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can not print correctly after convert QString to char *

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  • M Offline
    M Offline
    Mozzie
    wrote on 26 Feb 2020, 17:01 last edited by
    #1

    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

    J H 2 Replies Last reply 26 Feb 2020, 17:18
    0
    • C Offline
      C Offline
      Christian Ehrlicher
      Lifetime Qt Champion
      wrote on 26 Feb 2020, 17:12 last edited by Christian Ehrlicher
      #2

      @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.

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

      J M 2 Replies Last reply 26 Feb 2020, 17:23
      6
      • R Offline
        R Offline
        Rondog
        wrote on 26 Feb 2020, 17:14 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
        • M Mozzie
          26 Feb 2020, 17:01

          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

          J Offline
          J Offline
          JonB
          wrote on 26 Feb 2020, 17:18 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?

          C 1 Reply Last reply 26 Feb 2020, 17:20
          0
          • J JonB
            26 Feb 2020, 17:18

            @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?

            C Offline
            C Offline
            Christian Ehrlicher
            Lifetime Qt Champion
            wrote on 26 Feb 2020, 17:20 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
            • C Christian Ehrlicher
              26 Feb 2020, 17:12

              @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.

              J Offline
              J Offline
              JonB
              wrote on 26 Feb 2020, 17:23 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 26 Feb 2020, 17:24
              0
              • J JonB
                26 Feb 2020, 17:23

                @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 26 Feb 2020, 17:24 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

                J 1 Reply Last reply 26 Feb 2020, 17:28
                1
                • VRoninV VRonin
                  26 Feb 2020, 17:24

                  @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

                  J Offline
                  J Offline
                  JonB
                  wrote on 26 Feb 2020, 17:28 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 26 Feb 2020, 17:35
                  0
                  • J JonB
                    26 Feb 2020, 17:28

                    @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 26 Feb 2020, 17:35 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.

                    J 1 Reply Last reply 26 Feb 2020, 17:39
                    2
                    • aha_1980A aha_1980
                      26 Feb 2020, 17:35

                      Hi @JonB,

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

                      Regards

                      J Offline
                      J Offline
                      JonB
                      wrote on 26 Feb 2020, 17:39 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 26 Feb 2020, 17:48
                      0
                      • C Offline
                        C Offline
                        Christian Ehrlicher
                        Lifetime Qt Champion
                        wrote on 26 Feb 2020, 17:40 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

                        J 1 Reply Last reply 26 Feb 2020, 17:43
                        1
                        • C Christian Ehrlicher
                          26 Feb 2020, 17:40

                          @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.

                          J Offline
                          J Offline
                          JonB
                          wrote on 26 Feb 2020, 17:43 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
                          • J JonB
                            26 Feb 2020, 17:39

                            @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 26 Feb 2020, 17:48 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
                            • M Mozzie
                              26 Feb 2020, 17:01

                              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

                              H Offline
                              H Offline
                              hskoglund
                              wrote on 26 Feb 2020, 17:50 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 :-)

                              J aha_1980A M 3 Replies Last reply 26 Feb 2020, 17:55
                              1
                              • C Christian Ehrlicher
                                26 Feb 2020, 17:12

                                @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.

                                M Offline
                                M Offline
                                Mozzie
                                wrote on 26 Feb 2020, 17:52 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
                                C 1 Reply Last reply 26 Feb 2020, 18:35
                                0
                                • H hskoglund
                                  26 Feb 2020, 17:50

                                  @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 :-)

                                  J Offline
                                  J Offline
                                  JonB
                                  wrote on 26 Feb 2020, 17:55 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?

                                  H 1 Reply Last reply 26 Feb 2020, 18:03
                                  3
                                  • H hskoglund
                                    26 Feb 2020, 17:50

                                    @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 26 Feb 2020, 18:01 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
                                    • J JonB
                                      26 Feb 2020, 17:55

                                      @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?

                                      H Offline
                                      H Offline
                                      hskoglund
                                      wrote on 26 Feb 2020, 18:03 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
                                      • H hskoglund
                                        26 Feb 2020, 17:50

                                        @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 :-)

                                        M Offline
                                        M Offline
                                        Mozzie
                                        wrote on 26 Feb 2020, 18:08 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?

                                        H 1 Reply Last reply 26 Feb 2020, 18:11
                                        0
                                        • M Mozzie
                                          26 Feb 2020, 18:08

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

                                          H Offline
                                          H Offline
                                          hskoglund
                                          wrote on 26 Feb 2020, 18:11 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
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