QNetworkAccessManager doesn't work

Volebab

It just doesn't output the finished.

QNetworkAccessManager manager;

QNetworkRequest request;
request.setUrl(QUrl("http://www.google.com"));

auto response = manager.get(request);

connect(response, &QNetworkReply::finished, [] () {
    qDebug() << "Finished";
});

Paul Colby

Hi @Volebab,

It looks like your QNetworkAccessManager is being destroyed as soon as you create the request (ie as it goes out of scope).

Try:

QNetworkAccessManager * manager = new QNetworkAccessManager(this);

You need to ensure that the manager lives for as long as you want to be using the request you created with it.

Cheers.

Volebab

@Paul-Colby You are right, thank you.
How to ensure that this pointer isn't going to leak? I mean, I want to use it in a class, how to make sure that it's going to be deleted?

Paul Colby

@Paul-Colby You are right, thank you.

You're welcome :)

How to ensure that this pointer isn't going to leak?

There's lots of options, but you'll notice that in the QNetworkAccessManager constructor, I passed in this

QNetworkAccessManager * manager = new QNetworkAccessManager(this);

Assuming this is a QObject-derived class, then this sets up parent-child relationship such that when this gets destroyed, its destructor will also destroy manager.

Have a read of Object Trees & Ownership - Construction/Destruction Order of QObjects.

Cheers.

Volebab

@Paul-Colby I just noticed that if I try to use an actual slot it doesn't work anymore.

connect(response, &QNetworkReply::finished, this, &MyClass::getResult);

The getResult is never called.

Paul Colby

@Volebab said:

connect(response, &QNetworkReply::finished, this, &MyClass::getResult);

Looks okay to me.

Can you show us the updated code that sets up the request, as well as the implementation of the getResult function?

Volebab

@Paul-Colby

webservice.hpp:

#ifndef WEBSERVICE_HPP
#define WEBSERVICE_HPP

#include <QObject>
#include <QNetworkAccessManager>
#include <QNetworkReply>

class Webservice : public QObject
{
    Q_OBJECT
public:
    explicit Webservice(QObject *parent = 0);
    virtual void ~Webservice();

public slots:
    void sendRequest();
    
private slots:
    void getResult();
    
private:
    QNetworkAccessManager *manager;
    QNetworkReply *response;
};

#endif // WEBSERVICE_HPP

webservice.cpp:

#include "webservice.hpp"

Webservice::Webservice(QObject *parent) : QObject(parent)
{
    manager = new QNetworkAccessManager(this);
}

void Webservice::~Webservice()
{
    manager->deleteLater();
}

void Webservice::sendRequest()
{
    QNetworkRequest request;
    request.setUrl(QUrl("http://httpin.org/headers"));
    
    response = manager->get(request);

    connect(response, &QNetworkReply::finished, this, &Webservice::getResult);
}

void Webservice::getResult()
{
    qDebug() << "finished";
    qDebug() << response->readAll();
    response->deleteLater();
}

Volebab

I fixed it, first: The address was incorrect to it would never call finished but error signal, and second: it's not needed to create an instance of QNetworkAccessManager at all, it will work just fine with a stack allocation.