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Interfaces must inherit QObject?!

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  • idlefrogI idlefrog

    Given a couple of classes such as:

    #include <QObject>
    
    class ICake
    {
    public:
    
        ICake(){}
    };
    
    class VictoriaSpongeCake : public ICake, public QObject
    {
        Q_OBJECT
    
    public:
        VictoriaSpongeCake():ICake(){}
    };
    

    why do I get errors such as:

    /home/j/develop/qt/projects/build-InheritInterface-Desktop_Qt_5_15_2_GCC_64bit-Debug/moc_victoriaspongecake.cpp:67: error: ‘staticMetaObject’ is not a member of ‘ICake’
    moc_victoriaspongecake.cpp:67:41: error: ‘staticMetaObject’ is not a member of ‘ICake’
       67 |     QMetaObject::SuperData::link<ICake::staticMetaObject>(),
          |                                         ^~~~~~~~~~~~~~~~
    

    Of course the intention here is for concrete 'cakes' (errhum...sorry) to implement some signals & slots.

    So if I change ICake to inherit like this:

    class ICake: public QObject
    

    It all compiles without a problem.

    JonBJ Offline
    JonBJ Offline
    JonB
    wrote on last edited by
    #3

    @idlefrog said in Interfaces must inherit QObject?!:

    It all compiles without a problem.

    I am surprised. Maybe you will have runtime problems. You will find plenty of articles out there telling you in Qt you must not inherit from QObject multiple times.

    idlefrogI 1 Reply Last reply
    0
    • KroMignonK KroMignon

      @idlefrog said in Interfaces must inherit QObject?!:

      Of course the intention here is for concrete 'cakes' (errhum...sorry) I want to implement some signals & slots.

      I think QObject should be the first class in inheritance list:

      class VictoriaSpongeCake : public QObject, public ICake
      {
          Q_OBJECT
      
      public:
          VictoriaSpongeCake(QObject *parent = nullptr):QObject(parent), ICake(){}
      };
      
      idlefrogI Offline
      idlefrogI Offline
      idlefrog
      wrote on last edited by
      #4

      @KroMignon Yes,.. that did the job! Wouldn't have thought the inheritance order would matter... clearly it does. Thanks!

      1 Reply Last reply
      0
      • JonBJ JonB

        @idlefrog said in Interfaces must inherit QObject?!:

        It all compiles without a problem.

        I am surprised. Maybe you will have runtime problems. You will find plenty of articles out there telling you in Qt you must not inherit from QObject multiple times.

        idlefrogI Offline
        idlefrogI Offline
        idlefrog
        wrote on last edited by
        #5

        @JonB Yes, I also thought I would end up (eventually) in a catch 22 situation with diamond inheritance all over the shop! Thank goodness a simple inheritance reordering did the trick! :)

        KroMignonK 1 Reply Last reply
        0
        • idlefrogI idlefrog

          @JonB Yes, I also thought I would end up (eventually) in a catch 22 situation with diamond inheritance all over the shop! Thank goodness a simple inheritance reordering did the trick! :)

          KroMignonK Offline
          KroMignonK Offline
          KroMignon
          wrote on last edited by
          #6

          @idlefrog said in Interfaces must inherit QObject?!:

          Thank goodness a simple inheritance reordering did the trick! :)

          I don't remember where I found this in Qt documentation (and not remember why), but QObject (or a QObject based class) must be the first class in inheritance list.

          It is an old maxim of mine that when you have excluded the impossible, whatever remains, however improbable, must be the truth. (Sherlock Holmes)

          JonBJ 1 Reply Last reply
          0
          • KroMignonK KroMignon

            @idlefrog said in Interfaces must inherit QObject?!:

            Thank goodness a simple inheritance reordering did the trick! :)

            I don't remember where I found this in Qt documentation (and not remember why), but QObject (or a QObject based class) must be the first class in inheritance list.

            JonBJ Offline
            JonBJ Offline
            JonB
            wrote on last edited by JonB
            #7

            @KroMignon
            I should like a reference for this. So far as I know multiple inheritance from QObject is not supported. See for example QObject Multiple Inheritance. This is required by moc, perhaps we will see where the OP gets over time.

