idea across? — CharonX, Jul 29 '22 at 07:56

Qt C++ unable to connect to QTcpServer using Telnet

1 Answers1