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connect() new style syntax return result

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  • JonBJ JonB

    In answering https://forum.qt.io/topic/118027/how-to-connect-menu-action-with-existing-slot-programatically/5, I came across https://doc.qt.io/qt-5/qobject.html#connect::

    The function returns a QMetaObject::Connection that represents a handle to a connection if it successfully connects the signal to the slot. The connection handle will be invalid if it cannot create the connection, for example, if QObject is unable to verify the existence of either signal or method, or if their signatures aren't compatible. You can check if the handle is valid by casting it to a bool.

    I have yet to see any code which bothers to check the result, Certainly if people insist on using the old-style SIGNAL/SLOT() macros they should be doing this, as witness that post.

    Question: if I stick to new-style compile-time connect(), are there any/many circumstances when it could still fail? In this case how safe am I assuming the connect() will have succeeded without bothering to check all my connect() run-time return results?

    J.HilkJ Offline
    J.HilkJ Offline
    J.Hilk
    Moderators
    wrote on last edited by
    #2

    @JonB
    well, take this example:

    int main(int argc, char *argv[])
    {
        QApplication a(argc, argv);
    
        QLineEdit e;
        QLabel *l;
        QMetaObject::Connection c = QObject::connect(&e, &QLineEdit::textChanged, l, &QLabel::setText);
    
        qDebug() << c;
    
        return a.exec();
    }
    

    it compiles fine, but connection fails, because the QLabel does technically not exist during the connect call


    Be aware of the Qt Code of Conduct, when posting : https://forum.qt.io/topic/113070/qt-code-of-conduct


    Q: What's that?
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    JonBJ 1 Reply Last reply
    4
    • J.HilkJ J.Hilk

      @JonB
      well, take this example:

      int main(int argc, char *argv[])
      {
          QApplication a(argc, argv);
      
          QLineEdit e;
          QLabel *l;
          QMetaObject::Connection c = QObject::connect(&e, &QLineEdit::textChanged, l, &QLabel::setText);
      
          qDebug() << c;
      
          return a.exec();
      }
      

      it compiles fine, but connection fails, because the QLabel does technically not exist during the connect call

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

      @J-Hilk
      Yep.

      In that case, I want to see all the code you have ever written even with the new style syntax where, as a good citizen, you run-time check the return result of every connect() you ever perform? And a promise that you never fail to do so?

      J.HilkJ 1 Reply Last reply
      1
      • JonBJ JonB

        @J-Hilk
        Yep.

        In that case, I want to see all the code you have ever written even with the new style syntax where, as a good citizen, you run-time check the return result of every connect() you ever perform? And a promise that you never fail to do so?

        J.HilkJ Offline
        J.HilkJ Offline
        J.Hilk
        Moderators
        wrote on last edited by
        #4

        @JonB :P

        sure, no it's the only case that I can think of, where the connect "could fail"
        you asked for any case 😉


        Be aware of the Qt Code of Conduct, when posting : https://forum.qt.io/topic/113070/qt-code-of-conduct


        Q: What's that?
        A: It's blue light.
        Q: What does it do?
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        JonBJ 1 Reply Last reply
        0
        • J.HilkJ J.Hilk

          @JonB :P

          sure, no it's the only case that I can think of, where the connect "could fail"
          you asked for any case 😉

          JonBJ Offline
          JonBJ Offline
          JonB
          wrote on last edited by
          #5

          @J-Hilk
          I asked an intentional question: I know you to be a good citizen, so do you check all your new-style connect()s at runtime? I should like to know....

          J.HilkJ KroMignonK S 3 Replies Last reply
          0
          • JonBJ JonB

            @J-Hilk
            I asked an intentional question: I know you to be a good citizen, so do you check all your new-style connect()s at runtime? I should like to know....

            J.HilkJ Offline
            J.HilkJ Offline
            J.Hilk
            Moderators
            wrote on last edited by
            #6

            @JonB said in connect() new style syntax return result:

            so do you check all your new-style connect()s at runtime?

            tbth
            not a single one😅


            Be aware of the Qt Code of Conduct, when posting : https://forum.qt.io/topic/113070/qt-code-of-conduct


            Q: What's that?
            A: It's blue light.
            Q: What does it do?
            A: It turns blue.

