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can not print correctly after convert QString to char *

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  • R Offline
    R Offline
    Rondog
    wrote on last edited by
    #3

    The one version I think you have a problem with is this:

    char* p = s.toUtf8().data();
    qDebug() << p;
    

    There is a temporary object created and 'p' is a pointer to something that no longer exists when you try to output it on the next line.

    1 Reply Last reply
    5
    • MozzieM Mozzie

      Env: vs2017 , Qt 5.14.0, Qt 5.12.6
      code:

      		QString s = "hello world";
      		qDebug() << s;
      		qDebug() << s.toUtf8().data();
      
      		char* p = s.toUtf8().data();
      		qDebug() << p;
      
      		QByteArray b = s.toUtf8();
      		p = b.data();
      		qDebug() << p;
      
      

      output:

      "hello world"
      hello world
      ????????????????????????????????????????????7
      hello world
      

      can somebody explain this, I'd be appreciate

      JonBJ Offline
      JonBJ Offline
      JonB
      wrote on last edited by
      #4

      @Mozzie
      For your

      		char* p = s.toUtf8().data();
      		qDebug() << p;
      
      ????????
      

      case. https://doc.qt.io/qt-5/qbytearray.html#data

      Returns a pointer to the data stored in the byte array.
      The pointer remains valid as long as the byte array isn't reallocated or destroyed.

      I think it is, between the two lines. That s.toUtf8() looks temporary-ish to me. Use it on one ;line, or give it permanent variable QByteArray b = s.toUtf8(); like you do afterwards, and I think you remove its temporary-ness.

      Note

      For read-only access, constData() is faster because it never causes a deep copy to occur.

      I don't know, but try:

      const char* p = s.toUtf8().data();
      

      Any difference in output?

      Christian EhrlicherC 1 Reply Last reply
      0
      • JonBJ JonB

        @Mozzie
        For your

        		char* p = s.toUtf8().data();
        		qDebug() << p;
        
        ????????
        

        case. https://doc.qt.io/qt-5/qbytearray.html#data

        Returns a pointer to the data stored in the byte array.
        The pointer remains valid as long as the byte array isn't reallocated or destroyed.

        I think it is, between the two lines. That s.toUtf8() looks temporary-ish to me. Use it on one ;line, or give it permanent variable QByteArray b = s.toUtf8(); like you do afterwards, and I think you remove its temporary-ness.

        Note

        For read-only access, constData() is faster because it never causes a deep copy to occur.

        I don't know, but try:

        const char* p = s.toUtf8().data();
        

        Any difference in output?

        Christian EhrlicherC Offline
        Christian EhrlicherC Offline
        Christian Ehrlicher
        Lifetime Qt Champion
        wrote on last edited by
        #5

        @JonB said in can not print correctly after convert QString to char *:

        Any difference in output?

        For sure not.

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

        1 Reply Last reply
        0
        • Christian EhrlicherC Christian Ehrlicher

          @Mozzie said in can not print correctly after convert QString to char *:

          char* p = s.toUtf8().data();

          C++ basics - you're creating a temporary here so p points to garbage after this statement.

          JonBJ Offline
          JonBJ Offline
          JonB
          wrote on last edited by JonB
          #6

          @Christian-Ehrlicher said in can not print correctly after convert QString to char *:

          C++ basics - you're creating a temporary here so p points to garbage after this statement.

          OK then, let's pick you up on the exactitiudes of this. https://doc.qt.io/qt-5/qbytearray.html#data states:

          The pointer remains valid as long as the byte array isn't reallocated or destroyed.

          Are you saying the s.toUtf8() is returning a temporary, or going .data() is a temporary?

          VRoninV 1 Reply Last reply
          0
          • JonBJ JonB

            @Christian-Ehrlicher said in can not print correctly after convert QString to char *:

            C++ basics - you're creating a temporary here so p points to garbage after this statement.

            OK then, let's pick you up on the exactitiudes of this. https://doc.qt.io/qt-5/qbytearray.html#data states:

            The pointer remains valid as long as the byte array isn't reallocated or destroyed.

            Are you saying the s.toUtf8() is returning a temporary, or going .data() is a temporary?

            VRoninV Offline
            VRoninV Offline
            VRonin
            wrote on last edited by VRonin
            #7

            @JonB said in can not print correctly after convert QString to char *:

            Are you saying the s.toUtf8() is returning a temporary, or going .data() is a temporary?

