How to delete QTcpSocket properly

YuriQ

Hello.
I'm getting 'access violation' error while using this code:

Project file:

QT       += core gui network

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

TARGET = TcpSocketTest
TEMPLATE = app


SOURCES += main.cpp\
        mainwindow.cpp

HEADERS  += mainwindow.h

main.cpp:

#include "mainwindow.h"
#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();

    return a.exec();
}

mainwindow.h:

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QWidget>
#include <QPushButton>
#include <QHBoxLayout>
#include <QTcpSocket>

class MainWindow : public QWidget
{
    Q_OBJECT
public:
    MainWindow(QWidget* parent = 0);
private slots:
    void onClicked();
    void onConnected();
private:
    QPushButton* btn = nullptr;
    QTcpSocket* socket = nullptr;
};

#endif // MAINWINDOW_H

mainwindow.cpp:

#include "mainwindow.h"

MainWindow::MainWindow(QWidget *parent)
    : QWidget(parent)
{
    setFixedWidth(320);
    setWindowTitle("TCP connection test");

    setLayout(new QHBoxLayout);
    btn = new QPushButton("Start test");
    connect(btn, &QPushButton::clicked, this, &MainWindow::onClicked);
    layout()->addWidget(btn);
}

void MainWindow::onClicked()
{
    btn->setEnabled(false);

    // I want to instantiate QTcpSocket on the heap
    Q_ASSERT(socket == nullptr);
    socket = new QTcpSocket(this);

    Q_ASSERT(connect(socket, &QTcpSocket::connected, this, &MainWindow::onConnected));
    socket->connectToHost("google.com", 80, QIODevice::ReadWrite);
}

void MainWindow::onConnected()
{
    Q_ASSERT(socket != nullptr);

    // I want to delete QTcpSocket as soon as I don't need it.
    delete socket;
    socket = nullptr;

    btn->setEnabled(true);
}

Please help me fix it.

Environment: Windows 8.1 x64, Qt 5.4.1.

SGaist

Hi,

Use socket->deleteLater() That will schedule the object delete through the even loop so that any pending events for the object will be removed from the event queue and it can be safely deleted.

YuriQ

@SGaist Got it. So I changed onConnected function:

void MainWindow::onConnected()
{
    Q_ASSERT(socket != nullptr);

    socket->disconnect();
    socket->disconnectFromHost();
    socket->deleteLater();
    // Don't need to delete it manually
    // because parent will delete it automatically
    socket = nullptr;

    btn->setEnabled(true);
}

Is it looks good now?

YuriQ

@YuriQ Well, I think it is.

SGaist

The code is good but the comment is false, it's not the parent that will delete socket, it's the event loop at the next occasion.