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[solved] MetaObject not referencing slots in derived class

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  • mrjjM Offline
    mrjjM Offline
    mrjj
    Lifetime Qt Champion
    wrote on last edited by
    #2

    Hi
    I wonder if Derived class need

    Q_OBJECT

    too ?

    kshegunovK 1 Reply Last reply
    3
    • mrjjM mrjj

      Hi
      I wonder if Derived class need

      Q_OBJECT

      too ?

      kshegunovK Offline
      kshegunovK Offline
      kshegunov
      Moderators
      wrote on last edited by
      #3

      @mrjj

      I wonder if Derived class need

      It does, so why do you wonder? :)

      Read and abide by the Qt Code of Conduct

      mrjjM 1 Reply Last reply
      0
      • kshegunovK kshegunov

        @mrjj

        I wonder if Derived class need

        It does, so why do you wonder? :)

        mrjjM Offline
        mrjjM Offline
        mrjj
        Lifetime Qt Champion
        wrote on last edited by
        #4

        @kshegunov

        I often just wonder :)
        nah, i never used
        metaObject()->methodCount();

        so was not 100% sure.

        kshegunovK 1 Reply Last reply
        0
        • mrjjM mrjj

          @kshegunov

          I often just wonder :)
          nah, i never used
          metaObject()->methodCount();

          so was not 100% sure.

          kshegunovK Offline
          kshegunovK Offline
          kshegunov
          Moderators
          wrote on last edited by
          #5

          @mrjj
          QObject::metaObject() returns the QMetaObject associated with the class (it's a static member, and the method provides virtualization over the class tree). If you don't have the Q_OBJECT macro, there's no staticMetaObject member for the class, and consequently there's no metaObject() override, thus you get the parent's staticMetaObject (or the last class that had the macro). Like in this case, there can't be reflection for the derived type, because the needed meta information isn't generated (i.e. the missing macro); the best you get is the information for the parent class.

          Although, a need for this kind of introspection is rather rare ...

          Read and abide by the Qt Code of Conduct

          mrjjM 1 Reply Last reply
          2
          • kshegunovK kshegunov

            @mrjj
            QObject::metaObject() returns the QMetaObject associated with the class (it's a static member, and the method provides virtualization over the class tree). If you don't have the Q_OBJECT macro, there's no staticMetaObject member for the class, and consequently there's no metaObject() override, thus you get the parent's staticMetaObject (or the last class that had the macro). Like in this case, there can't be reflection for the derived type, because the needed meta information isn't generated (i.e. the missing macro); the best you get is the information for the parent class.

            Although, a need for this kind of introspection is rather rare ...

            mrjjM Offline
            mrjjM Offline
            mrjj
            Lifetime Qt Champion
            wrote on last edited by
            #6

            @kshegunov
            ahh, i never really examined the macro.
            So thats how it works.
            Its very cool.
            I wish it was a pure c++ feature :)

            kshegunovK 1 Reply Last reply
            0
            • mrjjM mrjj

              @kshegunov
              ahh, i never really examined the macro.
              So thats how it works.
              Its very cool.
              I wish it was a pure c++ feature :)

              kshegunovK Offline
              kshegunovK Offline
              kshegunov
              Moderators
              wrote on last edited by kshegunov
              #7

              @mrjj
              Actually it is, somewhat. See here. The moc is needed to generate the meta information about the class, and the methods implementations, but for the static members or virtual functions declarations we have the good ol' preprocessor. ;)

              Read and abide by the Qt Code of Conduct

              mrjjM 1 Reply Last reply
              0
              • X Offline
                X Offline
                XavierLL
                wrote on last edited by
                #8

                Resolved!

                I tried with the Q_OBJECT before but didn't rerun qmake... my fault :) and the errors I was obtaining made me think that maybe I could only use Q_OBJECT in classes directly derived from QObject, didn't think deeply about it.

                I am just experimenting with all the metadata classes for a property based system UI.

                Thanks a lot!

