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can not print correctly after convert QString to char *

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  • 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
                              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
                                  1
                                  • JonBJ JonB

                                    @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 Offline
                                    MozzieM Offline
                                    Mozzie
                                    wrote on last edited by
                                    #25

                                    @JonB
                                    nullptr is a c++11 key word, you can still use NULL, but NULL is defined as 0, sometimes it may cause some problem.

                                    such as:

                                    
                                    void test(int *p)
                                    {
                                    	qDebug() << "int *";
                                    }
                                    void test(int i)
                                    {
                                    	qDebug() << "int";
                                    }
                                    test(NULL);
                                    test(nullptr);
                                    

                                    output

                                    int
                                    int *
                                    
                                    JonBJ 1 Reply Last reply
                                    1
                                    • MozzieM Mozzie

                                      @JonB
                                      nullptr is a c++11 key word, you can still use NULL, but NULL is defined as 0, sometimes it may cause some problem.

                                      such as:

                                      
                                      void test(int *p)
                                      {
                                      	qDebug() << "int *";
                                      }
                                      void test(int i)
                                      {
                                      	qDebug() << "int";
                                      }
                                      test(NULL);
                                      test(nullptr);
                                      

                                      output

                                      int
                                      int *
                                      
                                      JonBJ Offline
                                      JonBJ Offline
                                      JonB
                                      wrote on last edited by JonB
                                      #26

                                      @Mozzie
                                      I know this :) That's why I was picking @Christian-Ehrlicher on his use of nullptr in his C program, it was just intended for amusement ;-)

                                      1 Reply Last reply
                                      1
                                      • MozzieM Mozzie

                                        @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 Offline
                                        Christian EhrlicherC Offline
                                        Christian Ehrlicher
                                        Lifetime Qt Champion
                                        wrote on last edited by
                                        #27

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

                                        does this right?

                                        Yes, exactly.

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

                                        MozzieM 1 Reply Last reply
                                        0
                                        • Christian EhrlicherC Christian Ehrlicher

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

                                          does this right?

                                          Yes, exactly.

                                          MozzieM Offline
                                          MozzieM Offline
                                          Mozzie
                                          wrote on last edited by
                                          #28

                                          @Christian-Ehrlicher
                                          thanks.

                                          1 Reply Last reply
                                          0

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