            The suggested solution there shows "the Qt way" of implementing what I think @idlefrog is trying to achieve, avoiding the multiple inheritance.

            idlefrogI JKSHJ 2 Replies Last reply
            1
            • JonBJ JonB

              @KroMignon
              I should like a reference for this. So far as I know multiple inheritance from QObject is not supported. See for example QObject Multiple Inheritance. This is required by moc, perhaps we will see where the OP gets over time.

              The suggested solution there shows "the Qt way" of implementing what I think @idlefrog is trying to achieve, avoiding the multiple inheritance.

              idlefrogI Offline
              idlefrogI Offline
              idlefrog
              wrote on last edited by
              #8

              @JonB Thanks for the link... it's a interesting reference. The composition solution might be handy, although looks a bit un-Qt-ish if there is such a thing! :)

              1 Reply Last reply
              0
              • JonBJ JonB

                @KroMignon
                I should like a reference for this. So far as I know multiple inheritance from QObject is not supported. See for example QObject Multiple Inheritance. This is required by moc, perhaps we will see where the OP gets over time.

                The suggested solution there shows "the Qt way" of implementing what I think @idlefrog is trying to achieve, avoiding the multiple inheritance.

                JKSHJ Offline
                JKSHJ Offline
                JKSH
                Moderators
                wrote on last edited by JKSH
                #9

                @KroMignon said in Interfaces must inherit QObject?!:

                I don't remember where I found this in Qt documentation (and not remember why), but QObject (or a QObject based class) must be the first class in inheritance list.

                ...

                @JonB said in Interfaces must inherit QObject?!:

                I should like a reference for this.

                https://doc.qt.io/qt-5/moc.html#multiple-inheritance-requires-qobject-to-be-first

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

                JonBJ 1 Reply Last reply
                4
                • JKSHJ JKSH

                  @KroMignon said in Interfaces must inherit QObject?!:

                  I don't remember where I found this in Qt documentation (and not remember why), but QObject (or a QObject based class) must be the first class in inheritance list.

                  ...

                  @JonB said in Interfaces must inherit QObject?!:

                  I should like a reference for this.

                  https://doc.qt.io/qt-5/moc.html#multiple-inheritance-requires-qobject-to-be-first

                  JonBJ Offline
                  JonBJ Offline
                  JonB
                  wrote on last edited by JonB
                  #10

                  @JKSH said in Interfaces must inherit QObject?!:

                  https://doc.qt.io/qt-5/moc.html#multiple-inheritance-requires-qobject-to-be-first

                  If you are using multiple inheritance, moc assumes that the first inherited class is a subclass of QObject. Also, be sure that only the first inherited class is a QObject.

                  Thank you for this reference. However, unless I read it wrong, this does not address the @idlefrog's case or @KroMignon's solution.

                  In the example in the docs

                  class SomeClass : public QObject, public OtherClass
                  

                  Here there is no indication that the second class, OtherClass, itself inherits QObject. And it states explicitly

                  Also, be sure that only the first inherited class is a QObject.

                  @idlefrog is asking specifically about multiple inheritance where both classes come from QObject, and that is what i was trying to answer.

                  I see that several members, including @kshegunov, have up-voted your post. I would welcome clarification on this issue, please, as my understand is that multiple inheritance where both involve QObject is not supported, regardless of ordering?

                  KroMignonK kshegunovK 2 Replies Last reply
                  0
                  • JonBJ JonB

                    @JKSH said in Interfaces must inherit QObject?!:

                    https://doc.qt.io/qt-5/moc.html#multiple-inheritance-requires-qobject-to-be-first

                    If you are using multiple inheritance, moc assumes that the first inherited class is a subclass of QObject. Also, be sure that only the first inherited class is a QObject.

                    Thank you for this reference. However, unless I read it wrong, this does not address the @idlefrog's case or @KroMignon's solution.

                    In the example in the docs

                    class SomeClass : public QObject, public OtherClass
                    

                    Here there is no indication that the second class, OtherClass, itself inherits QObject. And it states explicitly

                    Also, be sure that only the first inherited class is a QObject.

                    @idlefrog is asking specifically about multiple inheritance where both classes come from QObject, and that is what i was trying to answer.

                    I see that several members, including @kshegunov, have up-voted your post. I would welcome clarification on this issue, please, as my understand is that multiple inheritance where both involve QObject is not supported, regardless of ordering?

                    KroMignonK Offline
                    KroMignonK Offline
                    KroMignon
                    wrote on last edited by
                    #11

                    @JonB said in Interfaces must inherit QObject?!:

                    I see that several members, including @kshegunov, have up-voted your post. I would welcome clarification on this issue, please, as my understand is that multiple inheritance where both involve QObject is not supported, regardless of ordering?

