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;
}