            JonBJ 1 Reply Last reply
            1
            • J.HilkJ J.Hilk

              @JonB said in connect() new style syntax return result:

              so do you check all your new-style connect()s at runtime?

              tbth
              not a single one😅

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

              @J-Hilk
              Exactly ;-)

              Though from now on I shall be strongly recommending it for those who insist on old-style, run-time behaviour.

              Other than your (correct) example, can you think of any others, assuming the signaller/slotter are, say, correct? I'm trying to understand whether there is anything during the (new style) connect() which could still fail, even if my code looks right?

              J.HilkJ 1 Reply Last reply
              0
              • JonBJ JonB

                @J-Hilk
                Exactly ;-)

                Though from now on I shall be strongly recommending it for those who insist on old-style, run-time behaviour.

                Other than your (correct) example, can you think of any others, assuming the signaller/slotter are, say, correct? I'm trying to understand whether there is anything during the (new style) connect() which could still fail, even if my code looks right?

                J.HilkJ Offline
                J.HilkJ Offline
                J.Hilk
                Moderators
                wrote on last edited by
                #8

                @JonB I'm not sure in the case of custom argument types (enums, classes etc) when you send them via Qt::QueuedConnection I think those will also fail silently, If you failed to correctly register the types with the meta system.

                If you forget a copy constructor for your custom class you should get a completive error


                Be aware of the Qt Code of Conduct, when posting : https://forum.qt.io/topic/113070/qt-code-of-conduct


                Q: What's that?
                A: It's blue light.
                Q: What does it do?
                A: It turns blue.

                JonBJ 1 Reply Last reply
                1
                • J.HilkJ J.Hilk

                  @JonB I'm not sure in the case of custom argument types (enums, classes etc) when you send them via Qt::QueuedConnection I think those will also fail silently, If you failed to correctly register the types with the meta system.

                  If you forget a copy constructor for your custom class you should get a completive error

                  JonBJ Offline
                  JonBJ Offline
                  JonB
                  wrote on last edited by JonB
                  #9

                  @J-Hilk
                  Thanks, I'll keep this open for a day or so, in case it inspires interesting comments when the other experts finally get out of bed :) I can't believe they would ever fail to check a return result which might fail......

                  I now feel "unclean" with all my existing connect() code :( I hadn't even looked at the return result, all the examples out there and what is posted on this forum never bother so I hadn't investigated....

                  J.HilkJ 1 Reply Last reply
                  1
                  • JonBJ JonB

                    @J-Hilk
                    Thanks, I'll keep this open for a day or so, in case it inspires interesting comments when the other experts finally get out of bed :) I can't believe they would ever fail to check a return result which might fail......

                    I now feel "unclean" with all my existing connect() code :( I hadn't even looked at the return result, all the examples out there and what is posted on this forum never bother so I hadn't investigated....

                    J.HilkJ Offline
                    J.HilkJ Offline
                    J.Hilk
                    Moderators
                    wrote on last edited by
                    #10

                    @JonB aren't you glad that c++17 introduced [[nodiscard]]

                    😉 I'm, but I doubt QObject::connect will ever geht that attribute, but could be fun for a April fools joke, to submit such a patch 🤣


                    Be aware of the Qt Code of Conduct, when posting : https://forum.qt.io/topic/113070/qt-code-of-conduct


                    Q: What's that?
                    A: It's blue light.
                    Q: What does it do?
                    A: It turns blue.

                    JonBJ 1 Reply Last reply
                    1
                    • JonBJ JonB

                      @J-Hilk
                      I asked an intentional question: I know you to be a good citizen, so do you check all your new-style connect()s at runtime? I should like to know....

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

                      @JonB said in connect() new style syntax return result:

                      I know you to be a good citizen, so do you check all your new-style connect()s at runtime? I should like to know....

                      If you are using new-style connect(), the return value is not that relevant.
                      The only interesting use case I see, is to use it to disconnect a specific slots/lambda function.

