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QTConcurent and lambda in QT6

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  • SGaistS Offline
    SGaistS Offline
    SGaist
    Lifetime Qt Champion
    wrote on last edited by
    #2

    Hi,

    Might be a silly question but is m_pool a pointer ?

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    Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

    J 1 Reply Last reply
    0
    • SGaistS SGaist

      Hi,

      Might be a silly question but is m_pool a pointer ?

      J Offline
      J Offline
      JulsPower
      wrote on last edited by
      #3

      @SGaist
      Hi
      its a QTheadPool declared in the .h (not as a pointer)

      Christian EhrlicherC 1 Reply Last reply
      0
      • J JulsPower

        @SGaist
        Hi
        its a QTheadPool declared in the .h (not as a pointer)

        Christian EhrlicherC Offline
        Christian EhrlicherC Offline
        Christian Ehrlicher
        Lifetime Qt Champion
        wrote on last edited by
        #4

        @JulsPower said in QTConcurent and lambda in QT6:

        its a QTheadPool declared in the .h (not as a pointer)

        But QtConcurrent::run() needs a pointer as shown in the documentation.

        But your code inside the lambda is... strange. You must open a database in the same thread as you're using the connection.

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        1
        • Christian EhrlicherC Christian Ehrlicher

          @JulsPower said in QTConcurent and lambda in QT6:

          its a QTheadPool declared in the .h (not as a pointer)

          But QtConcurrent::run() needs a pointer as shown in the documentation.

          But your code inside the lambda is... strange. You must open a database in the same thread as you're using the connection.

          J Offline
          J Offline
          JulsPower
          wrote on last edited by
          #5

          @Christian-Ehrlicher
          I missed that about the pointer it does compile now (sending the reference of m_pool) to run function

          and if you read the article I sent youll see that qthreadpool is configured to use only 1 thread that doesn't expire. so it take care of having always the same thread use the connection to the database

          Christian EhrlicherC 1 Reply Last reply
          0
          • J JulsPower

            @Christian-Ehrlicher
            I missed that about the pointer it does compile now (sending the reference of m_pool) to run function

            and if you read the article I sent youll see that qthreadpool is configured to use only 1 thread that doesn't expire. so it take care of having always the same thread use the connection to the database

            Christian EhrlicherC Offline
            Christian EhrlicherC Offline
            Christian Ehrlicher
            Lifetime Qt Champion
            wrote on last edited by
            #6

            This usecase is weird - use a dedicated QThread then. As soon as someone else increases the thread count you will get into big trouble.

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            • J Offline
              J Offline
              JulsPower
              wrote on last edited by
              #7

              Now for my other question which is more to educate me than fix something

              when I tried in my main thread to use db.open().then [] () function to get the return of the open function at a later point.
              I get the correct return value, but I tried to show a QMessageBox::critical in case of error
              it shows but the program crash in sigsev shortly after.

              why is that? the then function is executed in the main thread right? not by the database qthreadpool one?

              JonBJ 1 Reply Last reply
              0
              • Christian EhrlicherC Christian Ehrlicher

                This usecase is weird - use a dedicated QThread then. As soon as someone else increases the thread count you will get into big trouble.

                J Offline
                J Offline
                JulsPower
                wrote on last edited by
                #8

                @Christian-Ehrlicher said in QTConcurent and lambda in QT6:

                This usecase is weird - use a dedicated QThread then. As soon as someone else increases the thread count you will get into big trouble.

                ill take that into consideration for sure I find how with he used qthreadpool instead of qthread

                I don't see how someone could increase the threadcount those its limited to 1 by: m_pool.setMaxThreadCount(1);

                Christian EhrlicherC 1 Reply Last reply
                0
                • J JulsPower

                  Now for my other question which is more to educate me than fix something

                  when I tried in my main thread to use db.open().then [] () function to get the return of the open function at a later point.
                  I get the correct return value, but I tried to show a QMessageBox::critical in case of error
                  it shows but the program crash in sigsev shortly after.

                  why is that? the then function is executed in the main thread right? not by the database qthreadpool one?

