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    Solved can not print correctly after convert QString to char *

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    • JonB
      JonB @Christian Ehrlicher last edited by 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?

      VRonin 1 Reply Last reply Reply Quote 0
      • VRonin
        VRonin @JonB last edited by 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

        "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

        JonB 1 Reply Last reply Reply Quote 1
        • JonB
          JonB @VRonin last edited by

          @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_1980 1 Reply Last reply Reply Quote 0
          • aha_1980
            aha_1980 Lifetime Qt Champion @JonB last edited by

            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.

            JonB 1 Reply Last reply Reply Quote 2
            • JonB
              JonB @aha_1980 last edited by 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_1980 jsulm 2 Replies Last reply Reply Quote 0
              • Christian Ehrlicher
                Christian Ehrlicher Lifetime Qt Champion last edited by

                @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

                JonB 1 Reply Last reply Reply Quote 1
                • JonB
                  JonB @Christian Ehrlicher last edited by

                  @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 Reply Quote 0
                  • aha_1980
                    aha_1980 Lifetime Qt Champion @JonB last edited by

                    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 Reply Quote 1
                    • hskoglund
                      hskoglund @Mozzie last edited by

                      @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 :-)

                      JonB aha_1980 Mozzie 3 Replies Last reply Reply Quote 1
                      • Mozzie
                        Mozzie @Christian Ehrlicher last edited by

                        @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 Ehrlicher 1 Reply Last reply Reply Quote 0
                        • JonB
                          JonB @hskoglund last edited by 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?

                          hskoglund 1 Reply Last reply Reply Quote 3
                          • aha_1980
                            aha_1980 Lifetime Qt Champion @hskoglund last edited by

                            @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 Reply Quote 4
                            • hskoglund
                              hskoglund @JonB last edited by hskoglund

                              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 Reply Quote 1
                              • Mozzie
                                Mozzie @hskoglund last edited by

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

                                hskoglund 1 Reply Last reply Reply Quote 0
                                • hskoglund
                                  hskoglund @Mozzie last edited by

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

                                  1 Reply Last reply Reply Quote 0
                                  • Christian Ehrlicher
                                    Christian Ehrlicher Lifetime Qt Champion @Mozzie last edited by

                                    @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
                                    

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                                    JonB Mozzie 2 Replies Last reply Reply Quote 3
                                    • JonB
                                      JonB @Christian Ehrlicher last edited by 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.

                                      Mozzie 1 Reply Last reply Reply Quote 0
                                      • jsulm
                                        jsulm Lifetime Qt Champion @JonB last edited by

                                        This post is deleted!
                                        1 Reply Last reply Reply Quote 0
                                        • Mozzie
                                          Mozzie @Christian Ehrlicher last edited by

                                          @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 Ehrlicher 1 Reply Last reply Reply Quote 1
                                          • Mozzie
                                            Mozzie @JonB last edited by

                                            @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 *
                                            
                                            JonB 1 Reply Last reply Reply Quote 1
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