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Trouble assigning value to boolean in the struct

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  • hskoglundH hskoglund

    Hi, perhaps value() is read-only, what happens if you try:

    current.fields[ui->editSectionNum->currentIndex()].permanent=(state>0);
    
    aha_1980A Offline
    aha_1980A Offline
    aha_1980
    Lifetime Qt Champion
    wrote on last edited by
    #5

    @hskoglund

    Hi, perhaps value() is read-only

    Not perhaps, it is const.

    That's what Christian wanted to say with his link.

    Regards

    Qt has to stay free or it will die.

    1 Reply Last reply
    1
    • Christian EhrlicherC Offline
      Christian EhrlicherC Offline
      Christian Ehrlicher
      Lifetime Qt Champion
      wrote on last edited by
      #6

      @hskoglund said in Trouble assigning value to boolean in the struct:

      perhaps value() is read-only

      It's even worse - it returns a copy.

      Qt Online Installer direct download: https://download.qt.io/official_releases/online_installers/
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      • JonBJ Offline
        JonBJ Offline
        JonB
        wrote on last edited by JonB
        #7

        @artwaw said in Trouble assigning value to boolean in the struct:
        If const is the cause of the error (and I don't doubt it is) for

        current.fields.value(ui->editSectionNum->currentIndex()).permanent=(state>0);
        

        why is there no error for

        current.fields.value(ui->editSectionNum->currentIndex()).pattern=ui->sectionEdit->text();
        

        ? That was the OP's query. That's what I don't see, it should produce the same error.

        1 Reply Last reply
        1
        • hskoglundH Offline
          hskoglundH Offline
          hskoglund
          wrote on last edited by
          #8

          I suspect it's because the const is only 1 layer deep, i.e. it protects integers and booleans in that struct from being reassigned, but since QStrings are pointers, that const only protects reassignment of the pointer, not what the pointer points to, i.e. QStrings data().

          1 Reply Last reply
          0
          • Christian EhrlicherC Offline
            Christian EhrlicherC Offline
            Christian Ehrlicher
            Lifetime Qt Champion
            wrote on last edited by Christian Ehrlicher
            #9

            @JonB said in Trouble assigning value to boolean in the struct:

            it should produce the same error.

            In theory it's a temporary so it should compile. In practice the compiler prevents you from shooting into your foot - at least when the left hand side is a POD. As soon as it's a class the compiler will not complain - blame the compiler :)

            struct blub
            {
                int a = 0;
                int b = 0;
                blub(int _a, int _b) : a(_a), b(_b) {}
                blub() = default;
            };
            struct foo
            {
                std::string stdStr;
                QString str;
                QPointF p;
                blub myBlub;
                int myInt = -1;
                bool myBool = false;
            };
            
            QVector<foo> fooVec;
            
            void doSomething()
            {
                int i = 5;
                fooVec.value(i).stdStr = "str";
                fooVec.value(i).str = "str";
                fooVec.value(i).p = QPointF(1,2);
                fooVec.value(i).myBlub = blub(1,2);
                fooVec.value(i).myInt = 2;  // gives compiler error
                fooVec.value(i).myBool = true;  // dito
            }
            

            Qt Online Installer direct download: https://download.qt.io/official_releases/online_installers/
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            kshegunovK 1 Reply Last reply
            3
            • Christian EhrlicherC Offline
              Christian EhrlicherC Offline
              Christian Ehrlicher
              Lifetime Qt Champion
              wrote on last edited by
              #10

              @hskoglund said in Trouble assigning value to boolean in the struct:

              it's because the const

              QVector::value() does not return a const object!

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              hskoglundH 1 Reply Last reply
              0
              • Christian EhrlicherC Christian Ehrlicher

                @hskoglund said in Trouble assigning value to boolean in the struct:

                it's because the const

                QVector::value() does not return a const object!

                hskoglundH Offline
                hskoglundH Offline
                hskoglund
                wrote on last edited by
                #11

                @Christian-Ehrlicher Hmmm, but the docs says that API is const:

                T QVector::value(int i) const
                
                kshegunovK 1 Reply Last reply
                0
                • Christian EhrlicherC Christian Ehrlicher

                  @JonB said in Trouble assigning value to boolean in the struct:

                  it should produce the same error.