                Xllr

                1 Reply Last reply
                1
                • kshegunovK kshegunov

                  @mrjj
                  Actually it is, somewhat. See here. The moc is needed to generate the meta information about the class, and the methods implementations, but for the static members or virtual functions declarations we have the good ol' preprocessor. ;)

                  mrjjM Offline
                  mrjjM Offline
                  mrjj
                  Lifetime Qt Champion
                  wrote on last edited by
                  #9

                  @kshegunov
                  oh. pretty neat.
                  I just wish it could list
                  members variables + type too :)

                  I never used the Qt metasystem. (directly)
                  Can moc be used to extract (plain) variables or does it has to be properties?

                  kshegunovK 1 Reply Last reply
                  0
                  • X Offline
                    X Offline
                    XavierLL
                    wrote on last edited by
                    #10

                    Not sure, because I have been working with this for 2 days, but I think they must be properties.

                    1 Reply Last reply
                    0
                    • mrjjM mrjj

                      @kshegunov
                      oh. pretty neat.
                      I just wish it could list
                      members variables + type too :)

                      I never used the Qt metasystem. (directly)
                      Can moc be used to extract (plain) variables or does it has to be properties?

                      kshegunovK Offline
                      kshegunovK Offline
                      kshegunov
                      Moderators
                      wrote on last edited by
                      #11

                      @mrjj said:

                      I just wish it could list
                      members variables + type too

                      Well, this wouldn't be very helpful, as you already know what member variables you have (and some of them may be private), or as in the usual case you only have a PIMPL pointer. You can however get the declared properties (such as declared with the Q_PROPERTY macro) and this is used extensively, e.g. in QML.

                      Read and abide by the Qt Code of Conduct

                      mrjjM 1 Reply Last reply
                      1
                      • kshegunovK kshegunov

                        @mrjj said:

                        I just wish it could list
                        members variables + type too

                        Well, this wouldn't be very helpful, as you already know what member variables you have (and some of them may be private), or as in the usual case you only have a PIMPL pointer. You can however get the declared properties (such as declared with the Q_PROPERTY macro) and this is used extensively, e.g. in QML.

                        mrjjM Offline
                        mrjjM Offline
                        mrjj
                        Lifetime Qt Champion
                        wrote on last edited by
                        #12

                        @kshegunov
                        well it would be extremely helpful for creating boilerplate code for serialization and
                        trace systems & module tests and all kind of code gen.
                        Without adding ANYTHING to the source code as all serialization
                        frameworks i have seen does.

                        Maybe in c++32 :)

                        kshegunovK 1 Reply Last reply
                        0
                        • mrjjM mrjj

                          @kshegunov
                          well it would be extremely helpful for creating boilerplate code for serialization and
                          trace systems & module tests and all kind of code gen.
                          Without adding ANYTHING to the source code as all serialization
                          frameworks i have seen does.

                          Maybe in c++32 :)

                          kshegunovK Offline
                          kshegunovK Offline
                          kshegunov
                          Moderators
                          wrote on last edited by
                          #13

                          @mrjj said:

                          well it would be extremely helpful for creating boilerplate code for serialization

                          You have QDataStream for that. My suspicion is that moc's source will just explode if you start adding more and more parsing features. It already does a lot, e.g. RTTI without compiler RTTI and of course the signal-slot mechanism. If you need to wrap some boilerplate code you can always use a combination of the preprocessor with virtualization (similarly to what Qt does). I, personally, use a virtual stream operator for such things:

                          class MyClass
                          {
                              friend QDataStream & operator << (QDataStream &, const MyClass &);
                              friend QDataStream & operator >> (QDataStream &, MyClass &);
                          
                          protected:
                              virtual bool serialize(QDataStream &) = 0;
                              virtual bool deserialize(QDataStream &) = 0;
                          }
                          
                          inline QDataStream & operator << (QDataStream & out, const MyClass & obj)
                          {
                              obj.serialize(out);
                              return out;
                          }
                          
                          inline QDataStream & operator >> (QDataStream & in, MyClass & obj)
                          {
                              obj.deserialize(in);
                              return in;
                          }
                          

                          Read and abide by the Qt Code of Conduct

                          mrjjM 1 Reply Last reply
                          2
                          • kshegunovK kshegunov

                            @mrjj said:

                            well it would be extremely helpful for creating boilerplate code for serialization

                            You have QDataStream for that. My suspicion is that moc's source will just explode if you start adding more and more parsing features. It already does a lot, e.g. RTTI without compiler RTTI and of course the signal-slot mechanism. If you need to wrap some boilerplate code you can always use a combination of the preprocessor with virtualization (similarly to what Qt does). I, personally, use a virtual stream operator for such things:

                            class MyClass
                            {
                                friend QDataStream & operator << (QDataStream &, const MyClass &);
                                friend QDataStream & operator >> (QDataStream &, MyClass &);
                            
                            protected:
                                virtual bool serialize(QDataStream &) = 0;
                                virtual bool deserialize(QDataStream &) = 0;
                            }
                            
                            inline QDataStream & operator << (QDataStream & out, const MyClass & obj)
                            {
                                obj.serialize(out);
                                return out;
                            }
                            
                            inline QDataStream & operator >> (QDataStream & in, MyClass & obj)
                            {
                                obj.deserialize(in);
                                return in;
                            }
                            
                            mrjjM Offline
                            mrjjM Offline
                            mrjj
                            Lifetime Qt Champion
                            wrote on last edited by
                            #14

                            @kshegunov
                            Well I guess it then again boils down to
                            out << var1 << var2 << var3 pr class which is what
                            i would like NOT to have to ever write. :)

                            So if could

                            for ( all member vars : curvar)
                            out << curvar;

                            make me very happy.

                            It seems Qt properties would allow such thing ? :)

                            kshegunovK 1 Reply Last reply
                            0
                            • mrjjM mrjj

                              @kshegunov
                              Well I guess it then again boils down to
                              out << var1 << var2 << var3 pr class which is what
                              i would like NOT to have to ever write. :)

                              So if could

                              for ( all member vars : curvar)
                              out << curvar;

                              make me very happy.

                              It seems Qt properties would allow such thing ? :)

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

                              @mrjj

                              It seems Qt properties would allow such thing ? :)

                              Yes, you can list them, but then again you have to declare them with Q_PROPERTY, and also you might want to save internal data that's not exposed through a property ... which could pose a significant problem.

                              I prefer the mentioned method, because I can delegate to the parent. Consider the following example:

                              class MyClassImpl : public MyClass
                              {
                                  // ...
                              
                              protected:
                                  bool serialize(QDataStream & out) override
                                  {
                                      out << x;
                                      return true;
                                  }
                              
                                  bool deserialize(QDataStream & in) override
                                  {
                                      in >> x;
                                      return true;
                                  }
                              
                              private:
                                  int x;
                              }
                              
                              class MyDerivedClassImpl : public MyClassImpl
                              {
                                  // ...
                              
                              protected:
                                  bool serialize(QDataStream & out) override
                                  {
                                      MyClassImpl::serialize(out);
                                      out << y << str;
                                      return true;
                                  }
                              
                                  bool deserialize(QDataStream & in) override
                                  {
                                      MyClassImpl::deserialize(in);
                                      out >> y >> str;
                                      return true;
                                  }
                              
                              private:
                                  double y;
                                  QString str;
                              }
                              

                              Read and abide by the Qt Code of Conduct

                              mrjjM 1 Reply Last reply
                              0
                              • kshegunovK kshegunov

                                @mrjj

                                It seems Qt properties would allow such thing ? :)

                                Yes, you can list them, but then again you have to declare them with Q_PROPERTY, and also you might want to save internal data that's not exposed through a property ... which could pose a significant problem.

                                I prefer the mentioned method, because I can delegate to the parent. Consider the following example:

                                class MyClassImpl : public MyClass
                                {
                                    // ...
                                
                                protected:
                                    bool serialize(QDataStream & out) override
                                    {
                                        out << x;
                                        return true;
                                    }
                                
                                    bool deserialize(QDataStream & in) override
                                    {
                                        in >> x;
                                        return true;
                                    }
                                
                                private:
                                    int x;
                                }
                                
                                class MyDerivedClassImpl : public MyClassImpl
                                {
                                    // ...
                                
                                protected:
                                    bool serialize(QDataStream & out) override
                                    {
                                        MyClassImpl::serialize(out);
                                        out << y << str;
                                        return true;
                                    }
                                
                                    bool deserialize(QDataStream & in) override
                                    {
                                        MyClassImpl::deserialize(in);
                                        out >> y >> str;
                                        return true;
                                    }
                                
                                private:
                                    double y;
                                    QString str;
                                }
                                
                                mrjjM Offline
                                mrjjM Offline
                                mrjj
                                Lifetime Qt Champion
                                wrote on last edited by
                                #16

                                @kshegunov
                                Yeah, 50% is not Qt enabled so would be an issue.
                                Im just daydreaming :)

                                Hmm, that is actually neat.

                                1 Reply Last reply
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