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Signal/slot and const parameters

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  • KroMignonK Offline
    KroMignonK Offline
    KroMignon
    wrote on last edited by
    #1

    Hi dear C++ gurus,

    I have perhaps a silly question about Signal / Slots connection.

    When I declare a signal or a slots in my class, is it a good or stupid idea to add "const &" to parameters?
    Do I have to add "const &" also to connected slots?

    Eg:

    class DataCollrecor: public QObject
    {
        Q_OBJECT
    
    ...
    signals:
        void newMessage(const MessageInfo& message);
    };
    
    class Listener : public QObject
    {
        Q_OBJECT
    
    ...
    public slots:
        void onNewMessage(const MessageInfo& message);
    };
    

    Is this a good or bad practice?

    Regards

    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 aha_1980A 2 Replies Last reply
    0
    • KroMignonK KroMignon

      Hi dear C++ gurus,

      I have perhaps a silly question about Signal / Slots connection.

      When I declare a signal or a slots in my class, is it a good or stupid idea to add "const &" to parameters?
      Do I have to add "const &" also to connected slots?

      Eg:

      class DataCollrecor: public QObject
      {
          Q_OBJECT
      
      ...
      signals:
          void newMessage(const MessageInfo& message);
      };
      
      class Listener : public QObject
      {
          Q_OBJECT
      
      ...
      public slots:
          void onNewMessage(const MessageInfo& message);
      };
      

      Is this a good or bad practice?

      Regards

      JonBJ Offline
      JonBJ Offline
      JonB
      wrote on last edited by JonB
      #2

      @KroMignon
      I'm not a C++ Guru, but I would say it is a good idea. If it's a const & then make it so (like you show, in both signal & slot). Qt in-built signals/slots do this (I don't know know if they do it everywhere, but they do do it).

      1 Reply Last reply
      1
      • KroMignonK KroMignon

        Hi dear C++ gurus,

        I have perhaps a silly question about Signal / Slots connection.

        When I declare a signal or a slots in my class, is it a good or stupid idea to add "const &" to parameters?
        Do I have to add "const &" also to connected slots?

        Eg:

        class DataCollrecor: public QObject
        {
            Q_OBJECT
        
        ...
        signals:
            void newMessage(const MessageInfo& message);
        };
        
        class Listener : public QObject
        {
            Q_OBJECT
        
        ...
        public slots:
            void onNewMessage(const MessageInfo& message);
        };
        

        Is this a good or bad practice?

        Regards

        aha_1980A Offline
        aha_1980A Offline
        aha_1980
        Lifetime Qt Champion
        wrote on last edited by aha_1980
        #3

        @KroMignon said in Signal/slot and const parameters:

        When I declare a signal or a slots in my class, is it a good or stupid idea to add "const &" to parameters?

        Good idea for signals as well as slots :)

        (As long as the parameter has a size > sizeof(pointer)

        Qt has to stay free or it will die.

        KroMignonK 1 Reply Last reply
        3
        • aha_1980A aha_1980

          @KroMignon said in Signal/slot and const parameters:

          When I declare a signal or a slots in my class, is it a good or stupid idea to add "const &" to parameters?

          Good idea for signals as well as slots :)

          (As long as the parameter has a size > sizeof(pointer)

          KroMignonK Offline
          KroMignonK Offline
          KroMignon
          wrote on last edited by
          #4

          @aha_1980 Thanks for your attention, but does I need to have "const &" for each Signal and Slot or does it only made sense for Slots?

          And when I use QMetaObject::invokeMethod(), do I need to add "const &" in Q_ARG() ?

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

          1 Reply Last reply
          0
          • VRoninV Offline
            VRoninV Offline
            VRonin
            wrote on last edited by
            #5

            It makes sense to add it in the declaration of both signals and slots. Qt will take care automatically of the lifecycle for arguments passed across threads.
            SGNAL() and SLOT() all support both the versions with and without const & but it's faster at execution if you leave it out.

            So following your example:

            • void newMessage(const MessageInfo& message);
            • void onNewMessage(const MessageInfo& message);
            • connect(collector,SIGNAL(newMessage(MessageInfo)),listener,SLOT(onNewMessage(MessageInfo));

            "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

            1 Reply Last reply
            5
            • sierdzioS Offline
              sierdzioS Offline
              sierdzio
              Moderators
              wrote on last edited by
              #6

              I also recommend declaring signals as const:

              signals:
                  void newMessage(const MessageInfo& message) const;
              

              Every signal can be const (always), it may help compiler optimize stuff, and it allows you to emit a signal also from const methods. So you can win (a small bit) and don't loose anything.