                      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
                      • J.HilkJ J.Hilk

                        @JonB aren't you glad that c++17 introduced [[nodiscard]]

                        😉 I'm, but I doubt QObject::connect will ever geht that attribute, but could be fun for a April fools joke, to submit such a patch 🤣

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

                        @J-Hilk
                        Excellent! I'm sure all existing code would welcome this attribute being auto-added ;-)

                        1 Reply Last reply
                        0
                        • KroMignonK KroMignon

                          @JonB said in connect() new style syntax return result:

                          I know you to be a good citizen, so do you check all your new-style connect()s at runtime? I should like to know....

                          If you are using new-style connect(), the return value is not that relevant.
                          The only interesting use case I see, is to use it to disconnect a specific slots/lambda function.

                          JonBJ Offline
                          JonBJ Offline
                          JonB
                          wrote on last edited by
                          #13

                          @KroMignon
                          Indeed! Nonetheless, @J-Hilk's example of an uninitialized variable might have been picked up. Though I suppose you will say we can never guard against, say, bad variables in this way....

                          Being an old C programmer, perhaps I should define connect() as a macro with a Q_ASSERT wrapper or similar on the return result...? ;-) [<-- Note the "wink", I am aware what you will think of that :) ]

                          KroMignonK 1 Reply Last reply
                          0
                          • JonBJ JonB

                            @KroMignon
                            Indeed! Nonetheless, @J-Hilk's example of an uninitialized variable might have been picked up. Though I suppose you will say we can never guard against, say, bad variables in this way....

                            Being an old C programmer, perhaps I should define connect() as a macro with a Q_ASSERT wrapper or similar on the return result...? ;-) [<-- Note the "wink", I am aware what you will think of that :) ]

                            KroMignonK Offline
                            KroMignonK Offline
                            KroMignon
                            wrote on last edited by
                            #14

                            @JonB said in connect() new style syntax return result:

                            Being an old C programmer, perhaps I should define connect() as a macro with a Q_ASSERT wrapper or similar on the return result...? ;-) [<-- Note the "wink", I am aware what you will think of that :) ]

                            I am also a far older (embedded) C programmer as a C++ programmer ;-)
                            But I am not sure this is the best idea for checking connect() return value...

                            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

                              @JonB said in connect() new style syntax return result:

                              Being an old C programmer, perhaps I should define connect() as a macro with a Q_ASSERT wrapper or similar on the return result...? ;-) [<-- Note the "wink", I am aware what you will think of that :) ]

                              I am also a far older (embedded) C programmer as a C++ programmer ;-)
                              But I am not sure this is the best idea for checking connect() return value...

                              JonBJ Offline
                              JonBJ Offline
                              JonB
                              wrote on last edited by
                              #15

                              @KroMignon said in connect() new style syntax return result:

                              But I am not sure this is the best idea for checking connect() return value...

                              It is when you don't want to change any lines of code, other than adding one #define :)

                              1 Reply Last reply
                              0
                              • JonBJ JonB

                                In answering https://forum.qt.io/topic/118027/how-to-connect-menu-action-with-existing-slot-programatically/5, I came across https://doc.qt.io/qt-5/qobject.html#connect::

                                The function returns a QMetaObject::Connection that represents a handle to a connection if it successfully connects the signal to the slot. The connection handle will be invalid if it cannot create the connection, for example, if QObject is unable to verify the existence of either signal or method, or if their signatures aren't compatible. You can check if the handle is valid by casting it to a bool.

                                I have yet to see any code which bothers to check the result, Certainly if people insist on using the old-style SIGNAL/SLOT() macros they should be doing this, as witness that post.

                                Question: if I stick to new-style compile-time connect(), are there any/many circumstances when it could still fail? In this case how safe am I assuming the connect() will have succeeded without bothering to check all my connect() run-time return results?

                                Pablo J. RoginaP Offline
                                Pablo J. RoginaP Offline
                                Pablo J. Rogina
                                wrote on last edited by
                                #16

                                @JonB said in connect() new style syntax return result:

                                Certainly if people insist on using the old-style SIGNAL/SLOT() macros

                                One of the things I see regarding old-style approach, is the lots and lots of examples still around using it. So for newcomers to Qt, having that available to start with is somehow misleading...

