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can not print correctly after convert QString to char *

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  • 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 Online
                      hskoglundH Online
                      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 Online
                              hskoglundH Online
                              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 Online
                                  hskoglundH Online
                                  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
                                      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?

                                        jsulmJ Offline
                                        jsulmJ Offline
                                        jsulm
                                        Lifetime Qt Champion
                                        wrote on last edited by
                                        #23
                                        This post is deleted!
                                        1 Reply Last reply
                                        0
                                        • 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
                                          
                                          MozzieM Offline
                                          MozzieM Offline
                                          Mozzie
                                          wrote on last edited by
                                          #24

                                          @Christian-Ehrlicher
                                          thanks, it helped a lot.
                                          and i have a hunch

                                          {//main stack
                                          	QString s = "hello world";
                                          	char* p = nullptr;
                                          	{// toUtf8()
                                          		QByteArray b = s.toUtf8();
                                          		{// data();
                                          			p = b.data();
                                          			qDebug() << p; // does this is same as "qDebug() << s.toUtf8().data();"
                                          		}
                                          	}
                                          	// b is freed
                                          	qDebug() << p; // this is same as "char * p = s.toUtf8().data(); qDebug() << p;"
                                          }
                                          

                                          does this right?

                                          Christian EhrlicherC 1 Reply Last reply
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