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Fetch text from a site as QString

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  • R realroot

    If I need to act on signals() does that mean that I have to connect my C++ class to do that?

    connect(manager, &QNetworkAccessManager::finished,
            this, &MyClass::replyFinished);
    

    To fix the lifetime can I do this?

    QString s = QString::fromUtf8(reply->readAll());
    
    JonBJ Offline
    JonBJ Offline
    JonB
    wrote on last edited by JonB
    #9

    @realroot
    Yes, after your connect() (or you could have connected the reply object) you should be able to reply->readAll() in slot.

    No to second, that's not the issue. The reply needs to outlive where you do the get(), till (at least) the finished(). You won't want to use a QScopedPointer, that will destroy it. And of course the QNAM must also be kept in existence.

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    1
    • R Offline
      R Offline
      realroot
      wrote on last edited by
      #10
      QNetworkAccessManager *manager = new QNetworkAccessManager(this);
         connect(manager, &QNetworkAccessManager::finished,
             this, &MyClass::replyFinished);
      
         manager->get(QNetworkRequest(QUrl(
             "https://...file")));
      
      // Class public slot function:
      
      void replyFinished(QNetworkReply *reply) {
         if (reply->error() == QNetworkReply::NoError) {
             QByteArray data = reply->readAll();
             QFile file(<file>);
             if (file.open(QIODevice::WriteOnly)) {
                 QTextStream out(&file);
                 out << data;
                 file.close();
                 doSomething(<file>);
             }
         }
         reply->deleteLater();
      }
      

      Thanks, this is working. Is it safe now?

      1 Reply Last reply
      0
      • SGaistS Offline
        SGaistS Offline
        SGaist
        Lifetime Qt Champion
        wrote on last edited by
        #11

        When are you creating your manager object ? Based only on your code, it seems you will be creating it many times though you only need one instance during the lifetime of your application.

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

        1 Reply Last reply
        1
        • R Offline
          R Offline
          realroot
          wrote on last edited by
          #12

          In the function there is no more code:

          void MyClass::downloadFile() {
          QNetworkAccessManager *manager = new QNetworkAccessManager(this);
             connect(manager, &QNetworkAccessManager::finished,
                 this, &MyClass::replyFinished);
          
             manager->get(QNetworkRequest(QUrl(
                 "https://...file")));
          }
          

          Should I make a manager instance as private member of MyClass?

          JonBJ 1 Reply Last reply
          0
          • R realroot

            In the function there is no more code:

            void MyClass::downloadFile() {
            QNetworkAccessManager *manager = new QNetworkAccessManager(this);
               connect(manager, &QNetworkAccessManager::finished,
                   this, &MyClass::replyFinished);
            
               manager->get(QNetworkRequest(QUrl(
                   "https://...file")));
            }
            

            Should I make a manager instance as private member of MyClass?

            JonBJ Offline
            JonBJ Offline
            JonB
            wrote on last edited by JonB
            #13

            @realroot said in Fetch text from a site as QString:

            In the function there is no more code:

            It is not a question of whether this function has more code. It is a question of whether MyClass::replyFinished() completes all processing of the reply/downloading the file. Which I imagine it does.

            Should I make a manager instance as private member of MyClass?

            Yes. And do not allocate it more than once! You could alternatively allocate manager within the class instead of with new, i.e. QNetworkAccessManager manager; as a member variable would work. And don't forget you need to call reply->deleteLater() in MyClass::replyFinished(QNetworkReply *reply). As per https://doc.qt.io/qt-6/qnetworkaccessmanager.html#details

            Note: After the request has finished, it is the responsibility of the user to delete the QNetworkReply object at an appropriate time. Do not directly delete it inside the slot connected to finished(). You can use the deleteLater() function.

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            • SGaistS Offline
              SGaistS Offline
              SGaist
              Lifetime Qt Champion
              wrote on last edited by
              #14

              In addition to what @JonB wrote, the fact that you pass a parent to your manager object only ensures that it will get destroyed when the parent gets destroyed.