                    I up-voted @JKSH post, because it gives the link to documentation I talking about.
                    But, the reply to your question: according to documentation QObject multiple inheritance (diamond inheritance) is not allowed/possible.

                    It is an old maxim of mine that when you have excluded the impossible, whatever remains, however improbable, must be the truth. (Sherlock Holmes)

                    JonBJ 1 Reply Last reply
                    1
                    • KroMignonK KroMignon

                      @JonB said in Interfaces must inherit QObject?!:

                      I see that several members, including @kshegunov, have up-voted your post. I would welcome clarification on this issue, please, as my understand is that multiple inheritance where both involve QObject is not supported, regardless of ordering?

                      I up-voted @JKSH post, because it gives the link to documentation I talking about.
                      But, the reply to your question: according to documentation QObject multiple inheritance (diamond inheritance) is not allowed/possible.

                      JonBJ Offline
                      JonBJ Offline
                      JonB
                      wrote on last edited by
                      #12

                      @KroMignon Thanks @Kro, understood, I'm just seeking clarification because this is all getting a bit muddy.... :)

                      1 Reply Last reply
                      0
                      • idlefrogI Offline
                        idlefrogI Offline
                        idlefrog
                        wrote on last edited by
                        #13

                        It seems to me that for a class where you inherited perhaps a set of interfaces and one QObject, you can't inherit any other QObject class. e.g.

                        class RealCake: public QObject, public ICake, public IEdible{}
                        
                        class PlasticCake: public QObject, public ICake {}
                        
                        class JokeCake: public PlasticCake, public RealCake{}
                        

                        The JokeCake is not allowed in the world of Qt. Pity!

                        kshegunovK JonBJ 2 Replies Last reply
                        0
                        • JonBJ JonB

                          @JKSH said in Interfaces must inherit QObject?!:

                          https://doc.qt.io/qt-5/moc.html#multiple-inheritance-requires-qobject-to-be-first

                          If you are using multiple inheritance, moc assumes that the first inherited class is a subclass of QObject. Also, be sure that only the first inherited class is a QObject.

                          Thank you for this reference. However, unless I read it wrong, this does not address the @idlefrog's case or @KroMignon's solution.

                          In the example in the docs

                          class SomeClass : public QObject, public OtherClass
                          

                          Here there is no indication that the second class, OtherClass, itself inherits QObject. And it states explicitly

                          Also, be sure that only the first inherited class is a QObject.

                          @idlefrog is asking specifically about multiple inheritance where both classes come from QObject, and that is what i was trying to answer.

                          I see that several members, including @kshegunov, have up-voted your post. I would welcome clarification on this issue, please, as my understand is that multiple inheritance where both involve QObject is not supported, regardless of ordering?

                          kshegunovK Offline
                          kshegunovK Offline
                          kshegunov
                          Moderators
                          wrote on last edited by kshegunov
                          #14

                          @JonB said in Interfaces must inherit QObject?!:

                          I see that several members, including @kshegunov, have up-voted your post. I would welcome clarification on this issue, please, as my understand is that multiple inheritance where both involve QObject is not supported, regardless of ordering?

                          If both classes inherit from QObject you enter virtual-inheritance-land, where the night is dark and full of terrors ... and it's simply not supported by moc (last sentence of the mentioned link). Otherwise multiple inheritance is fine, good, expected and so on (provided you use it properly), and is supported both by the language and by Qt with the minor note about base class order.

                          Read and abide by the Qt Code of Conduct

                          1 Reply Last reply
                          0
                          • idlefrogI idlefrog

                            It seems to me that for a class where you inherited perhaps a set of interfaces and one QObject, you can't inherit any other QObject class. e.g.

                            class RealCake: public QObject, public ICake, public IEdible{}
                            
                            class PlasticCake: public QObject, public ICake {}
                            
                            class JokeCake: public PlasticCake, public RealCake{}
                            

                            The JokeCake is not allowed in the world of Qt. Pity!

                            kshegunovK Offline
                            kshegunovK Offline
                            kshegunov
                            Moderators
                            wrote on last edited by
                            #15

                            @idlefrog said in Interfaces must inherit QObject?!:

                            The JokeCake is not allowed in the world of Qt. Pity!

