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    Solved Real confusion about when to delete QNetworkReply object

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    • H
      hbatalha last edited by hbatalha

      I have a class FileDownloader that I have an instance of it in my main class and I use it to download some files by calling a method 'download'. But I am having a hard time understanding when to call deleteLater on the QNetworkReply object.
      Here is the part of the code that I consider relevant. if something is unclear I can post some code or even the entire class:

      void FileDownloader::download(QString url, QString dest_file, QString downloadName, FileDownloadRequestType fileDownloadRequestType)
      {
          this->show();
      
          connect(ui->pushButton, &QPushButton::clicked, [this] { this->close(); });
      
          ui->download_progressBar->setMinimum(0);
          ui->download_progressBar->setMaximum(0);
      
          ui->msg_label->setText(tr("Downloading ") + downloadName);
      
          reply = net.get(QNetworkRequest(QUrl(url)));
      
          downloadBegan = true;
          finished = false;
         
          if(QFile::exists(dest_file))
              QFile::remove(dest_file);
      
          QObject::connect(reply, &QNetworkReply::downloadProgress, [this](qint64 received, qint64 total)
          {
              qDebug() << received << " of " << total;
      
              ui->progress_label->setText(Util::displaySize(received) + " of " + Util::displaySize(total));
      
              ui->download_progressBar->setRange(0, total);
              ui->download_progressBar->setValue(received);
          });
      
          QObject::connect(&net, &QNetworkAccessManager::finished, [this, dest_file, fileDownloadRequestType](QNetworkReply* reply)
          {
              if(! reply->error())
              {
                  QByteArray data = reply->readAll();
      
                  QFile file(dest_file);           
      
                  if( ! file.open(QIODevice::ReadWrite | QIODevice::Truncate))
                      Util::displayErrorMessage(tr("Something went wrong"), tr("Could not save the file"), this, true); // closes the dialog
      
                  file.write(data);
                  file.close();
      
                  ui->msg_label->setText("Download Complete");
                  ui->progress_label->setText("");
      
                  finished = true;
      
                  exe = dest_file;
      
                  if(fileDownloadRequestType == FileDownloadRequestType::UPDATE)
                      handleUpdateDownloadFinished();
              }
      
              // reply->deleteLater();  // if I call it here instead it crashes my app every once in a while
          });
      }
      
      void FileDownloader::handleUpdateDownloadFinished()
      {
          ui->download_progressBar->setMinimum(0);
          ui->download_progressBar->setMaximum(0);
          ui->pushButton->setText(tr("Install Update"));
      
          disconnect(ui->pushButton, &QPushButton::clicked, nullptr, nullptr);
          connect(ui->pushButton, &QPushButton::clicked, this, &FileDownloader::install);
      }
      
      void FileDownloader::install()
      {
          QProcess::startDetached(exe);
          qApp->quit();
      }
      
      void FileDownloader::closeEvent(QCloseEvent *)
      {
          if(reply != nullptr && downloadBegan && ! reply->isFinished())
          {
              reply->abort();
          }
      
          reply->deleteLater();
      }
      

      Right now I calling reply->deleteLater(); in the closeEvent( I am really not sure if it is safe) which seems to be working fine. Before I was I calling it in the lambda function connected to the QNetworkAccessManager::finished signal but that was causing my app to crash often.

      I have read through many forums and topics about this and found different approaches. The most common one I found was calling the reply->deleteLater(); in a slot connected to QNetworkAccessManager::finished signal like this:

      void FileDownloader::finished(QNetworkReply* reply)
       {
          QByteArray = reply->readAll();
          reply->deleteLater();
          //do other stuff
      }
      

      Unless I am misunderstanding the doc, it says not to delete the object there.

      Note: Do not delete the object in the slot connected to the errorOccurred() or finished() signal. Use deleteLater().

      In some they use a QScopedPointer to store the reply .

      So I am not sure which is the correct way of deleting the reply.

      jsulm 1 Reply Last reply Reply Quote 0
      • jsulm
        jsulm Lifetime Qt Champion @hbatalha last edited by

        @hbatalha said in Real confusion about when to delete QNetworkReply object:

        Use deleteLater()

        And this is exactly what you are doing, right? So, what is the problem?

        https://forum.qt.io/topic/113070/qt-code-of-conduct

        SGaist 1 Reply Last reply Reply Quote 0
        • SGaist
          SGaist Lifetime Qt Champion @jsulm last edited by

          Hi,

          @jsulm the OP calls deleteLater at the wrong time and also keeps a local copy of the reply which is not needed and on which he might call deleteLater while the pointer is dangling.

          @hbatalha you should call deleteLater in your reply once the request is finished so in your case in the lambda that is connected to the QNAM finished signal.

          Interested in AI ? www.idiap.ch
          Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

          jsulm H 2 Replies Last reply Reply Quote 1
          • jsulm
            jsulm Lifetime Qt Champion @SGaist last edited by

            @SGaist Ah, I was looking at

            void FileDownloader::finished(QNetworkReply* reply)
             {
                QByteArray = reply->readAll();
                reply->deleteLater();
                //do other stuff
            }
            

            which is not the actual code.

            https://forum.qt.io/topic/113070/qt-code-of-conduct

            H 1 Reply Last reply Reply Quote 0
            • H
              hbatalha @jsulm last edited by

              @jsulm said in Real confusion about when to delete QNetworkReply object:

              which is not the actual code.

