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Please help with lambda sytax

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  • jsulmJ jsulm

    @AnneRanch What is list_2?
    Also, can you show the whole error?

    A Offline
    A Offline
    Anonymous_Banned275
    wrote on last edited by
    #12

    @jsulm said in Please help with lambda sytax:

    @AnneRanch What is list_2?
    Also, can you show the whole error?

    The original , wrong ";" , whole error was already posted.
    The ";" error was found and corrected.

    Now the error is

    /media/f/QT/Qt/QT/qtconnectivity/examples/bluetooth/CAT_BT_18112020/device.cpp:181: error: no matching function for call to 'DeviceDiscoveryDialog::connect(QListWidget*&, void (/media/f/QT/Qt/QT/qtconnectivity/examples/bluetooth/CAT_BT_18112020/device.cpp:181: error: no matching function for call to 'DeviceDiscoveryDialog::connect(QListWidget*&, void (QListWidget::*)(QListWidgetItem*), DeviceDiscoveryDialog*, DeviceDiscoveryDialog::DeviceDiscoveryDialog(QWidget*)::<lambda(QListWidgetItem*)>)'
         [this]( QListWidgetItem * pItem ) { this->ui->list_2->addItem ( pItem ); }) ;
                                                                                   ^::*)(QListWidgetItem*), DeviceDiscoveryDialog*, DeviceDiscoveryDialog::DeviceDiscoveryDialog(QWidget*)::<lambda(QListWidgetItem*)>)'
         [this]( QListWidgetItem * pItem ) { this->ui->list_2->addItem ( pItem ); }) ;
                                                                                   ^
    

    Both list's are QListWidget

    This whole exercise is my desire to learn how to code "connect". I am trying to duplicate QtDesigner WORKING "connect". See attached screen shot. I went too far and started with copying "item" from one dialog to "main dialog". That when things got convoluted with QOverload ( handling "item" as a string ) and lambda.

    793b5e8b-8d6e-4285-bab2-731312ab49e7-image.png

    So far I have learn that Intelisense and lambda do not mix - I cab "addItem" using Intelisense ( legal) but it there but compiler complains there is not "addItem ".
    "click" and "itemchanged" are not the same - no surprise here.

    I am still looking into lambda doc and my head is spinning.
    ( I am not sure why lambda "mixes " QListWidget and QListWidgetItem)

    This is still a learning exercise and I like to go back and copy the QtDesigner WORKING XML to plain C++ code and make it work in ONE dialog.
    Then tackle same and make it work between dialog and main dialog - using "new " connect style. ( It would be nice if QtDesigner did "new connect style " too. )
    Using lambda is last on my TODO list, but I am open to suggestions.

    Thanks

    Pl45m4P 1 Reply Last reply
    0
    • A Anonymous_Banned275

      @jsulm said in Please help with lambda sytax:

      @AnneRanch What is list_2?
      Also, can you show the whole error?

      The original , wrong ";" , whole error was already posted.
      The ";" error was found and corrected.

      Now the error is

      /media/f/QT/Qt/QT/qtconnectivity/examples/bluetooth/CAT_BT_18112020/device.cpp:181: error: no matching function for call to 'DeviceDiscoveryDialog::connect(QListWidget*&, void (/media/f/QT/Qt/QT/qtconnectivity/examples/bluetooth/CAT_BT_18112020/device.cpp:181: error: no matching function for call to 'DeviceDiscoveryDialog::connect(QListWidget*&, void (QListWidget::*)(QListWidgetItem*), DeviceDiscoveryDialog*, DeviceDiscoveryDialog::DeviceDiscoveryDialog(QWidget*)::<lambda(QListWidgetItem*)>)'
           [this]( QListWidgetItem * pItem ) { this->ui->list_2->addItem ( pItem ); }) ;
                                                                                     ^::*)(QListWidgetItem*), DeviceDiscoveryDialog*, DeviceDiscoveryDialog::DeviceDiscoveryDialog(QWidget*)::<lambda(QListWidgetItem*)>)'
           [this]( QListWidgetItem * pItem ) { this->ui->list_2->addItem ( pItem ); }) ;
                                                                                     ^
      

      Both list's are QListWidget

      This whole exercise is my desire to learn how to code "connect". I am trying to duplicate QtDesigner WORKING "connect". See attached screen shot. I went too far and started with copying "item" from one dialog to "main dialog". That when things got convoluted with QOverload ( handling "item" as a string ) and lambda.