              (Z(:^

              VRoninV 1 Reply Last reply
              2
              • sierdzioS sierdzio

                I also recommend declaring signals as const:

                signals:
                    void newMessage(const MessageInfo& message) const;
                

                Every signal can be const (always), it may help compiler optimize stuff, and it allows you to emit a signal also from const methods. So you can win (a small bit) and don't loose anything.

                VRoninV Offline
                VRoninV Offline
                VRonin
                wrote on last edited by VRonin
                #7

                @sierdzio said in Signal/slot and const parameters:

                I also recommend declaring signals as const

                Mixed feeling about this. Qt internally will const_cast the sender if the signal is const so it might be misleading to the user to assume the method is const. The moc of your signal will look something like this:

                // SIGNAL 0
                void DataCollrecor::newMessage(const MessageInfo & _t1)const
                {
                    void *_a[] = { Q_NULLPTR, const_cast<void*>(reinterpret_cast<const void*>(&_t1)) };
                    QMetaObject::activate(const_cast< DataCollrecor *>(this), &staticMetaObject, 0, _a);
                }
                

                "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

                sierdzioS 1 Reply Last reply
                0
                • KroMignonK Offline
                  KroMignonK Offline
                  KroMignon
                  wrote on last edited by
                  #8

                  Thanks @sierdzio and @VRonin for your inputs, just complete my questioning, what will happen if signal/slot parameter don't match with "const &".
                  To continue with my basic example:

                  • void newMessage(MessageInfo message);
                  • void onNewMessage(const MessageInfo& message);

                  or:

                  • void newMessage(const MessageInfo& message);
                  • void onNewMessage(MessageInfo message);

                  Is there a risk to loose data or getting software crashes?

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

                  VRoninV 1 Reply Last reply
                  0
                  • KroMignonK KroMignon

                    Thanks @sierdzio and @VRonin for your inputs, just complete my questioning, what will happen if signal/slot parameter don't match with "const &".
                    To continue with my basic example:

                    • void newMessage(MessageInfo message);
                    • void onNewMessage(const MessageInfo& message);

                    or:

                    • void newMessage(const MessageInfo& message);
                    • void onNewMessage(MessageInfo message);

                    Is there a risk to loose data or getting software crashes?

                    VRoninV Offline
                    VRoninV Offline
                    VRonin
                    wrote on last edited by
                    #9

                    @KroMignon said in Signal/slot and const parameters:

                    what will happen if signal/slot parameter don't match with "const &".

                    You are calling a useless copy constructor

                    Is there a risk to loose data or getting software crashes?

                    No, just an efficiency issue

                    "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

                    KroMignonK 1 Reply Last reply
                    5
                    • VRoninV VRonin

                      @KroMignon said in Signal/slot and const parameters:

                      what will happen if signal/slot parameter don't match with "const &".

                      You are calling a useless copy constructor

                      Is there a risk to loose data or getting software crashes?

                      No, just an efficiency issue

                      KroMignonK Offline
                      KroMignonK Offline
                      KroMignon
                      wrote on last edited by
                      #10

                      @VRonin and @sierdzio thanks a lot for your time.

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

                      1 Reply Last reply
                      1
                      • VRoninV VRonin

                        @sierdzio said in Signal/slot and const parameters:

                        I also recommend declaring signals as const

                        Mixed feeling about this. Qt internally will const_cast the sender if the signal is const so it might be misleading to the user to assume the method is const. The moc of your signal will look something like this:

                        // SIGNAL 0
                        void DataCollrecor::newMessage(const MessageInfo & _t1)const
                        {
                            void *_a[] = { Q_NULLPTR, const_cast<void*>(reinterpret_cast<const void*>(&_t1)) };
                            QMetaObject::activate(const_cast< DataCollrecor *>(this), &staticMetaObject, 0, _a);
                        }
                        
                        sierdzioS Offline
                        sierdzioS Offline
                        sierdzio
                        Moderators
                        wrote on last edited by
                        #11

                        @VRonin said in Signal/slot and const parameters:

                        Mixed feeling about this. Qt internally will const_cast the sender if the signal is const

                        :O I had no idea.

                        (Z(:^

                        1 Reply Last reply
                        0

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