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How do I safely call methods from another object?

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  • hskoglundH Online
    hskoglundH Online
    hskoglund
    wrote on last edited by
    #6

    It's safe if you're sure that you're only calling it when Object B is alive and well. One simple safeguard is to check for null pointer before using it:

    oid ObjectA::functionA(ObjectB* b)
    {
        ...
        if (nullptr != b)
            b->functionB();
        ...
    }
    
    1 Reply Last reply
    1
    • J Offline
      J Offline
      JohnFrom
      wrote on last edited by
      #7

      Thanks. On a side note, can we use signal slots?

      Sender: object A

      Signal:some sort of customized signal

      target: object B

      Slot: well I dont know what slot in this scenario...

      1 Reply Last reply
      0
      • hskoglundH Online
        hskoglundH Online
        hskoglund
        wrote on last edited by
        #8

        Sure, that's a great way to use signal and slots, they take care of that "object gone anxiety" for you, i.e. checking for null pointers and nasty stuff, like objects instanced in different threads etc.

        J 1 Reply Last reply
        0
        • hskoglundH hskoglund

          Sure, that's a great way to use signal and slots, they take care of that "object gone anxiety" for you, i.e. checking for null pointers and nasty stuff, like objects instanced in different threads etc.

          J Offline
          J Offline
          JohnFrom
          wrote on last edited by
          #9

          @hskoglund Hi sounding totally like a cheapskate and lazybum, could you be kind enough to provide an example using object A and B? You may make up function C and D as you go. I can't thank you enough! :)

          J.HilkJ hskoglundH 2 Replies Last reply
          0
          • J JohnFrom

            @hskoglund Hi sounding totally like a cheapskate and lazybum, could you be kind enough to provide an example using object A and B? You may make up function C and D as you go. I can't thank you enough! :)

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

            @JohnFrom
            I'm making use of @aha_1980 link collection thread

            and point you to the wiki page of the new signal sliot syntax, should give you everything you need to successfully connect to objects/classes:
            https://wiki.qt.io/New_Signal_Slot_Syntax

            and here the example form the docu:
            http://doc.qt.io/qt-5/signalsandslots.html#a-small-example


            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.

            J 1 Reply Last reply
            3
            • J JohnFrom

              @hskoglund Hi sounding totally like a cheapskate and lazybum, could you be kind enough to provide an example using object A and B? You may make up function C and D as you go. I can't thank you enough! :)

              hskoglundH Online
              hskoglundH Online
              hskoglund
              wrote on last edited by
              #11

              @JohnFrom I second @J-Hilk's post, that example from the docs is good, also there some signal/slot examples to be found in Qt Creator's Welcome pane as well.

              1 Reply Last reply
              2
              • J.HilkJ J.Hilk

                @JohnFrom
                I'm making use of @aha_1980 link collection thread

                and point you to the wiki page of the new signal sliot syntax, should give you everything you need to successfully connect to objects/classes:
                https://wiki.qt.io/New_Signal_Slot_Syntax

                and here the example form the docu:
                http://doc.qt.io/qt-5/signalsandslots.html#a-small-example

                J Offline
                J Offline
                JohnFrom
                wrote on last edited by
                #12

                @J.Hilk

                Thanks a whole bunch. This is how far I managed to get:

                connect(&sender, SIGNAL(newNodeAdded()), this, SLOT(addNewNodeBySignal()));

                where:

                sender is the signal sender object.

                newNodeAdded() is the void function declared after signal: in the relevant header file.

                "this" is the signal receiver, the object that's resident as I mentioned above.

                addNewNodeBySignal is the SLOT defined in the receiver's header file.

                Now I'm having the "sender not decleared in the scope" error, kind of right back where I started.

                Any smart workaround?

                jsulmJ 1 Reply Last reply
                0
                • J JohnFrom

                  @J.Hilk

                  Thanks a whole bunch. This is how far I managed to get:

                  connect(&sender, SIGNAL(newNodeAdded()), this, SLOT(addNewNodeBySignal()));

                  where:

                  sender is the signal sender object.

                  newNodeAdded() is the void function declared after signal: in the relevant header file.

                  "this" is the signal receiver, the object that's resident as I mentioned above.

                  addNewNodeBySignal is the SLOT defined in the receiver's header file.

