I have problem with sender() in QTcpSocket::disconnected signal

[Kirill Gusarev]

Hello. I make server and client apps in C++ Qt.
In server side I store sockets of connected clients in QVector<QTcpSocket*>.
I want to remove socket from QVector when I close a client's window. But in function for signal QTcpSocket::disconnected the sender() returns nullptr and I can't get any information of this lost socket.
Help pls :C
Sorry for mistakes.

[JonB]

@Kirill-Gusarev
If that is so you will need to use lambdas, to which you can pass a parameter of which socket is closing, instead of sender(), which you should avoid if possible.

[SGaist]

Hi and welcome to devnet,

Can you share the code where you handle the new sockets and their disconnection handling ?

[Kirill Gusarev]

@JonB thanks for answer. Can you show me example of this lambda? (I'm just not sure that I understand you correctly)

[JonB]

@Kirill-Gusarev
Actually I don't know why you need to know about the sender here anyway, and hence might have wanted a lambda? Don't you just want to remove this from your QVector<QTcpSocket*> in disconnected slot? @SGaist asked

Can you share the code where you handle the new sockets and their disconnection handling ?

[Kirill Gusarev]

@SGaist Hi!

Incoming connection:

void Server::incomingConnection(qintptr socketDescriptor)
{
    socket = new QTcpSocket();
    socket->setSocketDescriptor(socketDescriptor);
    connect(socket, &QTcpSocket::readyRead, this, &Server::slotReadyRead);
    // connect(socket, &QTcpSocket::disconnected, socket, &QTcpSocket::deleteLater);
    /*
    connect(socket, &QTcpSocket::disconnected, socket, [=]()
    {
        qDebug() << "Number of sockets:" << sockets.size();
        qDebug() << "SOCKET DISCONNECTED";
//        qDebug() << "Sockets removed:" << erase(sockets, sender());
        qDebug() << "Sockets removed:" << sockets.removeOne(sender());
        qDebug() << "Number of sockets:" << sockets.size();
//        qDebug() << sender()->objectName(); // Краш
//        qDebug() << ((QTcpSocket*)(sender()))->socketDescriptor(); // Краш
        qDebug() << qobject_cast<QTcpSocket*>(sender())->socketDescriptor();
    });
    */
    connect(socket, &QTcpSocket::disconnected, socket, [=]()
    {
        QTcpSocket* client = qobject_cast<QTcpSocket*>(sender());
        if (client)
        {
            qDebug() << "if";
            qDebug() << client;
        }
        else
        {
            qDebug() << "else";
            qDebug() << client->socketDescriptor(); // crash (Segmentation fault)
        }
    });

    sockets.push_back(socket);
    qDebug() << "client connected:" << socketDescriptor;
}

Function for "disconnected" signal:

connect(socket, &QTcpSocket::disconnected, socket, [=]()
    {
        QTcpSocket* client = qobject_cast<QTcpSocket*>(sender());
        if (client)
        {
            qDebug() << "if";
            qDebug() << client;
        }
        else
        {
            qDebug() << "else";
            qDebug() << client->socketDescriptor(); // crash (Segmentation fault)
        }
    });

[JonB]

@Kirill-Gusarev
So you already have a lambda anyway.
Why are you looking at sender() and not just using this?
Oh, I get it, this is server, you expect sender() to be client?
And even when it works, why would you want to access a socket's socketDescriptor() during disconnect? I don't know whether it's valid at that point.

sockets.push_back(socket);

So don't you just want to remove socket or this from sockets during disconnected? I'm not understanding why you need sender() (nor socketDescriptor()).

[Kirill Gusarev]

@JonB sorry i have a 10 minute messaging limit because i am a new user.

Yes, it is a Server-side code in Server:QTcpServer class.

Calling of ->socketDescriptor is a just example of crashing while trying to get any information from sender().

I just want to delete this socket from QVector. I thought i can take socket information from sender() :((, but I take only empty pointer(((

[JonB]

@Kirill-Gusarev
I have now answered a couple of times:

So don't you just want to remove socket or this from sockets during disconnected? I'm not understanding why you need sender()

[Kirill Gusarev]

@JonB said in I have problem with sender() in QTcpSocket::disconnected signal:

@Kirill-Gusarev
If that is so you will need to use lambdas, to which you can pass a parameter of which socket is closing, instead of sender(), which you should avoid if possible.

[socket](){}

[socket] - is it "pass a parameter of which socket is closing"?

[JonB]

@Kirill-Gusarev
Indeed! Specifically it is whatever value socket had at the time that connect() statement was executed. Which was immediately prior to sockets.push_back(socket);.

Or it would have been this if you had derived from QTcpSocket and just provided a slot to attach to disconnected. But with lambda you can pass it whatever variables you like without having to subclass.

[Kirill Gusarev]

@JonB thank you. Sorry for my mistakes and missunderstandings, my main language is russian.
[socket] - that helped. I solved the problem :]

I have updated code (added QTcpSocket *currentSocket):

void Server::incomingConnection(qintptr socketDescriptor)
{
    socket = new QTcpSocket();
    socket->setSocketDescriptor(socketDescriptor);
    connect(socket, &QTcpSocket::readyRead, this, &Server::slotReadyRead);
    QTcpSocket *currentSocket = socket;
    connect(socket, &QTcpSocket::disconnected, socket, [=]()
    {
        qDebug() << currentSocket << "socket removed:" << sockets.removeOne(currentSocket);
    });

    sockets.push_back(socket);
    qDebug() << "client connected" << socketDescriptor;
}