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Why is the capacity of qvector smaller than that of std:: vector?

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  • J Offline
    J Offline
    John Van
    wrote on last edited by John Van
    #1

    Hello,everyone.

    I got a probelm.
    QVector<char>ware0 (220011222);
    QVector<double>ware1 (275111222);
    In my computer, will the first line of code prompt 'Size must be greater than or equal to 0', and the second line of code will crash directly. But using std:: vector even if size increased by 10 times, there is no exception.
    The Qt version is 5.14.2

    Thanks

    Edit:
    Sorry, I made a mistake. I was planning to write the first line of code for 2200111222.
    In addition, on my computer, a size of 2147483623 is the maximum value that can be used, which should be due to the issue of int index being out of bounds.

    jsulmJ Christian EhrlicherC 2 Replies Last reply
    0
    • Christian EhrlicherC Online
      Christian EhrlicherC Online
      Christian Ehrlicher
      Lifetime Qt Champion
      wrote on last edited by
      #4

      @John-Van because in Qt5 'int' is used as index. This was changed in Qt6.
      Wonder why someone would need such a big container

      1 Reply Last reply
      0
      • J John Van

        Hello,everyone.

        I got a probelm.
        QVector<char>ware0 (220011222);
        QVector<double>ware1 (275111222);
        In my computer, will the first line of code prompt 'Size must be greater than or equal to 0', and the second line of code will crash directly. But using std:: vector even if size increased by 10 times, there is no exception.
        The Qt version is 5.14.2

        Thanks

        Edit:
        Sorry, I made a mistake. I was planning to write the first line of code for 2200111222.
        In addition, on my computer, a size of 2147483623 is the maximum value that can be used, which should be due to the issue of int index being out of bounds.

        jsulmJ Offline
        jsulmJ Offline
        jsulm
        Lifetime Qt Champion
        wrote on last edited by
        #2

        @John-Van said in Why is the capacity of qvector smaller than that of std:: vector?:

        QVector<char>ware0 (220011222);

        Works just fine for me. Where exactly do you see this error? I guess in QtCreator editor? Or is it compiler error?
        Also, what is your platform, OS and Qt version?

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

        JonBJ 1 Reply Last reply
        1
        • J John Van

          Hello,everyone.

          I got a probelm.
          QVector<char>ware0 (220011222);
          QVector<double>ware1 (275111222);
          In my computer, will the first line of code prompt 'Size must be greater than or equal to 0', and the second line of code will crash directly. But using std:: vector even if size increased by 10 times, there is no exception.
          The Qt version is 5.14.2

          Thanks

          Edit:
          Sorry, I made a mistake. I was planning to write the first line of code for 2200111222.
          In addition, on my computer, a size of 2147483623 is the maximum value that can be used, which should be due to the issue of int index being out of bounds.

          Christian EhrlicherC Online
          Christian EhrlicherC Online
          Christian Ehrlicher
          Lifetime Qt Champion
          wrote on last edited by
          #3
          This post is deleted!
          jsulmJ 1 Reply Last reply
          0
          • Christian EhrlicherC Online
            Christian EhrlicherC Online
            Christian Ehrlicher
            Lifetime Qt Champion
            wrote on last edited by
            #4

            @John-Van because in Qt5 'int' is used as index. This was changed in Qt6.
            Wonder why someone would need such a big container

            1 Reply Last reply
            0
            • Christian EhrlicherC Christian Ehrlicher

              This post is deleted!

              jsulmJ Offline
              jsulmJ Offline
              jsulm
              Lifetime Qt Champion
              wrote on last edited by
              #5

              @Christian-Ehrlicher said in Why is the capacity of qvector smaller than that of std:: vector?:

              because in Qt5 'int' is used as index

              220.011.222 - fits perfectly into an int.

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

              Christian EhrlicherC 1 Reply Last reply
              0
              • jsulmJ jsulm

                @Christian-Ehrlicher said in Why is the capacity of qvector smaller than that of std:: vector?:

                because in Qt5 'int' is used as index

                220.011.222 - fits perfectly into an int.

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

                @jsulm oh, correct 🙂

                1 Reply Last reply
                0
                • jsulmJ jsulm

                  @John-Van said in Why is the capacity of qvector smaller than that of std:: vector?:

                  QVector<char>ware0 (220011222);

                  Works just fine for me. Where exactly do you see this error? I guess in QtCreator editor? Or is it compiler error?
                  Also, what is your platform, OS and Qt version?

                  JonBJ Offline
                  JonBJ Offline
                  JonB
                  wrote on last edited by JonB
                  #7

                  @jsulm
                  Also both lines fine for me under Linux, 64-bit, gcc 11.4, Qt 5.15, Creator 6.0.2.
                  I can multiply both sizes by x10 and also fine.
                  If I multiply both by x100 I get "overflow conversion from long int to int" error from compiler (not Creator), which is what I would expect.