                            Even if it were allowed, you're using it incorrectly. C++ is hard, man, it's just the way it is.

                            Read and abide by the Qt Code of Conduct

                            1 Reply Last reply
                            0
                            • idlefrogI idlefrog

                              It seems to me that for a class where you inherited perhaps a set of interfaces and one QObject, you can't inherit any other QObject class. e.g.

                              class RealCake: public QObject, public ICake, public IEdible{}
                              
                              class PlasticCake: public QObject, public ICake {}
                              
                              class JokeCake: public PlasticCake, public RealCake{}
                              

                              The JokeCake is not allowed in the world of Qt. Pity!

                              JonBJ Offline
                              JonBJ Offline
                              JonB
                              wrote on last edited by
                              #16

                              @idlefrog
                              Yes indeed that is precisely my understanding (unless mistaken). You must not inherit multiple times from QObejct, whether directly or indirectly, and regardless of order. Because of moc requirement/limitation.

                              I still refer you back to QObject Multiple Inheritance. Ah, hang on, I think the solution I had in mind is at https://stackoverflow.com/a/18113601/489865 "the Qt way of doing this is like this...." But now I see that includes

                              I am amazed that this actually works, but bummed out by the fact that it requires the old-style SIGNAL()/SLOT() syntax...

                              [Not my language!] Which is a bu**er :(

                              kshegunovK 1 Reply Last reply
                              0
                              • JonBJ JonB

                                @idlefrog
                                Yes indeed that is precisely my understanding (unless mistaken). You must not inherit multiple times from QObejct, whether directly or indirectly, and regardless of order. Because of moc requirement/limitation.

                                I still refer you back to QObject Multiple Inheritance. Ah, hang on, I think the solution I had in mind is at https://stackoverflow.com/a/18113601/489865 "the Qt way of doing this is like this...." But now I see that includes

                                I am amazed that this actually works, but bummed out by the fact that it requires the old-style SIGNAL()/SLOT() syntax...

                                [Not my language!] Which is a bu**er :(

                                kshegunovK Offline
                                kshegunovK Offline
                                kshegunov
                                Moderators
                                wrote on last edited by kshegunov
                                #17

                                @JonB said in Interfaces must inherit QObject?!:

                                I still refer you back to QObject Multiple Inheritance. Ah, hang on, I think the solution I had in mind is at https://stackoverflow.com/a/18113601/489865 "the Qt way of doing this is like this...." But now I see that includes

                                This is simply an abomination. Signals act polymorphically without the need to declare them as pure virtual in interfaces. Slots are a different kettle of fish, they work just fine with overriding.

                                (Edit)
                                Addendum:

                                Yes indeed that is precisely my understanding (unless mistaken). You must not inherit multiple times from QObejct, whether directly or indirectly, and regardless of order. Because of moc requirement/limitation.

                                Just to spill it as explicitly and bluntly as I possibly can:
                                You shall not derive a class from the same base multiple times (directly or indirectly) if you don't intimately know what's the difference between:

                                class Derived : public Base
                                

                                and

                                class Derived : virtual public Base
                                

                                Read and abide by the Qt Code of Conduct

                                JonBJ 1 Reply Last reply
                                3
                                • kshegunovK kshegunov

                                  @JonB said in Interfaces must inherit QObject?!:

                                  I still refer you back to QObject Multiple Inheritance. Ah, hang on, I think the solution I had in mind is at https://stackoverflow.com/a/18113601/489865 "the Qt way of doing this is like this...." But now I see that includes

                                  This is simply an abomination. Signals act polymorphically without the need to declare them as pure virtual in interfaces. Slots are a different kettle of fish, they work just fine with overriding.

                                  (Edit)
                                  Addendum:

                                  Yes indeed that is precisely my understanding (unless mistaken). You must not inherit multiple times from QObejct, whether directly or indirectly, and regardless of order. Because of moc requirement/limitation.

                                  Just to spill it as explicitly and bluntly as I possibly can:
                                  You shall not derive a class from the same base multiple times (directly or indirectly) if you don't intimately know what's the difference between:

                                  class Derived : public Base
                                  

                                  and

                                  class Derived : virtual public Base
                                  
                                  JonBJ Offline
                                  JonBJ Offline
                                  JonB
                                  wrote on last edited by JonB
                                  #18

                                  @kshegunov
                                  Yeah, I didn't notice the virtual on the signal. That's the trouble with assuming an accepted solution with 60 up-votes must be good!