      793b5e8b-8d6e-4285-bab2-731312ab49e7-image.png

      So far I have learn that Intelisense and lambda do not mix - I cab "addItem" using Intelisense ( legal) but it there but compiler complains there is not "addItem ".
      "click" and "itemchanged" are not the same - no surprise here.

      I am still looking into lambda doc and my head is spinning.
      ( I am not sure why lambda "mixes " QListWidget and QListWidgetItem)

      This is still a learning exercise and I like to go back and copy the QtDesigner WORKING XML to plain C++ code and make it work in ONE dialog.
      Then tackle same and make it work between dialog and main dialog - using "new " connect style. ( It would be nice if QtDesigner did "new connect style " too. )
      Using lambda is last on my TODO list, but I am open to suggestions.

      Thanks

      Pl45m4P Offline
      Pl45m4P Offline
      Pl45m4
      wrote on last edited by Pl45m4
      #13

      @AnneRanch

      Can you please clarify what your final goal is? It seems that some of your recent topics (incl. this one) are going in the wrong direction...

      You started here with an error in your connect statement, but we obviously can not know whether your connection makes sense in your case or not.

      If you want to transfer (i.e. "copy") items, you've clicked in one listWidget to the second listWidget, do what @JonB suggested here.

      addItem only works, if you have a "free" QListWidgetItem. So even if we correct your mistake, it wont help you or work at all.
      The item, that the clicked function returns, belongs already to one QListWidget, so you cant just add it to a second one.

      Changing your code inside lambda to this might work (not tested)

      // creates a copy of the clicked item, instead of trying to move it
      QListWidgetItem * newItem = new QListWidgetItem(pItem);
      ui->list_2->addItem( newItem );
      

      The connection as seen in your picture equals somewhat like this (lambda style):

      connect(ui->list, &QListWidget::currentTextChanged, [=](QString s){
      
      ui->plainTextEdit2->appendPlainText(s);
      });
      

      If debugging is the process of removing software bugs, then programming must be the process of putting them in.

      ~E. W. Dijkstra

      A 1 Reply Last reply
      0
      • Pl45m4P Pl45m4

        @AnneRanch

        Can you please clarify what your final goal is? It seems that some of your recent topics (incl. this one) are going in the wrong direction...

        You started here with an error in your connect statement, but we obviously can not know whether your connection makes sense in your case or not.

        If you want to transfer (i.e. "copy") items, you've clicked in one listWidget to the second listWidget, do what @JonB suggested here.

        addItem only works, if you have a "free" QListWidgetItem. So even if we correct your mistake, it wont help you or work at all.
        The item, that the clicked function returns, belongs already to one QListWidget, so you cant just add it to a second one.

        Changing your code inside lambda to this might work (not tested)

        // creates a copy of the clicked item, instead of trying to move it
        QListWidgetItem * newItem = new QListWidgetItem(pItem);
        ui->list_2->addItem( newItem );
        

        The connection as seen in your picture equals somewhat like this (lambda style):

        connect(ui->list, &QListWidget::currentTextChanged, [=](QString s){
        
        ui->plainTextEdit2->appendPlainText(s);
        });
        
        A Offline
        A Offline
        Anonymous_Banned275
        wrote on last edited by
        #14

        @Pl45m4 said in Please help with lambda sytax:

        @AnneRanch

        Can you please clarify what your final goal is? It seems that some of your recent topics (incl. this one) are going in the wrong direction...

        You started here with an error in your connect statement, but we obviously can not know whether your connection makes sense in your case or not.

        If you want to transfer (i.e. "copy") items, you've clicked in one listWidget to the second listWidget, do what @JonB suggested here.

        addItem only works, if you have a "free" QListWidgetItem. So even if we correct your mistake, it wont help you or work at all.
        The item, that the clicked function returns, belongs already to one QListWidget, so you cant just add it to a second one.

