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QMap iterator Bug??

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  • C Christian Ehrlicher
    1 Dec 2021, 06:26

    @IknowQT said in QMap iterator Bug??:

    Should ++ increase from begin?

    Don't know what this has to do with my comment above.

    m_pLogin->getAccountMgr().begin()

    This creates a temporary container and calls begin on it, then the container is destroyed and the iterator is dangling.
    Create a local copy of the container you want to iterate over.

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    IknowQT
    wrote on 1 Dec 2021, 06:35 last edited by
    #5

    @Christian-Ehrlicher

    QMap<QString, CLogin::AccountInfo>::iterator iter = m_pLogin->getAccountMgr().begin();
    			for (iter; iter != m_pLogin->getAccountMgr().end(); iter++)
    			{
    				UpdateTableWidget(iter);
    			}
    

    Does it mean I should do it like this?

    C 1 Reply Last reply 1 Dec 2021, 06:39
    0
    • I IknowQT
      1 Dec 2021, 06:35

      @Christian-Ehrlicher

      QMap<QString, CLogin::AccountInfo>::iterator iter = m_pLogin->getAccountMgr().begin();
      			for (iter; iter != m_pLogin->getAccountMgr().end(); iter++)
      			{
      				UpdateTableWidget(iter);
      			}
      

      Does it mean I should do it like this?

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      Christian Ehrlicher
      Lifetime Qt Champion
      wrote on 1 Dec 2021, 06:39 last edited by Christian Ehrlicher 12 Jan 2021, 06:40
      #6

      @IknowQT said in QMap iterator Bug??:

      Does it mean I should do it like this?

      No, since you still create two temporary container

      m_pLogin->getAccountMgr()...

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      I 1 Reply Last reply 1 Dec 2021, 06:41
      0
      • C Christian Ehrlicher
        1 Dec 2021, 06:39

        @IknowQT said in QMap iterator Bug??:

        Does it mean I should do it like this?

        No, since you still create two temporary container

        m_pLogin->getAccountMgr()...

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        IknowQT
        wrote on 1 Dec 2021, 06:41 last edited by
        #7

        @Christian-Ehrlicher

        Can you tell me in detail what to do?
        Can you tell me if you have an example code?

        C 1 Reply Last reply 1 Dec 2021, 06:41
        0
        • I IknowQT
          1 Dec 2021, 06:41

          @Christian-Ehrlicher

          Can you tell me in detail what to do?
          Can you tell me if you have an example code?

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          Christian Ehrlicher
          Lifetime Qt Champion
          wrote on 1 Dec 2021, 06:41 last edited by Christian Ehrlicher 12 Jan 2021, 06:43
          #8

          @IknowQT said in QMap iterator Bug??:

          Can you tell me in detail what to do?

          I already did:

          Create a local copy of the container you want to iterate over.

          Can you tell me if you have an example code?

          Maybe a c++ book would be a good start

          /edit: it's even described in the Qt documentation: https://doc.qt.io/qt-5/containers.html (Search for '// RIGHT' and '// WRONG')

          Qt Online Installer direct download: https://download.qt.io/official_releases/online_installers/
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          I 1 Reply Last reply 1 Dec 2021, 06:46
          0
          • C Christian Ehrlicher
            1 Dec 2021, 06:41

            @IknowQT said in QMap iterator Bug??:

            Can you tell me in detail what to do?

            I already did:

            Create a local copy of the container you want to iterate over.

            Can you tell me if you have an example code?

            Maybe a c++ book would be a good start

            /edit: it's even described in the Qt documentation: https://doc.qt.io/qt-5/containers.html (Search for '// RIGHT' and '// WRONG')

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            IknowQT
            wrote on 1 Dec 2021, 06:46 last edited by
            #9

            @Christian-Ehrlicher

            QMap<QString, CLogin::AccountInfo> test = m_pLogin->getAccountMgr();
            			QMap<QString, CLogin::AccountInfo>::iterator iter;// = test.begin();
            			for (iter = test.begin(); iter != test.end(); ++iter)
            			{
            				UpdateTableWidget(iter);
            			}
            

            Are you telling me to write it like this?
            I tested it and it worked well.