              What you currently have is a variant of memory leak since you create new instances of QNetworkAccessManager every time you call that function and they will only get destroyed when your MyClass instance will as well.

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

              1 Reply Last reply
              1
              • R Offline
                R Offline
                realroot
                wrote on last edited by
                #15

                I see thanks.
                If I declare it as QNetworkAccessManager manager; I have errors so I made it like this:

                private:
                    QNetworkAccessManager* m_manager = new QNetworkAccessManager(this);
                
                void MyClass::downloadFile() {
                   connect(m_manager, &QNetworkAccessManager::finished,
                       this, &MyClass::replyFinished);
                
                   manager->get(QNetworkRequest(QUrl(
                       "https://...file")));
                }
                
                void MyClass::replyFinished(QNetworkReply *reply) {
                   if (reply->error() == QNetworkReply::NoError) {
                       QByteArray data = reply->readAll();
                       QFile file(<file>);
                       if (file.open(QIODevice::WriteOnly)) {
                           QTextStream out(&file);
                           out << data;
                           file.close();
                           doSomething(<file>);
                       }
                   }
                   reply->deleteLater();
                }
                
                SGaistS 1 Reply Last reply
                0
                • R realroot

                  I see thanks.
                  If I declare it as QNetworkAccessManager manager; I have errors so I made it like this:

                  private:
                      QNetworkAccessManager* m_manager = new QNetworkAccessManager(this);
                  
                  void MyClass::downloadFile() {
                     connect(m_manager, &QNetworkAccessManager::finished,
                         this, &MyClass::replyFinished);
                  
                     manager->get(QNetworkRequest(QUrl(
                         "https://...file")));
                  }
                  
                  void MyClass::replyFinished(QNetworkReply *reply) {
                     if (reply->error() == QNetworkReply::NoError) {
                         QByteArray data = reply->readAll();
                         QFile file(<file>);
                         if (file.open(QIODevice::WriteOnly)) {
                             QTextStream out(&file);
                             out << data;
                             file.close();
                             doSomething(<file>);
                         }
                     }
                     reply->deleteLater();
                  }
                  
                  SGaistS Offline
                  SGaistS Offline
                  SGaist
                  Lifetime Qt Champion
                  wrote on last edited by
                  #16

                  @realroot said in Fetch text from a site as QString:

                  I see thanks.
                  If I declare it as QNetworkAccessManager manager; I have errors so I made it like this:

                  private:
                      QNetworkAccessManager* m_manager = new QNetworkAccessManager(this);
                  
                  void MyClass::downloadFile() {
                     connect(m_manager, &QNetworkAccessManager::finished,
                         this, &MyClass::replyFinished);
                  
                  
                  

                  Move that connect to the constructor of your class. Otherwise each time you call downloadFile you will create a new connection which means that the slot will be called an additional time.

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

                  1 Reply Last reply
                  2
                  • R Offline
                    R Offline
                    realroot
                    wrote on last edited by
                    #17

                    Then it should be so I believe:

                    class MyClass : public QAbstractListModel {
                        Q_OBJECT
                    public:
                        MyClass(QObject *parent = nullptr) : QAbstractListModel(parent) {
                            connect(m_manager, &QNetworkAccessManager::finished,
                                    this, &MyClass::downloadFinished);
                        }
                    private:
                        QNetworkAccessManager* m_manager = new QNetworkAccessManager(this);
                    
                    void MyClass::downloadFile() {
                        m_manager->get(QNetworkRequest(QUrl(
                           "https://...file")));
                    }
                    

                    It's working at least.

                    1 Reply Last reply
                    0
                    • R Offline
                      R Offline
                      realroot
                      wrote on last edited by
                      #18

                      If something is wrong let me know, thanks.

