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connect() new style syntax return result

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  • 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


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      JonBJ 1 Reply Last reply
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      • 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 ;-)

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          • 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
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            • 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
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              • 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
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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?

                  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
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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...

                    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
                        Add screenshots via postimage.org
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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.

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                            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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                            • 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....

                              S Offline
                              S Offline
                              SimonSchroeder
                              wrote on last edited by
                              #22

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

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

                              I have never written any checks and never seen any checks in any source code. In normal circumstances connects should be well structured and not occur everywhere in your source code. Connects for general QActions (menus, toolbars, etc.) we connect once at startup. When dialogs are constructed, we connect in the constructor (or the functions that creates a small dialog on demand). Rarely don't we have the object we'd like to connect. Then, there would not be any failures in connects when they succeded compiling in the new syntax. We still have a lot of old connects where it compiles even if the signal or slot name is wrong. However, Qt writes it out to the command line and this is where we catch these errors. So, in all our scenarios either the connect always succeeds or always fails. It does not depend on any runtime behaviour. This means, once a connect is known to work, we can forget about it.

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                              • SGaistS SGaist

                                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.

                                JKSHJ Offline
                                JKSHJ Offline
                                JKSH
                                Moderators
                                wrote on last edited by JKSH
                                #23

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

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

                                Correct. The old syntax lets us make connections between C++ code and QML code.

                                It also supports dynamic meta objects which the new syntax can't support: https://www.qtdeveloperdays.com/sites/default/files/QtDevDays2014US-DIY-moc.pdf (useful for language bindings)

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

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