                  JonBJ Online
                  JonBJ Online
                  JonB
                  wrote on last edited by JonB
                  #9

                  @JulsPower said in QTConcurent and lambda in QT6:

                  it shows but the program crash in sigsev shortly after.

                  Run under a debugger or show your code. And make sure you call no UI code from your secondary thread.

                  J 1 Reply Last reply
                  1
                  • J JulsPower

                    @Christian-Ehrlicher said in QTConcurent and lambda in QT6:

                    This usecase is weird - use a dedicated QThread then. As soon as someone else increases the thread count you will get into big trouble.

                    ill take that into consideration for sure I find how with he used qthreadpool instead of qthread

                    I don't see how someone could increase the threadcount those its limited to 1 by: m_pool.setMaxThreadCount(1);

                    Christian EhrlicherC Offline
                    Christian EhrlicherC Offline
                    Christian Ehrlicher
                    Lifetime Qt Champion
                    wrote on last edited by Christian Ehrlicher
                    #10

                    @JulsPower said in QTConcurent and lambda in QT6:

                    I don't see how someone could increase the threadcount those its limited to 1 by: m_pool.setMaxThreadCount(1);

                    Another developer who is modifying your code and don't know about such strange constraints for instance.

                    I get the correct return value, but I tried to show a QMessageBox::critical in case of error
                    it shows but the program crash in sigsev shortly after.

                    You must not show an ui element from outside the main (gui) thread.

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                    • Christian EhrlicherC Christian Ehrlicher

                      @JulsPower said in QTConcurent and lambda in QT6:

                      I don't see how someone could increase the threadcount those its limited to 1 by: m_pool.setMaxThreadCount(1);

                      Another developer who is modifying your code and don't know about such strange constraints for instance.

                      I get the correct return value, but I tried to show a QMessageBox::critical in case of error
                      it shows but the program crash in sigsev shortly after.

                      You must not show an ui element from outside the main (gui) thread.

                      J Offline
                      J Offline
                      JulsPower
                      wrote on last edited by
                      #11

                      @Christian-Ehrlicher said in QTConcurent and lambda in QT6:

                      You must not show an ui element from outside the main (gui) thread.

                      But the then function is run in the main thread that own the gui that shouldn't be the problem.

                      Christian EhrlicherC 1 Reply Last reply
                      0
                      • J JulsPower

                        @Christian-Ehrlicher said in QTConcurent and lambda in QT6:

                        You must not show an ui element from outside the main (gui) thread.

                        But the then function is run in the main thread that own the gui that shouldn't be the problem.

                        Christian EhrlicherC Offline
                        Christian EhrlicherC Offline
                        Christian Ehrlicher
                        Lifetime Qt Champion
                        wrote on last edited by
                        #12

                        @JulsPower said in QTConcurent and lambda in QT6:

                        But the then function is run in the main thread that own the gui that shouldn't be the problem.

                        You a) did not show us code and b) did not do what @JonB suggested so how should we help?

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                        1 Reply Last reply
                        2
                        • JonBJ JonB

                          @JulsPower said in QTConcurent and lambda in QT6:

                          it shows but the program crash in sigsev shortly after.

                          Run under a debugger or show your code. And make sure you call no UI code from your secondary thread.

                          J Offline
                          J Offline
                          JulsPower
                          wrote on last edited by
                          #13

                          @JonB said in QTConcurent and lambda in QT6:

                          @JulsPower said in QTConcurent and lambda in QT6:

                          it shows but the program crash in sigsev shortly after.

                          Run under a debugger or show your code. And make sure you call no UI code from your secondary thread.