            The former

            "La mort n'est rien, mais vivre vaincu et sans gloire, c'est mourir tous les jours"
            ~Napoleon Bonaparte

            On a crusade to banish setIndexWidget() from the holy land of Qt

            JonBJ 1 Reply Last reply
            1
            • VRoninV VRonin

              @JonB said in can not print correctly after convert QString to char *:

              Are you saying the s.toUtf8() is returning a temporary, or going .data() is a temporary?

              The former

              JonBJ Offline
              JonBJ Offline
              JonB
              wrote on last edited by
              #8

              @VRonin
              Fine. So I carefully read https://doc.qt.io/qt-5/qstring.html#toUtf8

              Returns a UTF-8 representation of the string as a QByteArray.

              @Christian-Ehrlicher says the question/code is "C++ basics". I do not see the word "temporary" there. In fact I search the whole of QString doc page and don't find it. So how do I know this, please?

              aha_1980A 1 Reply Last reply
              0
              • JonBJ JonB

                @VRonin
                Fine. So I carefully read https://doc.qt.io/qt-5/qstring.html#toUtf8

                Returns a UTF-8 representation of the string as a QByteArray.

                @Christian-Ehrlicher says the question/code is "C++ basics". I do not see the word "temporary" there. In fact I search the whole of QString doc page and don't find it. So how do I know this, please?

                aha_1980A Offline
                aha_1980A Offline
                aha_1980
                Lifetime Qt Champion
                wrote on last edited by
                #9

                Hi @JonB,

                as @Christian-Ehrlicher said, that is C++ basics: https://en.cppreference.com/w/cpp/language/lifetime

                Regards

                Qt has to stay free or it will die.

                JonBJ 1 Reply Last reply
                2
                • aha_1980A aha_1980

                  Hi @JonB,

                  as @Christian-Ehrlicher said, that is C++ basics: https://en.cppreference.com/w/cpp/language/lifetime

                  Regards

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

                  @aha_1980
                  Wow, OK, yes, I need to read! My problem is I have been "spoiled" by using C# and then Python/PyQt/PySide2 for so long now that I rarely have to think about this!

                  So let's take a basic, if my C++ holds up. If I write a function

                  QByteArray func()
                  {
                      QByteArray qb;
                      return qb;
                  }
                  

                  does that return such a "temporary object"? And that would be true for any class/struct I decalred and then returned in that fashion?

                  aha_1980A jsulmJ 2 Replies Last reply
                  0
                  • Christian EhrlicherC Offline
                    Christian EhrlicherC Offline
                    Christian Ehrlicher
                    Lifetime Qt Champion
                    wrote on last edited by
                    #11

                    @JonB said in can not print correctly after convert QString to char *:

                    does that return such a "temporary object"?

                    It's not about returning something. It's about the lifetime of an object.

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

                    JonBJ 1 Reply Last reply
                    1
                    • Christian EhrlicherC Christian Ehrlicher

                      @JonB said in can not print correctly after convert QString to char *:

                      does that return such a "temporary object"?

                      It's not about returning something. It's about the lifetime of an object.

                      JonBJ Offline
                      JonBJ Offline
                      JonB
                      wrote on last edited by
                      #12

                      @Christian-Ehrlicher

                      Temporary objects are created when a prvalue is materialized so that it can be used as a glvalue, which occurs (since C++17) in the following situations:

                      Lovely!

                      I also note its second item is:

                      returning a prvalue from a function

                      Is that where we are here? I'm not stupid, but I am clearly struggling to recognise which situations this applies in.... :(

                      1 Reply Last reply
                      0
                      • JonBJ JonB

                        @aha_1980
                        Wow, OK, yes, I need to read! My problem is I have been "spoiled" by using C# and then Python/PyQt/PySide2 for so long now that I rarely have to think about this!

                        So let's take a basic, if my C++ holds up. If I write a function

                        QByteArray func()
                        {
                            QByteArray qb;
                            return qb;
                        }
                        

                        does that return such a "temporary object"? And that would be true for any class/struct I decalred and then returned in that fashion?