              That was an example I created to show the most commom way I found being suggested to do in other forums

              1 Reply Last reply Reply Quote 0
              • H
                hbatalha @SGaist last edited by

                @SGaist

                you should call deleteLater in your reply once the request is finished

                If I do that the app will ocasionally crash

                the OP calls deleteLater at the wrong time and also keeps a local copy

                In my code where should I call it then?

                on which he might call deleteLater while the pointer is dangling.

                After a download is complete the next time will always be close the dialog so I think there won't be a case where that pointer is dangling unless I am missing something.
                But for for safety I could add condition to see it reply is null or not before calling deleteLater

                1 Reply Last reply Reply Quote 0
                • SGaist
                  SGaist Lifetime Qt Champion last edited by

                  What stack trace do you get when it crashes ?

                  Interested in AI ? www.idiap.ch
                  Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

                  H 1 Reply Last reply Reply Quote 0
                  • H
                    hbatalha @SGaist last edited by

                    @SGaist I get this
                    Screenshot_1.png

                    Which points to this line in the closeEvent ;

                     if(reply != nullptr && downloadBegan && ! reply->isFinished())
                    

                    This is the connect slot used:

                    QObject::connect(&net, &QNetworkAccessManager::finished, [this, dest_file, fileDownloadRequestType](QNetworkReply* reply)
                        {
                            if(! reply->error())
                            {
                                QByteArray data = reply->readAll();
                                reply->deleteLater();
                    
                                QFile file(dest_file);
                    
                                if( ! file.open(QIODevice::ReadWrite | QIODevice::Truncate))
                                    Util::displayErrorMessage(tr("Something went wrong"), tr("Could not save the file"), this, true);
                    
                                file.write(data);
                                file.close();
                    
                                ui->msg_label->setText(tr("Download Complete"));
                                ui->progress_label->setText("");
                    
                                finished = true;
                    
                                exe = dest_file;
                    
                                if(fileDownloadRequestType == FileDownloadRequestType::UPDATE)
                                    handleUpdateDownloadFinished();
                            }
                        });
                    

                    The difference here I call deleteLater right after QByteArray data = reply->readAll();

                    1 Reply Last reply Reply Quote 0
                    • SGaist
                      SGaist Lifetime Qt Champion last edited by

                      You know that deleteLater does not change that pointer to a nullptr value ?

                      Interested in AI ? www.idiap.ch
                      Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

                      H 1 Reply Last reply Reply Quote 1
                      • JoeCFD
                        JoeCFD last edited by JoeCFD

                        Use this to check if reply finishes before it is deleted.
                        connect( m_reply, &QNetworkReply::finished, **** );

                        Normally, you may do the following because it may finish immediately and the slot may not be called. Sigh!

                           if ( nullptr != m_reply ) { /* check reply is null or not first */
                               if ( m_reply->isRunning() ) { /* connect when it is still running */
                                   connect( m_reply, &QNetworkReply::finished,
                                            this,    &getFinished() );
                               }
                               else { /* not running(already finished) and call slot directly while connection is not needed */
                                   getFinished();
                               }
                           }
                        
                        H jeremy_k 2 Replies Last reply Reply Quote 0
                        • H
                          hbatalha @SGaist last edited by

                          @SGaist said in Real confusion about when to delete QNetworkReply object:

                          You know that deleteLater does not change that pointer to a nullptr value ?

                          Yeah, which is making me confused on why it's crashing the app. Any ideas?

                          I made some changes in these two functions:

                          void FileDownloader::download(QString url, QString dest_file, QString downloadName, FileDownloadRequestType fileDownloadRequestType)
                          {
                              this->show();
                          
                              downloadBegan = false;
                              finished = false;
                          
                              connect(ui->pushButton, &QPushButton::clicked, [this] { this->close(); });
                          
                              if(MAIN.preferences.theme == Preferences::Theme::DARK)
                              {
                                  ui->msg_label->setStyleSheet("color : white;");
                                  ui->progress_label->setStyleSheet("color : white;");
                              }
                              else  if(MAIN.preferences.theme == Preferences::Theme::LIGHT)
                              {
                                  ui->msg_label->setStyleSheet("color : black;");
                                  ui->progress_label->setStyleSheet("color : black;");
                              }
                          
                              ui->download_progressBar->setMinimum(0);
                              ui->download_progressBar->setMaximum(0);
                          
                              ui->msg_label->setText(tr("Downloading ") + downloadName);
                          
                              reply = net.get(QNetworkRequest(QUrl(url)));
                          
                              downloadBegan = true;
                              finished = false;
                          
                              if(QFile::exists(dest_file))
                                  QFile::remove(dest_file);
                          
                              QObject::connect(reply, &QNetworkReply::downloadProgress, [this](qint64 received, qint64 total)
                              {
                                  qDebug() << received << " of " << total;
                          