        Changing your code inside lambda to this might work (not tested)

        // creates a copy of the clicked item, instead of trying to move it
        QListWidgetItem * newItem = new QListWidgetItem(pItem);
        ui->list_2->addItem( newItem );
        

        The connection as seen in your picture equals somewhat like this (lambda style):

        connect(ui->list, &QListWidget::currentTextChanged, [=](QString s){
        
        ui->plainTextEdit2->appendPlainText(s);
        });
        

        In general terms - The objective is to copy an item from one widget to another.
        It is actually QString, not a generic "item" I want to copy.

        The QtDesigner does that with the screen shot of SIGNAL/SLOT I have posted. No problem.

        Yes, I do not know how to convert QtDesigner XML GUI to plain C code, but if QtDesigner apparently works without creating new "item" why is it necessary to create new item in code? But that is not main issue.

        To avoid further r misunderstanding - help me to duplicate XML in C code first . Then do the lambda and then copy between dialogs. Baby steps.

        I can just temporary delete the QtDesigner XML to test the plain C code.

        Perhaps screen shot what I have now?

        Eventually "automatically" copy each line of text from Bluetooth scanner dialog "nearby BT devices" to main window Tab2 "nearby BT devices".

        The unnamed text under "Connect" button , done by QtDesigner, is what I want to duplicate in code.

        6b8df14a-1471-4c70-9d2c-af9f17683f8d-image.png

        1 Reply Last reply
        0
        • A Offline
          A Offline
          Anonymous_Banned275
          wrote on last edited by
          #15

          Please note - I am posting this to illustrate the problem, please please please - NO comments on my programming style!

          My primary goal is to duplicate XML "connect" using code.
          The attached code and its variations generally compiles - that is one goal done.

          HOWEVER - it crashes during run time and I SUSPECT the problem is
          after
          clicking button 11 my code builds new instance of scanner dialog and it has build-in timers . The "list" is , in theory, filed AFTER each item individual timer expire.
          BUT
          my test connect is executed immediately after the first timer expires and the "item" is changed .

          I am asking - is it possible I have a timing problem?

          Here is my current - under construction connect to copy the "list" lines.
          All it does - just compiles and cannot be tested , in existing code , due to this issue.

          #ifdef BYPASS
              // interacts with pushing buitto 11
              // keeps crashing
              
              connect(
                          this->ui->list,
                          //          SIGNAL(ui->list->currentTextChanged()),
                          SIGNAL(ui->list->itemDoubleClicked(*pItem)),
                          //           SIGNAL(DDD->ui->list->itemChanged(TEST_LABEL)),
                          //           SIGNAL(itemClicked(QListWidgetItem *)),
                          //              SIGNAL(itemChanged(TEST_LABEL)),
                          this->ui->plainTextEdit,
                          //           SLOT(DDD->ui->plainTextEdit->appendPlainText(TEST_LABEL))
                          //             SLOT(appendPlainText(TEST_LABEL))
                          SLOT(ui->plainTextEdit->appendPlainText(*pItem)));
              //           );
              
          #endif
              
              
          

          This is how it tracks when working - without the code above

          QDEBUG TRACE 
          QDEBUG TRACE TASK 
          		 END imoplement Device DiscoiveryDialog  
          
          QDEBUG TRACE TASK 
          		 find nearby BT devices 
          
          file      /media/f/QT/QT_PROJECT/TEST_PROJECT_14/mainwindow_cat_tab.cpp
          function  on_pushButton_11_clicked
          @line     236
          TEMPORARY EXIT 
          /home/f/build-TEST_PROJECT-Desktop-Debug/TEST_PROJECT exited with code 0
          

          And this is failing track

          QDEBUG TRACE 
          QDEBUG TRACE TASK 
          		 START imoplement Device DiscoveryDialog  
          
          QDEBUG TRACE TASK 
          		 find nearby BT devices 
          
          file      /media/f/QT/QT_PROJECT/TEST_PROJECT_14/mainwindow_cat_tab.cpp
          function  on_pushButton_11_clicked
          @line     206
          TEMPORARY EXIT 
          TRACE SCAN FLOW 
          			 START TOK TRACE SCAN FLOW CONSTRUCTOR 
          			DeviceDiscoveryDialog::DeviceDiscoveryDialog(QWidget *parent)
          
          
          TEMPORARY exit  @line 
          133
          The program has unexpectedly finished.
          /home/f/build-TEST_PROJECT-Desktop-Debug/TEST_PROJECT crashed
          

          Again - all I am asking - is it possible I have a timing problem?

          jsulmJ JonBJ 3 Replies Last reply
          0
          • A Anonymous_Banned275

            Please note - I am posting this to illustrate the problem, please please please - NO comments on my programming style!