                          here is the code that open the db in the main thread (which call the thread to open the db in fact)

                              m_jdb->open().then([&](const bool &b){
                                  if(!b)
                                      QMessageBox::critical(this, "Error", "Couldn't connect to DB");
                              });
                          

                          80059239-e36d-4bd0-9f51-1488402eca2d-image.png
                          the debugger doesn't help me much here it look like a gui bug but if I breakpoint into the then function "this" is the mainwindow pointer

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                          • J Offline
                            J Offline
                            JulsPower
                            wrote on last edited by
                            #14

                            Here most of the related code
                            jdatabase.h

                            #ifndef JDATABASE_H
                            #define JDATABASE_H
                            #include <QThreadPool>
                            #include <QFuture>
                            #include <QSqlDatabase>
                            #include <QtConcurrent>
                            
                            
                            class JDatabase : public QObject
                            {
                                Q_OBJECT
                            
                            signals:
                                void Connected();
                                void FailToConnect();
                            public:
                                JDatabase(QWidget *parent = nullptr);
                                ~JDatabase();
                                QFuture<bool> open();
                            
                            private:
                                QThreadPool m_pool;
                            };
                            
                            #endif // JDATABASE_H
                            

                            jdatabase.cpp

                            #include "jdatabase.h"
                            
                            JDatabase::JDatabase(QWidget *parent)
                            {
                                // limit to one thread
                                m_pool.setMaxThreadCount(1);
                                // prevent automatic deletion and recreation
                                m_pool.setExpiryTimeout(-1);
                            }
                            
                            JDatabase::~JDatabase()
                            {
                            //    m_pool.deleteLater();
                            }
                            
                            QFuture<bool> JDatabase::open()
                            {
                            
                                //m_pool,
                                return QtConcurrent::run(&m_pool, [&]()
                                {
                                    QString connectString = "Driver={SQL Server};Server=192.168.21.174;Database=DBMachines;";
                            
                                    auto db = QSqlDatabase::addDatabase("QODBC");
                                    db.setUserName("Julien");
                                    db.setPassword("MB6262");
                                    db.setDatabaseName(connectString);
                                    auto s= db.connectOptions();
                                    db.open();
                                    if(db.isOpen())
                                        emit Connected();
                                    else
                                        emit FailToConnect();
                                    return db.isOpen();
                                });
                            }
                            

                            mainwindow.h declaration

                                JDatabase *m_jdb;
                            

                            and in mainwindow.cpp

                                //Connection base de donnée
                                m_jdb = new JDatabase(this);
                                connect(m_jdb, &JDatabase::Connected, this, [&](){QMessageBox::critical(this, "YAY", "Connecté, refresh"); RefreshDB();});
                            //    connect(m_jdb, &JDatabase::FailToConnect, this, [&](){QMessageBox::critical(this, "Erreur", "Impossible de ce connecter à la base de donnée");});
                                m_jdb->open().then([&](const bool &b){
                                    if(!b)
                                        QMessageBox::critical(this, "Error", "Couldn't connect to DB");
                                });
                            

                            the signal work as intended but I also tought that the then function would work

                            thanks guys

                            Christian EhrlicherC 1 Reply Last reply
                            0
                            • J JulsPower

                              Here most of the related code
                              jdatabase.h

                              #ifndef JDATABASE_H
                              #define JDATABASE_H
                              #include <QThreadPool>
                              #include <QFuture>
                              #include <QSqlDatabase>
                              #include <QtConcurrent>
                              
                              
                              class JDatabase : public QObject
                              {
                                  Q_OBJECT
                              
                              signals:
                                  void Connected();
                                  void FailToConnect();
                              public:
                                  JDatabase(QWidget *parent = nullptr);
                                  ~JDatabase();
                                  QFuture<bool> open();
                              
                              private:
                                  QThreadPool m_pool;
                              };
                              
                              #endif // JDATABASE_H
                              

                              jdatabase.cpp

                              #include "jdatabase.h"
                              
                              JDatabase::JDatabase(QWidget *parent)
                              {
                                  // limit to one thread
                                  m_pool.setMaxThreadCount(1);
                                  // prevent automatic deletion and recreation
                                  m_pool.setExpiryTimeout(-1);
                              }
                              