                                  ui->progress_label->setText(Util::displaySize(received) + " of " + Util::displaySize(total));
                          
                                  ui->download_progressBar->setRange(0, total);
                                  ui->download_progressBar->setValue(received);
                              });
                          
                              QObject::connect(&net, &QNetworkAccessManager::finished, [this, dest_file, fileDownloadRequestType](QNetworkReply* reply)
                              {
                                  finished = true;
                          
                                  if(! reply->error())
                                  {
                                      QByteArray data = reply->readAll();
                                      reply->deleteLater();
                          
                                      QFile file(dest_file);
                          
                                      if( ! file.open(QIODevice::ReadWrite | QIODevice::Truncate))
                                          Util::displayErrorMessage(tr("Something went wrong"), tr("Could not save the file"), this, true);
                          
                                      qDebug() << "Size written: " <<  file.write(data);
                                      file.close();
                          
                                      ui->msg_label->setText(tr("Download Complete"));
                                      ui->progress_label->setText("");
                          
                                      exe = dest_file;
                          
                                      if(fileDownloadRequestType == FileDownloadRequestType::UPDATE)
                                          handleUpdateDownloadFinished();
                                  }
                              });
                          }
                          
                          void FileDownloader::closeEvent(QCloseEvent *)
                          {
                              if( downloadBegan && ! finished)
                                  reply->abort();
                          }
                          

                          Instead of using the reply object to check if the download has finished I am using bool variables. I call deleteLater in the slot.

                          JKSH 1 Reply Last reply Reply Quote 0
                          • H
                            hbatalha @JoeCFD last edited by

                            @JoeCFD I didn't quite understand the code

                            JoeCFD 1 Reply Last reply Reply Quote 0
                            • JoeCFD
                              JoeCFD @hbatalha last edited by

                              @hbatalha added comments.

                              H 1 Reply Last reply Reply Quote 0
                              • fcarney
                                fcarney last edited by fcarney

                                We do something like this:

                                connect(reply, &QNetworkReply::finished, someobject, &SomeObject::someHandler); // or lambda
                                connect(reply, &QNetworkReply::finished, reply, &QObject::deleteLater);
                                

                                This keeps the lifetime of the reply separate from the "someobject".

                                C++ is a perfectly valid school of magic.

                                H 1 Reply Last reply Reply Quote 0
                                • JKSH
                                  JKSH Moderators @hbatalha last edited by

                                  @hbatalha said in Real confusion about when to delete QNetworkReply object:

                                  @SGaist said in Real confusion about when to delete QNetworkReply object:

                                  You know that deleteLater does not change that pointer to a nullptr value ?

                                  Yeah, which is making me confused on why it's crashing the app. Any ideas?

                                  1. Most important: Set FileDownloader::reply to nullptr after you call deleteLater().
                                  2. Second most important: Give your member variable a different name. Don't call it reply, since your lamba's parameter is also called reply. This causes shadowing (https://www.learncpp.com/cpp-tutorial/variable-shadowing-name-hiding/ )

                                  Qt Doc Search for browsers: forum.qt.io/topic/35616/web-browser-extension-for-improved-doc-searches

                                  1 Reply Last reply Reply Quote 3
                                  • jeremy_k
                                    jeremy_k last edited by

                                    For Qt 5.14 and later, theres QNetworkAccessmanager::setAutoDeleteReplies() which calls deleteLater() after emitting the finished signal.

                                    Asking a question about code? http://eel.is/iso-c++/testcase/

                                    H 1 Reply Last reply Reply Quote 2
                                    • H
                                      hbatalha @JoeCFD last edited by

                                      @JoeCFD Thanks I understand now.
                                      I didn't know that it could finish immediately and the slot may not be called. What would happen if that happens. I am curious.

                                      /* check reply is null or not first */

                                      Why would we do something like that. Is there any chance that the QNetworkAccessManager::get() may return a nullptr?

                                      JoeCFD 1 Reply Last reply Reply Quote 0
                                      • H
                                        hbatalha @fcarney last edited by

                                        @fcarney said in Real confusion about when to delete QNetworkReply object:

                                        connect(reply, &QNetworkReply::finished, reply, &QObject::deleteLater);

                                        So this is the same as @jeremy_k answer: QNetworkAccessmanager::setAutoDeleteReplies() ?

                                        1 Reply Last reply Reply Quote 0
                                        • H
                                          hbatalha @jeremy_k last edited by

                                          @jeremy_k So that will eradicate the need to call deleteLater?

                                          JKSH 1 Reply Last reply Reply Quote 0
                                          • JKSH
                                            JKSH Moderators @hbatalha last edited by JKSH

                                            @hbatalha said in Real confusion about when to delete QNetworkReply object:

                                            So that will eradicate the need to call deleteLater?

                                            Yes, that means you don't need to call deleteLater() yourself. But you must still set your pointer to nullptr.

                                            OR, avoid storing the QNetworkReply as a member variable.

                                            Qt Doc Search for browsers: forum.qt.io/topic/35616/web-browser-extension-for-improved-doc-searches

                                            H 1 Reply Last reply Reply Quote 0
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