                  Now I'm having the "sender not decleared in the scope" error, kind of right back where I started.

                  Any smart workaround?

                  jsulmJ Offline
                  jsulmJ Offline
                  jsulm
                  Lifetime Qt Champion
                  wrote on last edited by
                  #13

                  @JohnFrom Can you show your code?
                  You did not define sender in the context where you call connect.

                  https://forum.qt.io/topic/113070/qt-code-of-conduct

                  J 1 Reply Last reply
                  2
                  • jsulmJ jsulm

                    @JohnFrom Can you show your code?
                    You did not define sender in the context where you call connect.

                    J Offline
                    J Offline
                    JohnFrom
                    wrote on last edited by
                    #14

                    @jsulm You are absolutely correct. I knew what it meant. I just don't know the right way to solve it, other than static pointers or objects, both sound like colossal bad ideas.

                    Code-wise, I'll paste some here but it would be best if I provide some background:

                    First of all the sender is purposed to be a service, so I'm not sure it's the best of ideas to have it inherit Qobject and become a sender, what I'm trying to say is maybe the better way is to define a dedicated object inside this service, with its soul purpose being communicating with the outside world. I'm not sure if that's a good idea either.

                    The receiver on the other hand, is a typical Qobject, responsible for presenting the UI. So naturally it has tons of signals and slots.

                    And without further ado, here is the code:
                    Sender.h

                    class Sender: public someService, public QObject
                    {
                    
                         Q_OBJECT
                    
                    signals:
                        void newNodeAdded();
                    
                    };
                    

                    Sender.cpp

                    ```
                    
                    if( !node ){
                            node = new node(nodeId, nodeId);
                            TnodeStore.insertnode(node);
                            TDatabase.addnode(node);
                            emit newnodeAdded();
                        }
                    
                    
                    Receiver.h:
                    
                    
                    

                    class nodeView : public QWidget
                    {
                    Q_OBJECT

                    private slots:

                    void onRefreshnodeClicked(void);
                    };

                    
                    
                    Receiver.cpp:
                    
                    

                    connect((QObject*)sender, SIGNAL(newNodeAdded()), this, SLOT(onRefreshnodeClicked()));

                    
                    
                    
                    Note this code was updated with some errors rectified.
                    jsulmJ 1 Reply Last reply
                    0
                    • J JohnFrom

                      @jsulm You are absolutely correct. I knew what it meant. I just don't know the right way to solve it, other than static pointers or objects, both sound like colossal bad ideas.

                      Code-wise, I'll paste some here but it would be best if I provide some background:

                      First of all the sender is purposed to be a service, so I'm not sure it's the best of ideas to have it inherit Qobject and become a sender, what I'm trying to say is maybe the better way is to define a dedicated object inside this service, with its soul purpose being communicating with the outside world. I'm not sure if that's a good idea either.

                      The receiver on the other hand, is a typical Qobject, responsible for presenting the UI. So naturally it has tons of signals and slots.

                      And without further ado, here is the code:
                      Sender.h

                      class Sender: public someService, public QObject
                      {
                      
                           Q_OBJECT
                      
                      signals:
                          void newNodeAdded();
                      
                      };
                      

                      Sender.cpp

                      ```
                      
                      if( !node ){
                              node = new node(nodeId, nodeId);
                              TnodeStore.insertnode(node);
                              TDatabase.addnode(node);
                              emit newnodeAdded();
                          }
                      
                      
                      Receiver.h:
                      
                      
                      

                      class nodeView : public QWidget
                      {
                      Q_OBJECT

                      private slots:

                      void onRefreshnodeClicked(void);
                      };

                      
                      
                      Receiver.cpp:
                      
                      

                      connect((QObject*)sender, SIGNAL(newNodeAdded()), this, SLOT(onRefreshnodeClicked()));

                      
                      
                      
                      Note this code was updated with some errors rectified.
                      jsulmJ Offline
                      jsulmJ Offline
                      jsulm
                      Lifetime Qt Champion
                      wrote on last edited by
                      #15

                      @JohnFrom said in How do I safely call methods from another object?:

                      sender

                      where is this "sender" defined?
                      The code you posted does not help much as it is not complete.

                      https://forum.qt.io/topic/113070/qt-code-of-conduct

                      J 1 Reply Last reply
                      0
                      • jsulmJ jsulm

                        @JohnFrom said in How do I safely call methods from another object?:

                        sender

                        where is this "sender" defined?
                        The code you posted does not help much as it is not complete.