                  1 Reply Last reply
                  0
                  • J.HilkJ Offline
                    J.HilkJ Offline
                    J.Hilk
                    Moderators
                    wrote on last edited by
                    #8

                    QVector<double>ware1 (275111222)

                    actually crashes during runtime MSVC 2019 Qt 5.12.9

                    with 5.15.8 I get
                    e3c2e53e-00c8-4f84-be11-803a46c7b0aa-image.png

                    which is even more unusual.


                    Be aware of the Qt Code of Conduct, when posting : https://forum.qt.io/topic/113070/qt-code-of-conduct


                    Q: What's that?
                    A: It's blue light.
                    Q: What does it do?
                    A: It turns blue.

                    Christian EhrlicherC JonBJ 2 Replies Last reply
                    0
                    • J.HilkJ J.Hilk

                      QVector<double>ware1 (275111222)

                      actually crashes during runtime MSVC 2019 Qt 5.12.9

                      with 5.15.8 I get
                      e3c2e53e-00c8-4f84-be11-803a46c7b0aa-image.png

                      which is even more unusual.

                      Christian EhrlicherC Online
                      Christian EhrlicherC Online
                      Christian Ehrlicher
                      Lifetime Qt Champion
                      wrote on last edited by
                      #9

                      @J-Hilk std will crash also as you don't have 800MB of contiguous memory.

                      J.HilkJ S 2 Replies Last reply
                      1
                      • J.HilkJ J.Hilk

                        QVector<double>ware1 (275111222)

                        actually crashes during runtime MSVC 2019 Qt 5.12.9

                        with 5.15.8 I get
                        e3c2e53e-00c8-4f84-be11-803a46c7b0aa-image.png

                        which is even more unusual.

                        JonBJ Offline
                        JonBJ Offline
                        JonB
                        wrote on last edited by JonB
                        #10

                        @J-Hilk
                        OOI, if you click Abbrechen to debug do you get to see what line abort() is being called from?

                        @Christian-Ehrlicher
                        Since it's 200 million * double which is 8 bytes, I think it's actually 1.6GB contiguous which is required?
                        Still an awful lot, as you said one wonders why someone would want so much (and contiguous)....

                        1 Reply Last reply
                        0
                        • Christian EhrlicherC Christian Ehrlicher

                          @J-Hilk std will crash also as you don't have 800MB of contiguous memory.

                          J.HilkJ Offline
                          J.HilkJ Offline
                          J.Hilk
                          Moderators
                          wrote on last edited by J.Hilk
                          #11

                          @Christian-Ehrlicher said in Why is the capacity of qvector smaller than that of std:: vector?:

                          @J-Hilk std will crash also as you don't have 800MB of contiguous memory.

                          I tested that, that actually works with std::vector!

                          But you're right, thats 800 mb!!! I did not realise !!


                          Be aware of the Qt Code of Conduct, when posting : https://forum.qt.io/topic/113070/qt-code-of-conduct


                          Q: What's that?
                          A: It's blue light.
                          Q: What does it do?
                          A: It turns blue.

                          Christian EhrlicherC 1 Reply Last reply
                          0
                          • J.HilkJ J.Hilk

                            @Christian-Ehrlicher said in Why is the capacity of qvector smaller than that of std:: vector?:

                            @J-Hilk std will crash also as you don't have 800MB of contiguous memory.

                            I tested that, that actually works with std::vector!

                            But you're right, thats 800 mb!!! I did not realise !!

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

                            @J-Hilk said in Why is the capacity of qvector smaller than that of std:: vector?:

                            thats 800 mb!!! I did not realise !!

                            Then it will crash on first access when the memory needs to be allocated.

                            J.HilkJ 1 Reply Last reply
                            0
                            • Christian EhrlicherC Christian Ehrlicher

                              @J-Hilk said in Why is the capacity of qvector smaller than that of std:: vector?:

                              thats 800 mb!!! I did not realise !!

                              Then it will crash on first access when the memory needs to be allocated.

                              J.HilkJ Offline
                              J.HilkJ Offline
                              J.Hilk
                              Moderators
                              wrote on last edited by
                              #13

                              @Christian-Ehrlicher probably


                              Be aware of the Qt Code of Conduct, when posting : https://forum.qt.io/topic/113070/qt-code-of-conduct


                              Q: What's that?
                              A: It's blue light.
                              Q: What does it do?
                              A: It turns blue.

                              JoeCFDJ 1 Reply Last reply
                              0
                              • J.HilkJ J.Hilk

                                @Christian-Ehrlicher probably

                                JoeCFDJ Offline
                                JoeCFDJ Offline
                                JoeCFD
                                wrote on last edited by JoeCFD
                                #14

                                @J-Hilk no crash on Ubuntu 22.04.