                              JDatabase::~JDatabase()
                              {
                              //    m_pool.deleteLater();
                              }
                              
                              QFuture<bool> JDatabase::open()
                              {
                              
                                  //m_pool,
                                  return QtConcurrent::run(&m_pool, [&]()
                                  {
                                      QString connectString = "Driver={SQL Server};Server=192.168.21.174;Database=DBMachines;";
                              
                                      auto db = QSqlDatabase::addDatabase("QODBC");
                                      db.setUserName("Julien");
                                      db.setPassword("MB6262");
                                      db.setDatabaseName(connectString);
                                      auto s= db.connectOptions();
                                      db.open();
                                      if(db.isOpen())
                                          emit Connected();
                                      else
                                          emit FailToConnect();
                                      return db.isOpen();
                                  });
                              }
                              

                              mainwindow.h declaration

                                  JDatabase *m_jdb;
                              

                              and in mainwindow.cpp

                                  //Connection base de donnée
                                  m_jdb = new JDatabase(this);
                                  connect(m_jdb, &JDatabase::Connected, this, [&](){QMessageBox::critical(this, "YAY", "Connecté, refresh"); RefreshDB();});
                              //    connect(m_jdb, &JDatabase::FailToConnect, this, [&](){QMessageBox::critical(this, "Erreur", "Impossible de ce connecter à la base de donnée");});
                                  m_jdb->open().then([&](const bool &b){
                                      if(!b)
                                          QMessageBox::critical(this, "Error", "Couldn't connect to DB");
                                  });
                              

                              the signal work as intended but I also tought that the then function would work

                              thanks guys

                              Christian EhrlicherC Offline
                              Christian EhrlicherC Offline
                              Christian Ehrlicher
                              Lifetime Qt Champion
                              wrote on last edited by
                              #15

                              Take a look at the complete backtrace - I'm pretty sure it's not executed in the main thread as I don't see how this should be achieved.

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                              • Christian EhrlicherC Christian Ehrlicher

                                Take a look at the complete backtrace - I'm pretty sure it's not executed in the main thread as I don't see how this should be achieved.

                                J Offline
                                J Offline
                                JulsPower
                                wrote on last edited by
                                #16

                                @Christian-Ehrlicher said in QTConcurent and lambda in QT6:

                                Take a look at the complete backtrace - I'm pretty sure it's not executed in the main thread as I don't see how this should be achieved.

                                the breakpoint in the then lambda give me this
                                6fc3e6e3-4991-419d-8ada-88999763d3fb-image.png

                                then I run the app the msgbox shows
                                and as soon my pointer the into the box I get sigsev
                                4375a312-f5b9-4b05-b29e-125b08658306-image.png

                                how could I find if its the right thread showing the msgbox?, did I understand the then function correctly?

                                Christian EhrlicherC 1 Reply Last reply
                                0
                                • J JulsPower

                                  @Christian-Ehrlicher said in QTConcurent and lambda in QT6:

                                  Take a look at the complete backtrace - I'm pretty sure it's not executed in the main thread as I don't see how this should be achieved.

                                  the breakpoint in the then lambda give me this
                                  6fc3e6e3-4991-419d-8ada-88999763d3fb-image.png

                                  then I run the app the msgbox shows
                                  and as soon my pointer the into the box I get sigsev
                                  4375a312-f5b9-4b05-b29e-125b08658306-image.png

                                  how could I find if its the right thread showing the msgbox?, did I understand the then function correctly?