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                                J.HilkJ JonBJ 2 Replies Last reply
                                1
                                • Pablo J. RoginaP Pablo J. Rogina

                                  @JonB said in connect() new style syntax return result:

                                  Certainly if people insist on using the old-style SIGNAL/SLOT() macros

                                  One of the things I see regarding old-style approach, is the lots and lots of examples still around using it. So for newcomers to Qt, having that available to start with is somehow misleading...

                                  J.HilkJ Offline
                                  J.HilkJ Offline
                                  J.Hilk
                                  Moderators
                                  wrote on last edited by
                                  #17

                                  @Pablo-J-Rogina
                                  there's almost no situation where the qt5 syntax wouldn't work as well, so maybe marking it as deprecated could be an option. 🤷‍♂️


                                  Be aware of the Qt Code of Conduct, when posting : https://forum.qt.io/topic/113070/qt-code-of-conduct


                                  Q: What's that?
                                  A: It's blue light.
                                  Q: What does it do?
                                  A: It turns blue.

                                  Pablo J. RoginaP 1 Reply Last reply
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                                  • Pablo J. RoginaP Pablo J. Rogina

                                    @JonB said in connect() new style syntax return result:

                                    Certainly if people insist on using the old-style SIGNAL/SLOT() macros

                                    One of the things I see regarding old-style approach, is the lots and lots of examples still around using it. So for newcomers to Qt, having that available to start with is somehow misleading...

                                    JonBJ Offline
                                    JonBJ Offline
                                    JonB
                                    wrote on last edited by
                                    #18

                                    @Pablo-J-Rogina
                                    Exactly. Hence I often evangelise with noobs to get changed over. I realize it's hard with all the examples being old-style, and somehow they find new style --- (and lambdas) --- trickier. But judging by some of the answers where their old-style connect is wrong but they don't know it is till runtime problems, I think it will help them to move to new because they get compile-time error and importantly they get auto-completion of only suitable methods to use.

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                                    • J.HilkJ J.Hilk

                                      @Pablo-J-Rogina
                                      there's almost no situation where the qt5 syntax wouldn't work as well, so maybe marking it as deprecated could be an option. 🤷‍♂️

                                      Pablo J. RoginaP Offline
                                      Pablo J. RoginaP Offline
                                      Pablo J. Rogina
                                      wrote on last edited by
                                      #19

                                      @J-Hilk said in connect() new style syntax return result:

                                      there's almost no situation where the qt5 syntax wouldn't work as well,

                                      Please don't misunderstand me. I'm in full favor of the new syntax, the compile time checking is a great advantage.

                                      marking it as deprecated could be an option

                                      That would be great! Do we need to create a feature request?

                                      Upvote the answer(s) that helped you solve the issue
                                      Use "Topic Tools" button to mark your post as Solved
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                                      KroMignonK 1 Reply Last reply
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                                      • Pablo J. RoginaP Pablo J. Rogina

                                        @J-Hilk said in connect() new style syntax return result:

                                        there's almost no situation where the qt5 syntax wouldn't work as well,

                                        Please don't misunderstand me. I'm in full favor of the new syntax, the compile time checking is a great advantage.

                                        marking it as deprecated could be an option

                                        That would be great! Do we need to create a feature request?

                                        KroMignonK Offline
                                        KroMignonK Offline
                                        KroMignon
                                        wrote on last edited by
                                        #20

                                        @Pablo-J-Rogina said in connect() new style syntax return result:

                                        That would be great! Do we need to create a feature request?

                                        I don't agree with this!
                                        There are some situation where using old syntax has advantages.
                                        For example, suppose you have some classes with implements QObjects which all have a slot void doWork(void), you could connect to this slot without having to know from which class the instance is based.
                                        That's not possible with new syntax, also slots overload handling is not so easy to use.
                                        I agree that overloading slots is not a good code practice.

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

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                                        • SGaistS Offline
                                          SGaistS Offline
                                          SGaist
                                          Lifetime Qt Champion
                                          wrote on last edited by
                                          #21

                                          And IIRC, the "old" system is also what allows for things like QML to work its magic.

                                          The porting of the example is a good idea. The documentation team would be happy to get some help with that so you are more than welcome to submit patches. I think there are tasks related to that on the bug tracker to help organise.

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

                                          JKSHJ 1 Reply Last reply
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