                                #include <vector>
                                #include <iostream>
                                
                                int main()
                                {
                                    std::vector< double > ware1( 275111222 );
                                    for ( auto & value : ware1 ) {
                                           value = 1.0;
                                    }
                                    
                                    std::cout << ware1[ 0 ] << "  " << ware1.size() << std::endl;
                                    return 1;
                                }
                                

                                output:
                                1 275111222

                                Christian EhrlicherC 1 Reply Last reply
                                0
                                • JoeCFDJ JoeCFD

                                  @J-Hilk no crash on Ubuntu 22.04.

                                  #include <vector>
                                  #include <iostream>
                                  
                                  int main()
                                  {
                                      std::vector< double > ware1( 275111222 );
                                      for ( auto & value : ware1 ) {
                                             value = 1.0;
                                      }
                                      
                                      std::cout << ware1[ 0 ] << "  " << ware1.size() << std::endl;
                                      return 1;
                                  }
                                  

                                  output:
                                  1 275111222

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

                                  @JoeCFD said in Why is the capacity of qvector smaller than that of std:: vector?:

                                  o crash on Ubuntu 22.04

                                  We already discussed the reason... You have enough contiguous memory for this test.

                                  JoeCFDJ 1 Reply Last reply
                                  0
                                  • Christian EhrlicherC Christian Ehrlicher

                                    @JoeCFD said in Why is the capacity of qvector smaller than that of std:: vector?:

                                    o crash on Ubuntu 22.04

                                    We already discussed the reason... You have enough contiguous memory for this test.

                                    JoeCFDJ Offline
                                    JoeCFDJ Offline
                                    JoeCFD
                                    wrote on last edited by
                                    #16

                                    @Christian-Ehrlicher
                                    Ok. In that case, maybe it is better to add exception handling

                                    int main()
                                    {
                                        try { 
                                            std::vector< double > ware1( 275111222 );
                                            for ( auto & value : ware1 ) {
                                                   value = 1.0;
                                            }
                                        
                                            std::cout << ware1[ 0 ] << "  " << ware1.size() << std::endl;
                                        }
                                        catch( std::bad_alloc & ex ) {
                                            std::cerr << "bad allocation caught " << ex.what();
                                        }
                                        return 1;
                                    }
                                    
                                    JonBJ 1 Reply Last reply
                                    0
                                    • JoeCFDJ JoeCFD

                                      @Christian-Ehrlicher
                                      Ok. In that case, maybe it is better to add exception handling

                                      int main()
                                      {
                                          try { 
                                              std::vector< double > ware1( 275111222 );
                                              for ( auto & value : ware1 ) {
                                                     value = 1.0;
                                              }
                                          
                                              std::cout << ware1[ 0 ] << "  " << ware1.size() << std::endl;
                                          }
                                          catch( std::bad_alloc & ex ) {
                                              std::cerr << "bad allocation caught " << ex.what();
                                          }
                                          return 1;
                                      }
                                      
                                      JonBJ Offline
                                      JonBJ Offline
                                      JonB
                                      wrote on last edited by
                                      #17

                                      @JoeCFD Well, yes, but it doesn't actually get the OP anywhere, does it? :)

                                      JoeCFDJ 2 Replies Last reply
                                      0
                                      • JonBJ JonB

                                        @JoeCFD Well, yes, but it doesn't actually get the OP anywhere, does it? :)

                                        JoeCFDJ Offline
                                        JoeCFDJ Offline
                                        JoeCFD
                                        wrote on last edited by
                                        #18

                                        @JonB At least you know quickly where the problem is in a big project.

                                        1 Reply Last reply
                                        0
                                        • JonBJ JonB

                                          @JoeCFD Well, yes, but it doesn't actually get the OP anywhere, does it? :)

                                          JoeCFDJ Offline
                                          JoeCFDJ Offline
                                          JoeCFD
                                          wrote on last edited by
                                          #19

                                          @JonB And your app(especially GUI) will not crash. An error message can be displayed.

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                                          • J John Van has marked this topic as solved on
                                          • Christian EhrlicherC Christian Ehrlicher

                                            @J-Hilk std will crash also as you don't have 800MB of contiguous memory.

                                            S Offline
                                            S Offline
                                            SimonSchroeder
                                            wrote on last edited by
                                            #20

                                            @Christian-Ehrlicher said in Why is the capacity of qvector smaller than that of std:: vector?:

                                            std will crash also as you don't have 800MB of contiguous memory.

                                            "Contiguous" is usually not the problem. With a 64bit OS and paging you have a huge address space which can be used to create contiguous addresses. PC processors don't use 64bit pointers in hardware and IIRC it was only 52bit pointers initially. This still would give you 4 petabytes of address space per process. You only need enough free memory which can then be put anywhere inside that huge address space. You would have to produce some serious fragmentation (or other really huge data) to fill up that address space.

                                            However, if you only have 8 GB of RAM you'll quickly might run of space if you request 800MB and a lot of other things are also running.

                                            1 Reply Last reply
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