                                  Christian EhrlicherC Offline
                                  Christian EhrlicherC Offline
                                  Christian Ehrlicher
                                  Lifetime Qt Champion
                                  wrote on last edited by
                                  #17

                                  Take a breakpoint on 'b' and look at the backtrace to see if it is called from main() and/or check the currentThreadId() with the main .thread id (qApp->threadId())

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                                  • Christian EhrlicherC Christian Ehrlicher

                                    Take a breakpoint on 'b' and look at the backtrace to see if it is called from main() and/or check the currentThreadId() with the main .thread id (qApp->threadId())

                                    J Offline
                                    J Offline
                                    JulsPower
                                    wrote on last edited by
                                    #18

                                    @Christian-Ehrlicher said in QTConcurent and lambda in QT6:

                                    Take a breakpoint on 'b' and look at the backtrace to see if it is called from main() and/or check the currentThreadId() with the main .thread id (qApp->threadId())

                                    HI
                                    what is the "backtrace"?
                                    here is a breakpoint to b
                                    550d6237-72ff-4b4c-84d8-13b6d6706b41-image.png

                                    should the then part be executed in the main loop?

                                    Christian EhrlicherC 1 Reply Last reply
                                    0
                                    • J JulsPower

                                      @Christian-Ehrlicher said in QTConcurent and lambda in QT6:

                                      Take a breakpoint on 'b' and look at the backtrace to see if it is called from main() and/or check the currentThreadId() with the main .thread id (qApp->threadId())

                                      HI
                                      what is the "backtrace"?
                                      here is a breakpoint to b
                                      550d6237-72ff-4b4c-84d8-13b6d6706b41-image.png

                                      should the then part be executed in the main loop?

                                      Christian EhrlicherC Offline
                                      Christian EhrlicherC Offline
                                      Christian Ehrlicher
                                      Lifetime Qt Champion
                                      wrote on last edited by
                                      #19

                                      So as I already said in all of my previous post the message box is not drawn in the main thread. Fix it by e.g. using signals and slots or don't use threads at all.

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                                      Visit the Qt Academy at https://academy.qt.io/catalog

                                      J 1 Reply Last reply
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                                      • Christian EhrlicherC Christian Ehrlicher

                                        So as I already said in all of my previous post the message box is not drawn in the main thread. Fix it by e.g. using signals and slots or don't use threads at all.

                                        J Offline
                                        J Offline
                                        JulsPower
                                        wrote on last edited by
                                        #20

                                        @Christian-Ehrlicher said in QTConcurent and lambda in QT6:

                                        So as I already said in all of my previous post the message box is not drawn in the main thread. Fix it by e.g. using signals and slots or don't use threads at all.

                                        Hi
                                        I understand the signal and slot and know how to use them for what I want to do
                                        but I wanted to understand better the then function. from what I understood the qfuture would allow to execute code (lambda or a function call) at a later time when the result would have been received. but I don't get why it would be executed by the other thread.

                                        thanks again im just trying to understand how thing work (or should work) so I can use those function correctly.

                                        Christian EhrlicherC 1 Reply Last reply
                                        0
                                        • J JulsPower

                                          @Christian-Ehrlicher said in QTConcurent and lambda in QT6:

                                          So as I already said in all of my previous post the message box is not drawn in the main thread. Fix it by e.g. using signals and slots or don't use threads at all.

                                          Hi
                                          I understand the signal and slot and know how to use them for what I want to do
                                          but I wanted to understand better the then function. from what I understood the qfuture would allow to execute code (lambda or a function call) at a later time when the result would have been received. but I don't get why it would be executed by the other thread.

                                          thanks again im just trying to understand how thing work (or should work) so I can use those function correctly.

                                          Christian EhrlicherC Offline
                                          Christian EhrlicherC Offline
                                          Christian Ehrlicher
                                          Lifetime Qt Champion
                                          wrote on last edited by
                                          #21

                                          @JulsPower said in QTConcurent and lambda in QT6:

                                          but I don't get why it would be executed by the other thread.

                                          Because there is no other way - the future is ready and can't simply switch the thread to the main thread (and there is even no reason to do so) so it's executed in the future's thread.

                                          Qt Online Installer direct download: https://download.qt.io/official_releases/online_installers/
                                          Visit the Qt Academy at https://academy.qt.io/catalog

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