                  In theory it's a temporary so it should compile. In practice the compiler prevents you from shooting into your foot - at least when the left hand side is a POD. As soon as it's a class the compiler will not complain - blame the compiler :)

                  struct blub
                  {
                      int a = 0;
                      int b = 0;
                      blub(int _a, int _b) : a(_a), b(_b) {}
                      blub() = default;
                  };
                  struct foo
                  {
                      std::string stdStr;
                      QString str;
                      QPointF p;
                      blub myBlub;
                      int myInt = -1;
                      bool myBool = false;
                  };
                  
                  QVector<foo> fooVec;
                  
                  void doSomething()
                  {
                      int i = 5;
                      fooVec.value(i).stdStr = "str";
                      fooVec.value(i).str = "str";
                      fooVec.value(i).p = QPointF(1,2);
                      fooVec.value(i).myBlub = blub(1,2);
                      fooVec.value(i).myInt = 2;  // gives compiler error
                      fooVec.value(i).myBool = true;  // dito
                  }
                  
                  kshegunovK Offline
                  kshegunovK Offline
                  kshegunov
                  Moderators
                  wrote on last edited by kshegunov
                  #12

                  @Christian-Ehrlicher said in Trouble assigning value to boolean in the struct:

                  As soon as it's a class the compiler will not complain - blame the compiler

                  But don't rush to. The compiler has to generate the assignment for the trivial bool, hence it can see that the written is nonsense. On the other hand QString has both assignment operators defined already, so the compiler has to be really, really, really clever to understand that the written assignment is not what was intended. (for example the compiler has to verify that the destructor of mentioned temporary produces no side effects, which isn't trivial to begin with, to be able to then to infer that the assignment is wrong)

                  Read and abide by the Qt Code of Conduct

                  Christian EhrlicherC 1 Reply Last reply
                  0
                  • hskoglundH hskoglund

                    @Christian-Ehrlicher Hmmm, but the docs says that API is const:

                    T QVector::value(int i) const
                    
                    kshegunovK Offline
                    kshegunovK Offline
                    kshegunov
                    Moderators
                    wrote on last edited by
                    #13

                    @hskoglund said in Trouble assigning value to boolean in the struct:

                    but the docs says that API is const

                    The point, Henry, is not that the method is non-mutating, which is without a doubt correct, but that it returns a copy of the element, which copy is then modified.

                    Read and abide by the Qt Code of Conduct

                    hskoglundH 1 Reply Last reply
                    3
                    • kshegunovK kshegunov

                      @hskoglund said in Trouble assigning value to boolean in the struct:

                      but the docs says that API is const

                      The point, Henry, is not that the method is non-mutating, which is without a doubt correct, but that it returns a copy of the element, which copy is then modified.

                      hskoglundH Offline
                      hskoglundH Offline
                      hskoglund
                      wrote on last edited by
                      #14

                      @kshegunov Aha, you're right! If the method had a const at the beginning things would be different :-)

                      Christian EhrlicherC 1 Reply Last reply
                      4
                      • kshegunovK kshegunov

                        @Christian-Ehrlicher said in Trouble assigning value to boolean in the struct:

                        As soon as it's a class the compiler will not complain - blame the compiler

                        But don't rush to. The compiler has to generate the assignment for the trivial bool, hence it can see that the written is nonsense. On the other hand QString has both assignment operators defined already, so the compiler has to be really, really, really clever to understand that the written assignment is not what was intended. (for example the compiler has to verify that the destructor of mentioned temporary produces no side effects, which isn't trivial to begin with, to be able to then to infer that the assignment is wrong)

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

                        @kshegunov I thought this too, but even when the compiler generates all the operators (see my struct blub) it will not complain. So I think they simply did not implement it for classes.

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

                        kshegunovK 1 Reply Last reply
                        0
                        • Christian EhrlicherC Christian Ehrlicher

                          @kshegunov I thought this too, but even when the compiler generates all the operators (see my struct blub) it will not complain. So I think they simply did not implement it for classes.

                          kshegunovK Offline
                          kshegunovK Offline
                          kshegunov
                          Moderators
                          wrote on last edited by
                          #16

                          @Christian-Ehrlicher said in Trouble assigning value to boolean in the struct:

                          @kshegunov I thought this too, but even when the compiler generates all the operators (see my struct blub) it will not complain. So I think they simply did not implement it for classes.

                          Yes probably, and probably because it's nontrivial in the general case. While at the same time for an intrinsic type it's plain and simple to deduce.

                          Read and abide by the Qt Code of Conduct

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                          0
                          • hskoglundH hskoglund

                            @kshegunov Aha, you're right! If the method had a const at the beginning things would be different :-)

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

                            @hskoglund said in Trouble assigning value to boolean in the struct:

                            had a const at the beginning

                            But then the copy elision would not work afair.