            K 1 Reply Last reply 1 Dec 2021, 07:14
            0
            • I IknowQT
              1 Dec 2021, 06:46

              @Christian-Ehrlicher

              QMap<QString, CLogin::AccountInfo> test = m_pLogin->getAccountMgr();
              			QMap<QString, CLogin::AccountInfo>::iterator iter;// = test.begin();
              			for (iter = test.begin(); iter != test.end(); ++iter)
              			{
              				UpdateTableWidget(iter);
              			}
              

              Are you telling me to write it like this?
              I tested it and it worked well.

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              KroMignon
              wrote on 1 Dec 2021, 07:14 last edited by
              #10

              @IknowQT said in QMap iterator Bug??:

              Are you telling me to write it like this?

              What @Christian-Ehrlicher is telling you, is that you are creating 2 different container.
              Each time you are calling m_pLogin->getAccountMgr(), it will return a QMap instance.
              So each one has a different begin and end.
              So you are comparing things which cannot be compared.

              But yes, this is the right way to do, as it is given in Qt documentation:

              // store local copy of the map
              auto &test = m_pLogin->getAccountMgr();
              // iterate through the map
              for (auto iter = test.begin(); iter != test.end(); ++iter)
              {
                  UpdateTableWidget(iter);
              }
              

              It is an old maxim of mine that when you have excluded the impossible, whatever remains, however improbable, must be the truth. (Sherlock Holmes)

              1 Reply Last reply
              1
              • C Online
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                Christian Ehrlicher
                Lifetime Qt Champion
                wrote on 1 Dec 2021, 07:19 last edited by
                #11

                @KroMignon said in QMap iterator Bug??:

                So each one has a different begin and end.

                No, it's even worse - as I said above the two iterators are dangling because the returned containers are temporaries.

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                I 1 Reply Last reply 1 Dec 2021, 08:14
                0
                • C Christian Ehrlicher
                  1 Dec 2021, 07:19

                  @KroMignon said in QMap iterator Bug??:

                  So each one has a different begin and end.

                  No, it's even worse - as I said above the two iterators are dangling because the returned containers are temporaries.

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                  IknowQT
                  wrote on 1 Dec 2021, 08:14 last edited by
                  #12

                  @Christian-Ehrlicher

                  I think this topic is endless. Can you post some concrete example code?

                  J K 2 Replies Last reply 1 Dec 2021, 08:16
                  0
                  • I IknowQT
                    1 Dec 2021, 08:14

                    @Christian-Ehrlicher

                    I think this topic is endless. Can you post some concrete example code?

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                    jsulm
                    Lifetime Qt Champion
                    wrote on 1 Dec 2021, 08:16 last edited by
                    #13

                    @IknowQT said in QMap iterator Bug??:

                    I think this topic is endless. Can you post some concrete example code?

                    You already have it and you even wrote that it is working...

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

                    1 Reply Last reply
                    4
                    • I IknowQT
                      1 Dec 2021, 08:14

                      @Christian-Ehrlicher

                      I think this topic is endless. Can you post some concrete example code?

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                      KroMignon
                      wrote on 1 Dec 2021, 10:41 last edited by
                      #14

                      @IknowQT said in QMap iterator Bug??:

                      I think this topic is endless. Can you post some concrete example code?

                      Why endless? All has been explained.
                      It is not a QMap iterator bug. It is your code which is wrong.
                      Each time you are calling m_pLogin->getAccountMgr() is returns a new QMap instance.
                      Your code has many errors:

                      • first : in the initialization statement in the for loop you are setting up iter is a begin of the return QMap.
                      • second: this return QMap is temporary, which means it is destroyed directly after usage ==> iter becomes a dangling pointer!!!
                      • third: in the condition statement, comparing iter with another QMap instance (which also is temporary!!)