                        J Offline
                        J Offline
                        JohnFrom
                        wrote on last edited by JohnFrom
                        #16

                        @jsulm

                        Sorry!

                        The other parts of the code are really irrelevant and I don't want to have anything to do with the NDA so I'll have to pretty much manually quasi-obfuscate the code which is a lot of work.

                        "sender" is defined just like what you read, in Sender.cpp and Sender.h

                        I assume you were trying to say where it was instantiated? It's instantiated in a rather unconventional way:

                        SenderService *sender = NULL;
                            if( condition == A ){
                                sender = new senderServiceA();
                            }else if( condition == B ){
                                sender = new senderServiceB();
                            }else{
                                qDebug() << "Invalid condition!";
                            }
                        

                        Note that both senderServiceA and senderServiceB are "child classes" of SenderService.

                        That code above was instantiated as an object in

                        MainWindow class, which is a "child classes" of QMainWindow, and will be executed during run time.

                        However it is quite impossible for the UI (that persistent object I mentioned above, lives as long as the program is running) to access this service object. Shall I write another object dedicated in interconnecting the 2 objects as some sort of proxy, or shall I embed this "communication object" inside one of the 2 objects?

                        Or I should stick with basics, try to enable both objects to use slots? I'm reluctant to go down that route because I don't want either object to be able to directly access another and strangely enough, for object B to receive signal from object A, object A must be defined in the scope visible to B.

                        jsulmJ J.HilkJ 2 Replies Last reply
                        0
                        • J JohnFrom

                          @jsulm

                          Sorry!

                          The other parts of the code are really irrelevant and I don't want to have anything to do with the NDA so I'll have to pretty much manually quasi-obfuscate the code which is a lot of work.

                          "sender" is defined just like what you read, in Sender.cpp and Sender.h

                          I assume you were trying to say where it was instantiated? It's instantiated in a rather unconventional way:

                          SenderService *sender = NULL;
                              if( condition == A ){
                                  sender = new senderServiceA();
                              }else if( condition == B ){
                                  sender = new senderServiceB();
                              }else{
                                  qDebug() << "Invalid condition!";
                              }
                          

                          Note that both senderServiceA and senderServiceB are "child classes" of SenderService.

                          That code above was instantiated as an object in

                          MainWindow class, which is a "child classes" of QMainWindow, and will be executed during run time.

                          However it is quite impossible for the UI (that persistent object I mentioned above, lives as long as the program is running) to access this service object. Shall I write another object dedicated in interconnecting the 2 objects as some sort of proxy, or shall I embed this "communication object" inside one of the 2 objects?

                          Or I should stick with basics, try to enable both objects to use slots? I'm reluctant to go down that route because I don't want either object to be able to directly access another and strangely enough, for object B to receive signal from object A, object A must be defined in the scope visible to B.

                          jsulmJ Offline
                          jsulmJ Offline
                          jsulm
                          Lifetime Qt Champion
                          wrote on last edited by
                          #17

                          @JohnFrom said in How do I safely call methods from another object?:

                          SenderService *sender = NULL;

                          sender here is a LOCAL variable!
                          Do you call connect in same scope?
                          Posting such small pieces of code makes it hard for others to understand what you do and where the problem is...

                          https://forum.qt.io/topic/113070/qt-code-of-conduct

                          1 Reply Last reply
                          1
                          • J JohnFrom

                            @jsulm

                            Sorry!

                            The other parts of the code are really irrelevant and I don't want to have anything to do with the NDA so I'll have to pretty much manually quasi-obfuscate the code which is a lot of work.

                            "sender" is defined just like what you read, in Sender.cpp and Sender.h

                            I assume you were trying to say where it was instantiated? It's instantiated in a rather unconventional way:

                            SenderService *sender = NULL;
                                if( condition == A ){
                                    sender = new senderServiceA();
                                }else if( condition == B ){
                                    sender = new senderServiceB();
                                }else{
                                    qDebug() << "Invalid condition!";
                                }
                            

                            Note that both senderServiceA and senderServiceB are "child classes" of SenderService.

                            That code above was instantiated as an object in

                            MainWindow class, which is a "child classes" of QMainWindow, and will be executed during run time.