                        aha_1980A Offline
                        aha_1980A Offline
                        aha_1980
                        Lifetime Qt Champion
                        wrote on last edited by
                        #13

                        Hi @JonB,

                        if I take your example and do the following: QByteArray ba = func(); then ba lives until it goes out of scope. But if I do QByteArray hex = func().toHex() I have two conversations in one line. That is no problem here, as I take the result of func() and immediately call toHex() on it. But note that afterward neither the returned value of func() nor of toHex() exists anymore, only hex.

                        And that is the whole problem, with data() you access the raw data of an object that's lifetime is already over.

                        Regards

                        Qt has to stay free or it will die.

                        1 Reply Last reply
                        1
                        • MozzieM Mozzie

                          Env: vs2017 , Qt 5.14.0, Qt 5.12.6
                          code:

                          		QString s = "hello world";
                          		qDebug() << s;
                          		qDebug() << s.toUtf8().data();
                          
                          		char* p = s.toUtf8().data();
                          		qDebug() << p;
                          
                          		QByteArray b = s.toUtf8();
                          		p = b.data();
                          		qDebug() << p;
                          
                          

                          output:

                          "hello world"
                          hello world
                          ????????????????????????????????????????????7
                          hello world
                          

                          can somebody explain this, I'd be appreciate

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

                          @Mozzie You had a bit of bad luck, if you compile in Release mode instead of Debug it'll work fine

                          "hello world"
                          hello world
                          hello world
                          hello world
                          

                          And if you switch to MinGW compiler it'll work both in Debug and Release :-)

                          JonBJ aha_1980A MozzieM 3 Replies Last reply
                          1
                          • Christian EhrlicherC Christian Ehrlicher

                            @Mozzie said in can not print correctly after convert QString to char *:

                            char* p = s.toUtf8().data();

                            C++ basics - you're creating a temporary here so p points to garbage after this statement.

                            MozzieM Offline
                            MozzieM Offline
                            Mozzie
                            wrote on last edited by
                            #15

                            @Christian-Ehrlicher
                            Thank you very much, and thank other replyer.
                            I think i understand your reply, and I do fogot the temp object , maybe because I also use java a lot.

                            and i alse have a few questions:

                            1. where is the temp object in memory, stack or heap or somewhere else.
                            2. if it is on stack, it can not remain until the stack is finished
                            Christian EhrlicherC 1 Reply Last reply
                            0
                            • hskoglundH hskoglund

                              @Mozzie You had a bit of bad luck, if you compile in Release mode instead of Debug it'll work fine

                              "hello world"
                              hello world
                              hello world
                              hello world
                              

                              And if you switch to MinGW compiler it'll work both in Debug and Release :-)

                              JonBJ Offline
                              JonBJ Offline
                              JonB
                              wrote on last edited by JonB
                              #16

                              @hskoglund
                              Your findings are even more scary in view of the above conversation! :)

                              @aha_1980 , and others
                              I think I get it. Also that it's nothing to do with Qt specific classes. Not because of shared QByteArrays and stuff.

                              So to summarize: s.toUtf8() only "lasts" for the lifetime of the statement (probably rather expression) it is in. But if you go QByteArray b = s.toUtf8() then the b will persist OK as usual. Right?

                              hskoglundH 1 Reply Last reply
                              3
                              • hskoglundH hskoglund

                                @Mozzie You had a bit of bad luck, if you compile in Release mode instead of Debug it'll work fine

                                "hello world"
                                hello world
                                hello world
                                hello world
                                

                                And if you switch to MinGW compiler it'll work both in Debug and Release :-)

                                aha_1980A Offline
                                aha_1980A Offline
                                aha_1980
                                Lifetime Qt Champion
                                wrote on last edited by
                                #17

                                @hskoglund said in can not print correctly after convert QString to char *:

                                And if you switch to MinGW compiler it'll work both in Debug and Release :-)

                                Today. Tomorrow it will run away with your wife, bankrupt your workplace and aim for world domination.

                                t

                                Qt has to stay free or it will die.

                                1 Reply Last reply
                                4
                                • JonBJ JonB

                                  @hskoglund
                                  Your findings are even more scary in view of the above conversation! :)

                                  @aha_1980 , and others
                                  I think I get it. Also that it's nothing to do with Qt specific classes. Not because of shared QByteArrays and stuff.

                                  So to summarize: s.toUtf8() only "lasts" for the lifetime of the statement (probably rather expression) it is in. But if you go QByteArray b = s.toUtf8() then the b will persist OK as usual. Right?

