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    Forum Updated on Feb 6th

    Qt 5.5 - QList variable causes compilation to fail

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    • K
      knight556 last edited by

      Hi guys, I'm trying to include a QList member variable in my class, e.g.

      QList<QString> m_myList;

      I have included all the relevant classes, ie. <QList>, <QString>, etc, and I have QT += core in my make file, but the compilation always fails with the message:

      expected member name or ';' after declaration specifiers
      QList<QString> m_myList;
      –––––^

      I have tried everything I can think of to get it to work. I feel like I'm making a very fundamental mistake somewhere! Any ideas?

      1 Reply Last reply Reply Quote 0
      • dheerendra
        dheerendra Qt Champions 2022 last edited by

        Your error may be coming line previous to this line. Just check. I don't see any issue with above line of code.

        Dheerendra
        @Community Service
        Certified Qt Specialist
        http://www.pthinks.com

        1 Reply Last reply Reply Quote 4
        • K
          knight556 last edited by

          I wish it was that easy, but if that line is commented out, the rest of the program runs perfectly.

          mrjj 1 Reply Last reply Reply Quote 0
          • mrjj
            mrjj Lifetime Qt Champion @knight556 last edited by

            @knight556 said:

            And the
            #include <QList>
            #include <QString>

            are in the .H file ?

            with the class having the m_mylist?

            Are you using Creator and a pro file or how do you build the project ?

            1 Reply Last reply Reply Quote 1
            • K
              knight556 last edited by

              The includes are in the .cpp file, and yes I'm using Creator with a pro file. I've noticed that if I declare a QList within a class method then it compiles just fine, it's only when I try and declare it as a member variable that it complains.

              mrjj 1 Reply Last reply Reply Quote 0
              • SGaist
                SGaist Lifetime Qt Champion last edited by

                Hi and welcome to devnet,

                Not a direct answer but since you want a QList of QString, why not use QStringList ?

                Interested in AI ? www.idiap.ch
                Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

                1 Reply Last reply Reply Quote 1
                • mrjj
                  mrjj Lifetime Qt Champion @knight556 last edited by

                  @knight556
                  Hi

                  But if you declare m_list in the H file you must have
                  #include <QList>
                  #include <QString>

                  in the H file (and not cpp file ) or the type is unknown :)
                  Its not enough to include in CPP file.

                  I have noticed that if I declare a QList within a class method ...

                  Yes as you included the files in the cpp I guess.

                  K 1 Reply Last reply Reply Quote 1
                  • K
                    knight556 last edited by

                    This post is deleted!
                    1 Reply Last reply Reply Quote 0
                    • K
                      knight556 @mrjj last edited by

                      @mrjj Moving the includes to the .h file didn't help.

                      @SGaist Yes the QStringList does in fact work, but what I have failed to mention (for simplicity) is that this bug is occurring for all data types, eg. QList<int> fails as well. As it turns out I want to use my own custom class as the list type, but I can't hope to get that working until I get the QList working with more basic data types first!

                      Creator is clearly happy with QList being used, because if I start typing on the next line something like:
                      m_mylist.
                      the autocomplete/intellisense/whatever-it's-called will give me the option of append(), so something is working correctly!

                      mrjj 1 Reply Last reply Reply Quote 0
                      • mrjj
                        mrjj Lifetime Qt Champion @knight556 last edited by

                        @knight556

                        Oh ok. sorry for assuming it was that basic thing,

                        Can we try a simple test just to be sure compiler not funky?

                        If you create a new console project and then
                        right click -> add new -> Header file and call it thehappylist.h
                        and put code below in thehappylist.h

                        #ifndef THEHAPPYLIST
                        #define THEHAPPYLIST
                        
                        #include <QList>
                        #include <QString>
                        class Happy {
                        public:
                            QList<QString> m_list;
                        };
                        #endif // THEHAPPYLIST
                        

                        and then the main.cpp

                        #include "thehappylist.h"
                        int main(int argc, char *argv[])
                        {
                            Happy h;
                            h.m_list.append("test");
                        }
                        

                        And tell me if that compiles ?

                        1 Reply Last reply Reply Quote 1
                        • K
                          knight556 last edited by

                          @mrjj said:

                          #include "thehappylist.h"
                          int main(int argc, char *argv[])
                          {
                          Happy h;
                          h.m_list.append("test");
                          }

                          Hi, yes it does compile... which is frustrating because that doesn't help find the problem.

                          1 Reply Last reply Reply Quote 0
                          • mrjj
                            mrjj Lifetime Qt Champion last edited by

                            HI

                            Well, its not an include path thing then.
                            Since the mini sample works.

                            Would it be possible to post your code ?
                            I really can not get a clue of what could be wrong as you have the includes
                            and it all sounds ok.

                            1 Reply Last reply Reply Quote 1
                            • K
                              knight556 last edited by

                              I'm working on quite a large project (it's basically a DICOM image viewer), but I will attempt to create a minimal working example.

                              1 Reply Last reply Reply Quote 0
                              • K
                                knight556 last edited by

                                OK now this is weird. I'm not sure how I came to this, but instead of writing:

                                QList<QString> m_mylist;

                                I write:

                                ::QList<QString> m_mylist

                                It compiles and works... however Qt Creator doesn't seem to like it as the terms QList and QString are not coloured purple as normal.
                                Any ideas why this works?

                                JKSH 1 Reply Last reply Reply Quote 0
                                • JKSH
                                  JKSH Moderators @knight556 last edited by

                                  @knight556 said:

                                  OK now this is weird. I'm not sure how I came to this, but instead of writing:

                                  QList<QString> m_mylist;

                                  I write:

                                  ::QList<QString> m_mylist

                                  It compiles and works... however Qt Creator doesn't seem to like it as the terms QList and QString are not coloured purple as normal.
                                  Any ideas why this works?

                                  1. Did you redefine "QList" somewhere else in your code?
                                  2. Does this only happen in this particular class, or does it also happen if you add a QList member to other classses?

                                  Qt Doc Search for browsers: forum.qt.io/topic/35616/web-browser-extension-for-improved-doc-searches

                                  1 Reply Last reply Reply Quote 1
                                  • K
                                    knight556 last edited by

                                    Well that got me thinking, so I had a look and for some reason in another one of my classes I had listed a forward reference to QList. I have now deleted that line of code and the rest of the program compiles and works as expected, without needing the extra :: bits at the start of the QList declaration!
                                    So... clearly I don't understand forward references. Thanks for the help everyone!

                                    1 Reply Last reply Reply Quote 0
                                    • SGaist
                                      SGaist Lifetime Qt Champion last edited by

                                      It's pretty unusual to forward declare QList, why did you do that in the first place ?

                                      Interested in AI ? www.idiap.ch
                                      Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

                                      1 Reply Last reply Reply Quote 1
                                      • K
                                        knight556 last edited by

                                        I was learning Qt through a series of Pluralsight videos and the instructor was basically forward declaring every class he was gonna use, so I got into that habit. Time to go back and remove some of that rubbish.

                                        1 Reply Last reply Reply Quote 0
                                        • SGaist
                                          SGaist Lifetime Qt Champion last edited by

                                          Even QString ?

                                          Interested in AI ? www.idiap.ch
                                          Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

                                          1 Reply Last reply Reply Quote 1
                                          • K
                                            knight556 last edited by

                                            Yep even QString. Is forward referencing even necessary if you put all your includes in the header file? I always thought forward referencing was only needed if you had 2 classes that referenced one another, but of course 1 had to be defined before the other.

                                            JKSH 1 Reply Last reply Reply Quote 0
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