It seems setParent will cause crash

[Mozzie]

Hi,

Recently, I meet a very strange problem.
I am using Qt5.12 on windows10 with VS2017

my code is:
HttpClient.h

#ifndef HTTPCLIENT_H
#define HTTPCLIENT_H

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


class HttpClient : public QObject
{
	Q_OBJECT
protected:
	QNetworkAccessManager* m_networkManager = nullptr;
	QNetworkReply* m_reply = nullptr;
public:
	explicit HttpClient(QObject *parent = nullptr);
	virtual ~HttpClient();

	inline QNetworkReply* reply() const {return m_reply;}

	void start();
signals:
	void finished();
};

#endif // HTTPCLIENT_H

HttpClient.cpp

#include "HttpClient.h"

#include "NetworkAccessManager.h"

#include <QDebug>


HttpClient::HttpClient(QObject *parent) : QObject(parent)
{

}

HttpClient::~HttpClient()
{
#ifdef QT_DEBUG
	qDebug() << __FUNCTION__;
#endif

	//// when use m_networkManager = new NetworkAccessManager();
//	if (m_networkManager) {
//		m_networkManager->deleteLater();
//	}
}

void HttpClient::start()
{
#ifdef QT_DEBUG
	qDebug() << __FUNCTION__;
#endif
	{
		if (!m_networkManager) {
			// without "this" as parent, delete m_networkManager in destructor will not crash
			//m_networkManager = new NetworkAccessManager();
			// with "this" as parent, not delete m_networkManager in destructor will crash
			m_networkManager = new NetworkAccessManager(this);
		}
		QNetworkRequest request(QUrl("https://qt.io/"));
		m_reply = m_networkManager->get(request);

		// with "this" as parent, not delete m_networkManager in destructor, not setParent(m_reply),
		// delete HttpClient after finished() signal will not crash
		setParent(m_reply);
		connect(m_reply, &QNetworkReply::finished, this, &HttpClient::finished);

	}
}

call

	{
		HttpClient* client = new HttpClient;
		connect(client, &HttpClient::finished, this, [this, client](){
			qDebug() << __FUNCTION__ << __LINE__;
			auto reply = client->reply();
			//client->deleteLater();
			reply->deleteLater();
		});
		client->start();
	}

after calling m_networkManager = new NetworkAccessManager(this);, the m_networkManager will be a child of this(HttpClient), Then after calling setParent(m_reply); the HttpClient wiil be a child of reply, after the reply is finished, I called reply->deleteLater();, this should delete the three object one by one, but it will cause crash. If I do not use HttpClient as parent of the m_networkManager OR not use setParent(m_reply); then delete the m_networkManager in the destructor, it will not crash.

Is there anyone know why did this happened? Thank you.

[Christian Ehrlicher]

@Mozzie said in It seems setParent will cause crash:

Is there anyone know why did this happened? Thank you.

You create a circular dependency HttpClient -> NetworkAccessManager -> QNetworkReply.

But what do you try to achieve with this really bogus setParent() - call?

[jsulm]

@Mozzie said in It seems setParent will cause crash:

HttpClient

Why should HttpClient be child of the reply?
This sounds wrong.

[Mozzie]

@Christian-Ehrlicher

Thank you. I want to delete the HttpClient object automatically after the QNetworkReply called deleteLater()

[Mozzie]

@jsulm
I want to delete the HttpClient object automatically after the QNetworkReply called deleteLater()

[Christian Ehrlicher]

Then connect deleteLater() to QNetworkReply::destroyed()

[Mozzie]

@Christian-Ehrlicher
Thank you very much.