Exception after deletion QTcpSocket

[Heliconter]

Hi,
I wrote a simple Server class that accepts the first connection request, but rejects other. When the first connection is disconnected, the server can again accept the first another connection request.
I wrote a simple code to test the Server class. This code sets up connection to the Server, then it disconnects from and then reconnects to the Server.
But when I run Server application and test application I get an exception on the "a.exec();" line of main.cpp file of the Server application.
I suspect that it is somehow related with socket deletion, because when I remove the "delete m_socket;" line in the server.cpp this exception disappears.
What have I done wrong?

main.cpp of Server application
#include <QCoreApplication>
#include "server.h"

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    Server server;
    server.listen(QHostAddress::Any, 47326);

    return a.exec();
}

server.h
#ifndef SERVER_H
#define SERVER_H

#include <QTcpServer>
#include <QTcpSocket>

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

    void listen(QHostAddress hostAddress, quint16 port);

public slots:
    void handleConnectionRequest();
    void handleSocketDisconnetion();

private:
    QTcpServer *m_server;
    QTcpSocket *m_socket = nullptr;
};

#endif // SERVER_H

server.cpp
#include "server.h"

Server::Server(QObject *parent)
    : QObject(parent),
      m_server(new QTcpServer(this))
{
    connect(m_server, &QTcpServer::newConnection, this, &Server::handleConnectionRequest);
}

void Server::listen(QHostAddress hostAddress, quint16 port)
{
    m_server->listen(hostAddress, port);
}

void Server::handleConnectionRequest()
{    
    QTcpSocket *socket = m_server->nextPendingConnection();

    qInfo() << "New connection request entered";

    if (m_socket == nullptr)
    {
        m_socket = socket;
        connect(socket, &QTcpSocket::disconnected, this, &Server::handleSocketDisconnetion);
        qInfo() << "The connection request is accepted";
    }
    else
    {
        delete socket;
        qInfo() << "The connection request is rejected";
    }
}

void Server::handleSocketDisconnetion()
{
    delete m_socket;
    m_socket = nullptr;

    qInfo() << "Socket is deleted";
}

main.cpp of test application
#include <QCoreApplication>
#include <QTcpSocket>
#include <QHostAddress>
#include <QThread>

void connect(QTcpSocket &socket)
{
    socket.connectToHost(QHostAddress("127.0.0.1"), 47326);
    if (socket.waitForConnected(-1))
        qInfo() << "Connected";
    else
        qInfo() << socket.errorString();

    QThread::msleep(100);
}

void disconnect(QTcpSocket &socket)
{
    socket.disconnectFromHost();
    if (socket.state() == QAbstractSocket::UnconnectedState || socket.waitForDisconnected(-1))
        qInfo() << "Disconnected";
    else
        qInfo() << socket.errorString();

    QThread::msleep(100);
}

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    QTcpSocket socket;

    connect(socket);
    disconnect(socket);
    connect(socket);

    return a.exec();
}

[SGaist]

Hi and welcome to devnet,

You should rather use socker->deleteLater();. That will allow the event loop to handle pending actions on the socket.

[Heliconter]

@SGaist Thank you, deleteLater () eliminates the exception.
Could you give advice when should I use deleteLater() to delete a QObject?

[SGaist]

Each time you want to delete a connected QObject.

[VRonin]

@SGaist said in Exception after deletion QTcpSocket:

connected QObject.

Specifically, a QObject connected on the receiving end (i.e. the QObject you are deleting might be called "as a slot").
Calling deleteLater() after the thread event loop finished is a memory leak however

[Heliconter]

Probably I have found answers about using deleteLater().
https://stackoverflow.com/a/4889395
http://doc.qt.io/qt-5/qobject.html#deleteLater
http://doc.qt.io/qt-5/qobject.html#dtor.QObject

[VRonin]

To cover 90% of use cases:
if you are in the main thread and you use the classic main:

int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
//something
return a.exec();
}

then always use deleteLater() as it's 100% safe.