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can not print correctly after convert QString to char *

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  • 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 Online
          Christian EhrlicherC Online
          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 Online
                              Christian EhrlicherC Online
                              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 Online
                                          Christian EhrlicherC Online
                                          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

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