                      JonBJ 1 Reply Last reply
                      0
                      • R realroot

                        If something is wrong let me know, thanks.

                        JonBJ Offline
                        JonBJ Offline
                        JonB
                        wrote on last edited by JonB
                        #19

                        @realroot
                        It looks reasonable. Although your

                        QNetworkAccessManager* m_manager = new QNetworkAccessManager(this);
                        

                        will work personally I would do the new in the MyClass constructor, to the line above where you have moved the connect() like @SGaist said. But maybe that's just me. In any case I believe your code is now acceptable.

                        1 Reply Last reply
                        0
                        • R realroot has marked this topic as solved on
                        • R Offline
                          R Offline
                          realroot
                          wrote on last edited by
                          #20

                          @JonB Like this?

                          public:
                                      MyClass(QObject *parent = nullptr) : QAbstractListModel(parent) {
                                          m_manager = new QNetworkAccessManager(this);
                                          connect(m_manager, &QNetworkAccessManager::finished,
                                                  this, &MyClass::downloadFinished);
                                      }
                          
                          Pl45m4P 1 Reply Last reply
                          0
                          • R realroot

                            @JonB Like this?

                            public:
                                        MyClass(QObject *parent = nullptr) : QAbstractListModel(parent) {
                                            m_manager = new QNetworkAccessManager(this);
                                            connect(m_manager, &QNetworkAccessManager::finished,
                                                    this, &MyClass::downloadFinished);
                                        }
                            
                            Pl45m4P Offline
                            Pl45m4P Offline
                            Pl45m4
                            wrote on last edited by
                            #21

                            @realroot

                            Yeah, better, but why is everything in your header?
                            As you add more to your class, it will become pretty crowded and chaotic.


                            If debugging is the process of removing software bugs, then programming must be the process of putting them in.

                            ~E. W. Dijkstra

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                            • SGaistS Offline
                              SGaistS Offline
                              SGaist
                              Lifetime Qt Champion
                              wrote on last edited by
                              #22

                              C++ now allows to initialize class variable at declaration spot. It's easy and nice for base types however, it can make things harder to read for complex type.

                              Next, unless you have complex logic associated, you should use your class initializer list and after that the constructor. This will help the compiler optimize some things.

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

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                              1
                              • R Offline
                                R Offline
                                realroot
                                wrote on last edited by
                                #23

                                @Pl45m4 Functions ( downloadFile and replyFinished) are not in the header. I did not specify that but I did add MyClass scope e.g. void MyClass::downloadFile().

                                @SGaist So:

                                MyClass(QObject *parent = nullptr) : QAbstractListModel(parent), 
                                        m_manager(new QNetworkAccessManager(this)) {
                                        connect(m_manager, &QNetworkAccessManager::finished,
                                                this, &MyClass::downloadFinished);
                                }
                                
                                Pl45m4P 1 Reply Last reply
                                0
                                • R realroot

                                  @Pl45m4 Functions ( downloadFile and replyFinished) are not in the header. I did not specify that but I did add MyClass scope e.g. void MyClass::downloadFile().

                                  @SGaist So:

                                  MyClass(QObject *parent = nullptr) : QAbstractListModel(parent), 
                                          m_manager(new QNetworkAccessManager(this)) {
                                          connect(m_manager, &QNetworkAccessManager::finished,
                                                  this, &MyClass::downloadFinished);
                                  }
                                  
                                  Pl45m4P Offline
                                  Pl45m4P Offline
                                  Pl45m4
                                  wrote on last edited by
                                  #24

                                  @realroot said in Fetch text from a site as QString:

                                  @Pl45m4 Functions ( downloadFile and replyFinished) are not in the header. I did not specify that but I did add MyClass scope e.g. void MyClass::downloadFile().

                                  Don't know what this means, but might be okay.


                                  If debugging is the process of removing software bugs, then programming must be the process of putting them in.

                                  ~E. W. Dijkstra

                                  1 Reply Last reply
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