                      Solution for this ==> learn C++ basics like variable/objects lifetime! (cf. https://en.cppreference.com/w/cpp/language/lifetime)

                      It is an old maxim of mine that when you have excluded the impossible, whatever remains, however improbable, must be the truth. (Sherlock Holmes)

                      J 1 Reply Last reply 1 Dec 2021, 14:59
                      4
                      • K KroMignon
                        1 Dec 2021, 10:41

                        @IknowQT said in QMap iterator Bug??:

                        I think this topic is endless. Can you post some concrete example code?

                        Why endless? All has been explained.
                        It is not a QMap iterator bug. It is your code which is wrong.
                        Each time you are calling m_pLogin->getAccountMgr() is returns a new QMap instance.
                        Your code has many errors:

                        • first : in the initialization statement in the for loop you are setting up iter is a begin of the return QMap.
                        • second: this return QMap is temporary, which means it is destroyed directly after usage ==> iter becomes a dangling pointer!!!
                        • third: in the condition statement, comparing iter with another QMap instance (which also is temporary!!)

                        Solution for this ==> learn C++ basics like variable/objects lifetime! (cf. https://en.cppreference.com/w/cpp/language/lifetime)

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                        JoeCFD
                        wrote on 1 Dec 2021, 14:59 last edited by JoeCFD 12 Jan 2021, 15:00
                        #15

                        @KroMignon if m_pLogin->getAccountMgr() returns const Whatever &, his code will work as well. But it is not great to code like he does. I think the following loop for map may be simpler

                            std::map<std::string, int > test_map;
                            test_map[ "test1" ] = 1;
                            test_map[ "test2" ] = 2;
                            for ( const std::pair< std::string, int > & p : test_map ) {
                                std::cout << p.first << '\t' << p.second << std::endl;
                            }
                        
                        K 1 Reply Last reply 1 Dec 2021, 15:14
                        0
                        • J JoeCFD
                          1 Dec 2021, 14:59

                          @KroMignon if m_pLogin->getAccountMgr() returns const Whatever &, his code will work as well. But it is not great to code like he does. I think the following loop for map may be simpler

                              std::map<std::string, int > test_map;
                              test_map[ "test1" ] = 1;
                              test_map[ "test2" ] = 2;
                              for ( const std::pair< std::string, int > & p : test_map ) {
                                  std::cout << p.first << '\t' << p.second << std::endl;
                              }
                          
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                          KroMignon
                          wrote on 1 Dec 2021, 15:14 last edited by
                          #16

                          @JoeCFD said in QMap iterator Bug??:

                          if m_pLogin->getAccountMgr() returns const Whatever &, his code will work as well.

                          Yes, but that was not the point (I guess)

                          I think the following loop for map may be simpler

                          To be honest, there is no real difference. You are using std::map instead of QMap, that's all.
                          It is only a question of taste... Preferring std::map over QMap.
                          There is no need to use QMap::iterator to iterate through a QMap, it is only a possibility ;)

                          It is an old maxim of mine that when you have excluded the impossible, whatever remains, however improbable, must be the truth. (Sherlock Holmes)

                          J 1 Reply Last reply 1 Dec 2021, 15:55
                          2
                          • K KroMignon
                            1 Dec 2021, 15:14

                            @JoeCFD said in QMap iterator Bug??:

                            if m_pLogin->getAccountMgr() returns const Whatever &, his code will work as well.

                            Yes, but that was not the point (I guess)

                            I think the following loop for map may be simpler

                            To be honest, there is no real difference. You are using std::map instead of QMap, that's all.
                            It is only a question of taste... Preferring std::map over QMap.
                            There is no need to use QMap::iterator to iterate through a QMap, it is only a possibility ;)

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                            JoeCFD
                            wrote on 1 Dec 2021, 15:55 last edited by
                            #17

                            @KroMignon not only a taste issue. I believe std containers may be better optimized than Qt containers in most cases.

