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  4. [solved] MetaObject not referencing slots in derived class

[solved] MetaObject not referencing slots in derived class

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  • 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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