                                  It shows a lot of people are obviously struggling with this. How then can the OP here arrange to write an "interface" which inherits from QObject and apply it to classes which inherit from QObject too?

                                  Ohhhh, maybe I misunderstood? Only now do I see that OP did not originally say that ICake needed QObject itself. Only that he put it first in the class inheritance list before QObject and that upset moc. I get it now, and why public QObject, public ICake works here. OK, but what about if OP did want multiple QObject inheritance, any way without the "abomination" ?

                                  kshegunovK 1 Reply Last reply
                                  0
                                  • JonBJ JonB

                                    @kshegunov
                                    Yeah, I didn't notice the virtual on the signal. That's the trouble with assuming an accepted solution with 60 up-votes must be good!

                                    It shows a lot of people are obviously struggling with this. How then can the OP here arrange to write an "interface" which inherits from QObject and apply it to classes which inherit from QObject too?

                                    Ohhhh, maybe I misunderstood? Only now do I see that OP did not originally say that ICake needed QObject itself. Only that he put it first in the class inheritance list before QObject and that upset moc. I get it now, and why public QObject, public ICake works here. OK, but what about if OP did want multiple QObject inheritance, any way without the "abomination" ?

                                    kshegunovK Offline
                                    kshegunovK Offline
                                    kshegunov
                                    Moderators
                                    wrote on last edited by
                                    #19

                                    @JonB said in Interfaces must inherit QObject?!:

                                    OK, but what about if OP did want multiple QObject inheritnace, any way without the "abomination" ?

                                    He shan't do it. Also read the addendum I added to my previous post.

                                    Read and abide by the Qt Code of Conduct

                                    JonBJ 1 Reply Last reply
                                    0
                                    • kshegunovK kshegunov

                                      @JonB said in Interfaces must inherit QObject?!:

                                      OK, but what about if OP did want multiple QObject inheritnace, any way without the "abomination" ?

                                      He shan't do it. Also read the addendum I added to my previous post.

                                      JonBJ Offline
                                      JonBJ Offline
                                      JonB
                                      wrote on last edited by
                                      #20

                                      @kshegunov
                                      Is that a requirement of C++? The OP claimed

                                      class VictoriaSpongeCake : public ICake, public QObject
                                      

                                      So if I change ICake to inherit like this:

                                      class ICake: public QObject
                                      

                                      It all compiles without a problem.

                                      That is not quoting me, it is quoting him.

                                      kshegunovK JKSHJ 2 Replies Last reply
                                      0
                                      • JonBJ JonB

                                        @kshegunov
                                        Is that a requirement of C++? The OP claimed

                                        class VictoriaSpongeCake : public ICake, public QObject
                                        

                                        So if I change ICake to inherit like this:

                                        class ICake: public QObject
                                        

                                        It all compiles without a problem.

                                        That is not quoting me, it is quoting him.

                                        kshegunovK Offline
                                        kshegunovK Offline
                                        kshegunov
                                        Moderators
                                        wrote on last edited by kshegunov
                                        #21

                                        @JonB said in Interfaces must inherit QObject?!:

                                        Is that a requirement of C++?

                                        It is not a requirement, but there's a significant difference.
                                        Here's links, I know you love them sources:
                                        https://isocpp.org/wiki/faq/multiple-inheritance (What is the “dreaded diamond”? being key)
                                        https://en.cppreference.com/w/cpp/language/derived_class (Virtual base classes is relevant)

                                        Read and abide by the Qt Code of Conduct

                                        JonBJ 1 Reply Last reply
                                        3
                                        • kshegunovK kshegunov

                                          @JonB said in Interfaces must inherit QObject?!:

                                          Is that a requirement of C++?

                                          It is not a requirement, but there's a significant difference.
                                          Here's links, I know you love them sources:
                                          https://isocpp.org/wiki/faq/multiple-inheritance (What is the “dreaded diamond”? being key)
                                          https://en.cppreference.com/w/cpp/language/derived_class (Virtual base classes is relevant)

                                          JonBJ Offline
                                          JonBJ Offline
                                          JonB
                                          wrote on last edited by
                                          #22

                                          @kshegunov
                                          Yeah, thanks, this looks good. public virtual Base, that's a new one for me, I like it. The more complexity in C++ the better... ;-)

                                          1 Reply Last reply
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