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                            • hskoglundH Offline
                              hskoglundH Offline
                              hskoglund
                              wrote on last edited by hskoglund
                              #18

                              Thinking about this, if the intent is to modify the elements of the QVector, then using value() is a bad idea since you're working with a copy (even though it happens to work for QStrings in that struct).

                              But if the API was changed to have another const at the beginning, say:

                              const T QVector::value(int i) const
                              

                              I'm too lazy to test it, but wouldn't that leading const cause compilation errors also for QStrings assignments? If so that would help dispel confusion about the usage of value().

                              kshegunovK 1 Reply Last reply
                              1
                              • hskoglundH hskoglund

                                Thinking about this, if the intent is to modify the elements of the QVector, then using value() is a bad idea since you're working with a copy (even though it happens to work for QStrings in that struct).

                                But if the API was changed to have another const at the beginning, say:

                                const T QVector::value(int i) const
                                

                                I'm too lazy to test it, but wouldn't that leading const cause compilation errors also for QStrings assignments? If so that would help dispel confusion about the usage of value().

                                kshegunovK Offline
                                kshegunovK Offline
                                kshegunov
                                Moderators
                                wrote on last edited by
                                #19

                                @hskoglund said in Trouble assigning value to boolean in the struct:

                                I'm too lazy to test it, but wouldn't that leading const cause compilation errors also for QStrings assignments?

                                It would. I'm not sure if the compiler can RVO it correctly if you assign to a non-const however ... that needs actual looking at the asm output.

                                Read and abide by the Qt Code of Conduct

                                1 Reply Last reply
                                1
                                • Christian EhrlicherC Offline
                                  Christian EhrlicherC Offline
                                  Christian Ehrlicher
                                  Lifetime Qt Champion
                                  wrote on last edited by
                                  #20

                                  From my pov QVector::value() should be deprecated - it's a really weird function which returns a copy just for the reason that the caller does not do the bounds checking.
                                  I don't see a difference in the asm output in a single testcase.

                                  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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                                  • artwawA Offline
                                    artwawA Offline
                                    artwaw
                                    wrote on last edited by
                                    #21

                                    Thank you for the explanation - indeed, I saw in docs that value() is const but since it worked for the QString I just... assumed it works anyway. Silly of me.

                                    So, to make it work as intended I should use [index] notation, like @hskoglund said (I am somehow opposed to using it in the favour of methods).

                                    Since I already got everyones involved attention - the QVector (in this case) is supposed to hold less than 20 of items, set once and not being manipulated further. Is the choice of QVector over other container types a valid one? I read about container classes and differences between them and after that I rather tend to avoid QList.

                                    For more information please re-read.

                                    Kind Regards,
                                    Artur

                                    J.HilkJ 1 Reply Last reply
                                    0
                                    • artwawA artwaw

                                      Thank you for the explanation - indeed, I saw in docs that value() is const but since it worked for the QString I just... assumed it works anyway. Silly of me.

                                      So, to make it work as intended I should use [index] notation, like @hskoglund said (I am somehow opposed to using it in the favour of methods).

                                      Since I already got everyones involved attention - the QVector (in this case) is supposed to hold less than 20 of items, set once and not being manipulated further. Is the choice of QVector over other container types a valid one? I read about container classes and differences between them and after that I rather tend to avoid QList.

                                      J.HilkJ Offline
                                      J.HilkJ Offline
                                      J.Hilk
                                      Moderators
                                      wrote on last edited by
                                      #22

                                      @artwaw said in Trouble assigning value to boolean in the struct:

                                      after that I rather tend to avoid QList.

                                      👍 very good.

                                      If you want to stay in Qt - World, than there are not many other options available.
                                      A simple c/c++ array could be enough for your case?


                                      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.

                                      artwawA 1 Reply Last reply
                                      2
                                      • J.HilkJ J.Hilk

                                        @artwaw said in Trouble assigning value to boolean in the struct:

                                        after that I rather tend to avoid QList.

                                        👍 very good.

                                        If you want to stay in Qt - World, than there are not many other options available.
                                        A simple c/c++ array could be enough for your case?

                                        artwawA Offline
                                        artwawA Offline
                                        artwaw
                                        wrote on last edited by
                                        #23

                                        @J-Hilk Might be enough, of course. QVector seemed... just convenient.

                                        For more information please re-read.

                                        Kind Regards,
                                        Artur

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                                        0
                                        • artwawA Offline
                                          artwawA Offline
                                          artwaw
                                          wrote on last edited by
                                          #24

                                          Thank you all, I learned a lot from my mistake. Will pay more attention to docs and implications next time.

                                          For more information please re-read.

                                          Kind Regards,
                                          Artur

                                          1 Reply Last reply
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