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

                                  Yes! I's just luck that the bits are still around in Release mode. The Debug mode output of ??????? could happen in Release also some other day when the sun doesn't shin.e

                                  Anyway, one simple modification to make it waterproof could be:

                                  QString s = "hello world";
                                  qDebug() << s;
                                  qDebug() << s.toUtf8().data();
                                  
                                  QByteArray a = s.toUtf8();
                                  char* p = a.data();
                                  qDebug() << p;
                                  
                                  QByteArray b = s.toUtf8();
                                  p = b.data();
                                  qDebug() << p;
                                  

                                  Edit: too fast, didn't read the code in the 3d paragraph ! But they are both waterproof now :-)

                                  1 Reply Last reply
                                  1
                                  • hskoglundH hskoglund

                                    @Mozzie You had a bit of bad luck, if you compile in Release mode instead of Debug it'll work fine

                                    "hello world"
                                    hello world
                                    hello world
                                    hello world
                                    

                                    And if you switch to MinGW compiler it'll work both in Debug and Release :-)

                                    MozzieM Offline
                                    MozzieM Offline
                                    Mozzie
                                    wrote on last edited by
                                    #19

                                    @hskoglund
                                    that is interesting .
                                    i dont have test on linux or MinGW, maybe vs and MinGW is diffrent on deal with temp object?

                                    hskoglundH 1 Reply Last reply
                                    0
                                    • MozzieM Mozzie

                                      @hskoglund
                                      that is interesting .
                                      i dont have test on linux or MinGW, maybe vs and MinGW is diffrent on deal with temp object?

                                      hskoglundH Offline
                                      hskoglundH Offline
                                      hskoglund
                                      wrote on last edited by
                                      #20

                                      @Mozzie Actually MinGW works on Windows as well (I prefer it over MSVC2017 because MinGW compiles/builds my projects faster).

                                      1 Reply Last reply
                                      0
                                      • MozzieM Mozzie

                                        @Christian-Ehrlicher
                                        Thank you very much, and thank other replyer.
                                        I think i understand your reply, and I do fogot the temp object , maybe because I also use java a lot.

                                        and i alse have a few questions:

                                        1. where is the temp object in memory, stack or heap or somewhere else.
                                        2. if it is on stack, it can not remain until the stack is finished
                                        Christian EhrlicherC Offline
                                        Christian EhrlicherC Offline
                                        Christian Ehrlicher
                                        Lifetime Qt Champion
                                        wrote on last edited by
                                        #21

                                        @Mozzie said in can not print correctly after convert QString to char *:

                                        where is the temp object in memory, stack or heap or somewhere else.

                                        It's on the stack since you did not allocate it with new

                                        if it is on stack, it can not remain until the stack is finished

                                        No, this is not allowed since it's unnamed.

                                        It's also not c++ specific - you can do the same (in a little bit more obvious way) in C:

                                        int *myPtr = nullptr;
                                        {
                                          int a = 3;
                                          myPtr = &a;
                                          printf("%d\n", *myPtr);   // works fine
                                        }
                                        printf("%d\n", *myPtr);   // works on garbage and may eat kitten
                                        

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

                                        JonBJ MozzieM 2 Replies Last reply
                                        3
                                        • Christian EhrlicherC Christian Ehrlicher

                                          @Mozzie said in can not print correctly after convert QString to char *:

                                          where is the temp object in memory, stack or heap or somewhere else.

                                          It's on the stack since you did not allocate it with new

                                          if it is on stack, it can not remain until the stack is finished

                                          No, this is not allowed since it's unnamed.

                                          It's also not c++ specific - you can do the same (in a little bit more obvious way) in C:

                                          int *myPtr = nullptr;
                                          {
                                            int a = 3;
                                            myPtr = &a;
                                            printf("%d\n", *myPtr);   // works fine
                                          }
                                          printf("%d\n", *myPtr);   // works on garbage and may eat kitten
                                          
                                          JonBJ Offline
                                          JonBJ Offline
                                          JonB
                                          wrote on last edited by JonB
                                          #22

                                          @Christian-Ehrlicher said in can not print correctly after convert QString to char *:

                                          int *myPtr = nullptr;

                                          Never heard of nullptr in C ;-) NULL was much nicer to read anyway.

                                          MozzieM 1 Reply Last reply
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