                            However it is quite impossible for the UI (that persistent object I mentioned above, lives as long as the program is running) to access this service object. Shall I write another object dedicated in interconnecting the 2 objects as some sort of proxy, or shall I embed this "communication object" inside one of the 2 objects?

                            Or I should stick with basics, try to enable both objects to use slots? I'm reluctant to go down that route because I don't want either object to be able to directly access another and strangely enough, for object B to receive signal from object A, object A must be defined in the scope visible to B.

                            J.HilkJ Offline
                            J.HilkJ Offline
                            J.Hilk
                            Moderators
                            wrote on last edited by J.Hilk
                            #18

                            @JohnFrom said in How do I safely call methods from another object?:

                            Or I should stick with basics, try to enable both objects to use slots? I'm reluctant to go down that route because I don't want either object to be able to directly access another and strangely enough, for object B to receive signal from object A, object A must be defined in the scope visible to B.

                            whats that supposed to mean?

                            For QObject::connect to work, at least the Sender-object needs to have QObject as its base class, sender and receiver, if you're using qt4 syntax.

                            Also Signal & Slot connections do not violate c++ standards/rules, you can connect only to functions and/or slots outside the scope where connect is called, that are public.

                            if object A lives in object C, and object B lives in object C, than you can use QObject::connect in object C to connect A & B without A knowing about B or visa versa.


                            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.

                            jsulmJ 1 Reply Last reply
                            0
                            • J.HilkJ J.Hilk

                              @JohnFrom said in How do I safely call methods from another object?:

                              Or I should stick with basics, try to enable both objects to use slots? I'm reluctant to go down that route because I don't want either object to be able to directly access another and strangely enough, for object B to receive signal from object A, object A must be defined in the scope visible to B.

                              whats that supposed to mean?

                              For QObject::connect to work, at least the Sender-object needs to have QObject as its base class, sender and receiver, if you're using qt4 syntax.

                              Also Signal & Slot connections do not violate c++ standards/rules, you can connect only to functions and/or slots outside the scope where connect is called, that are public.

                              if object A lives in object C, and object B lives in object C, than you can use QObject::connect in object C to connect A & B without A knowing about B or visa versa.

                              jsulmJ Offline
                              jsulmJ Offline
                              jsulm
                              Lifetime Qt Champion
                              wrote on last edited by
                              #19

                              @J.Hilk I think the problem is that he declares sender als local variable and when he tries to connect "sender" is undefined. This kind of mistake is one of the most common here :-)

                              https://forum.qt.io/topic/113070/qt-code-of-conduct

                              J.HilkJ 1 Reply Last reply
                              1
                              • jsulmJ jsulm

                                @J.Hilk I think the problem is that he declares sender als local variable and when he tries to connect "sender" is undefined. This kind of mistake is one of the most common here :-)

                                J.HilkJ Offline
                                J.HilkJ Offline
                                J.Hilk
                                Moderators
                                wrote on last edited by
                                #20

                                @jsulm possible, we simply don't know enough of the actual code to say for sure.

                                However, am I wrong here, or is "xxxx not decleared in the scope error " not usually a sign of missing includes ?
                                I tried to use connect on a null pointer recently.
                                It compiled, it run, it did not crash, I only got a warning " cannot connect to nullptr".

                                More infos needed!


                                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.

                                jsulmJ 1 Reply Last reply
                                0
                                • J.HilkJ J.Hilk

                                  @jsulm possible, we simply don't know enough of the actual code to say for sure.

                                  However, am I wrong here, or is "xxxx not decleared in the scope error " not usually a sign of missing includes ?
                                  I tried to use connect on a null pointer recently.
                                  It compiled, it run, it did not crash, I only got a warning " cannot connect to nullptr".

                                  More infos needed!

                                  jsulmJ Offline
                                  jsulmJ Offline
                                  jsulm
                                  Lifetime Qt Champion
                                  wrote on last edited by
                                  #21

                                  @J.Hilk

                                  connect((QObject*)sender, SIGNAL(newNodeAdded()), this, SLOT(onRefreshnodeClicked()));
                                  

                                  "sender not decleared in the scope"

                                  https://forum.qt.io/topic/113070/qt-code-of-conduct

                                  1 Reply Last reply
                                  0

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