                            C K 2 Replies Last reply 1 Dec 2021, 16:00
                            0
                            • J JoeCFD
                              1 Dec 2021, 15:55

                              @KroMignon not only a taste issue. I believe std containers may be better optimized than Qt containers in most cases.

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                              Christian Ehrlicher
                              Lifetime Qt Champion
                              wrote on 1 Dec 2021, 16:00 last edited by
                              #18

                              @JoeCFD said in QMap iterator Bug??:

                              I believe

                              ... so nonsense without any real background.
                              And the above usecase which the topic is about (and your posts simply ignore) would be much slower with std::map than QMap since the contents of the std::map would have been copied.

                              Qt Online Installer direct download: https://download.qt.io/official_releases/online_installers/
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                              • J JoeCFD
                                1 Dec 2021, 15:55

                                @KroMignon not only a taste issue. I believe std containers may be better optimized than Qt containers in most cases.

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                                KroMignon
                                wrote on 1 Dec 2021, 16:02 last edited by
                                #19

                                @JoeCFD said in QMap iterator Bug??:

                                I believe std containers may be better optimized than Qt containers in most cases.

                                Not according to this benchmark done by woboq => https://woboq.com/blog/qmap_qhash_benchmark.html
                                or this stackoverflow entry => https://stackoverflow.com/questions/1668259/stl-or-qt-containers

                                It is an old maxim of mine that when you have excluded the impossible, whatever remains, however improbable, must be the truth. (Sherlock Holmes)

                                J 1 Reply Last reply 1 Dec 2021, 16:40
                                4
                                • K KroMignon
                                  1 Dec 2021, 16:02

                                  @JoeCFD said in QMap iterator Bug??:

                                  I believe std containers may be better optimized than Qt containers in most cases.

                                  Not according to this benchmark done by woboq => https://woboq.com/blog/qmap_qhash_benchmark.html
                                  or this stackoverflow entry => https://stackoverflow.com/questions/1668259/stl-or-qt-containers

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                                  JoeCFD
                                  wrote on 1 Dec 2021, 16:40 last edited by
                                  #20

                                  @KroMignon said in QMap iterator Bug??:

                                  @JoeCFD said in QMap iterator Bug??:

                                  I believe std containers may be better optimized than Qt containers in most cases.

                                  Not according to this benchmark done by woboq => https://woboq.com/blog/qmap_qhash_benchmark.html <=== done in 2012 ages old.
                                  or this stackoverflow entry => https://stackoverflow.com/questions/1668259/stl-or-qt-containers read comments: 35

                                  C 1 Reply Last reply 1 Dec 2021, 16:52
                                  0
                                  • J JoeCFD
                                    1 Dec 2021, 16:40

                                    @KroMignon said in QMap iterator Bug??:

                                    @JoeCFD said in QMap iterator Bug??:

                                    I believe std containers may be better optimized than Qt containers in most cases.

                                    Not according to this benchmark done by woboq => https://woboq.com/blog/qmap_qhash_benchmark.html <=== done in 2012 ages old.
                                    or this stackoverflow entry => https://stackoverflow.com/questions/1668259/stl-or-qt-containers read comments: 35

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                                    Christian Ehrlicher
                                    Lifetime Qt Champion
                                    wrote on 1 Dec 2021, 16:52 last edited by Christian Ehrlicher 12 Jan 2021, 16:52
                                    #21

                                    @JoeCFD When you're at the point where you really need to worry about a speed difference between those implementations you should write your own implementation of the problematic container. You will never notice if a running program is using c++ or Qt or boost or whatever containers.

                                    Qt Online Installer direct download: https://download.qt.io/official_releases/online_installers/
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