            My primary goal is to duplicate XML "connect" using code.
            The attached code and its variations generally compiles - that is one goal done.

            HOWEVER - it crashes during run time and I SUSPECT the problem is
            after
            clicking button 11 my code builds new instance of scanner dialog and it has build-in timers . The "list" is , in theory, filed AFTER each item individual timer expire.
            BUT
            my test connect is executed immediately after the first timer expires and the "item" is changed .

            I am asking - is it possible I have a timing problem?

            Here is my current - under construction connect to copy the "list" lines.
            All it does - just compiles and cannot be tested , in existing code , due to this issue.

            #ifdef BYPASS
                // interacts with pushing buitto 11
                // keeps crashing
                
                connect(
                            this->ui->list,
                            //          SIGNAL(ui->list->currentTextChanged()),
                            SIGNAL(ui->list->itemDoubleClicked(*pItem)),
                            //           SIGNAL(DDD->ui->list->itemChanged(TEST_LABEL)),
                            //           SIGNAL(itemClicked(QListWidgetItem *)),
                            //              SIGNAL(itemChanged(TEST_LABEL)),
                            this->ui->plainTextEdit,
                            //           SLOT(DDD->ui->plainTextEdit->appendPlainText(TEST_LABEL))
                            //             SLOT(appendPlainText(TEST_LABEL))
                            SLOT(ui->plainTextEdit->appendPlainText(*pItem)));
                //           );
                
            #endif
                
                
            

            This is how it tracks when working - without the code above

            QDEBUG TRACE 
            QDEBUG TRACE TASK 
            		 END imoplement Device DiscoiveryDialog  
            
            QDEBUG TRACE TASK 
            		 find nearby BT devices 
            
            file      /media/f/QT/QT_PROJECT/TEST_PROJECT_14/mainwindow_cat_tab.cpp
            function  on_pushButton_11_clicked
            @line     236
            TEMPORARY EXIT 
            /home/f/build-TEST_PROJECT-Desktop-Debug/TEST_PROJECT exited with code 0
            

            And this is failing track

            QDEBUG TRACE 
            QDEBUG TRACE TASK 
            		 START imoplement Device DiscoveryDialog  
            
            QDEBUG TRACE TASK 
            		 find nearby BT devices 
            
            file      /media/f/QT/QT_PROJECT/TEST_PROJECT_14/mainwindow_cat_tab.cpp
            function  on_pushButton_11_clicked
            @line     206
            TEMPORARY EXIT 
            TRACE SCAN FLOW 
            			 START TOK TRACE SCAN FLOW CONSTRUCTOR 
            			DeviceDiscoveryDialog::DeviceDiscoveryDialog(QWidget *parent)
            
            
            TEMPORARY exit  @line 
            133
            The program has unexpectedly finished.
            /home/f/build-TEST_PROJECT-Desktop-Debug/TEST_PROJECT crashed
            

            Again - all I am asking - is it possible I have a timing problem?

            jsulmJ Offline
            jsulmJ Offline
            jsulm
            Lifetime Qt Champion
            wrote on last edited by jsulm
            #16

            @AnneRanch said in Please help with lambda sytax:

            connect(
            this->ui->list,
            // SIGNAL(ui->list->currentTextChanged()),
            SIGNAL(ui->list->itemDoubleClicked(*pItem)),
            // SIGNAL(DDD->ui->list->itemChanged(TEST_LABEL)),
            // SIGNAL(itemClicked(QListWidgetItem *)),
            // SIGNAL(itemChanged(TEST_LABEL)),
            this->ui->plainTextEdit,
            // SLOT(DDD->ui->plainTextEdit->appendPlainText(TEST_LABEL))
            // SLOT(appendPlainText(TEST_LABEL))
            SLOT(ui->plainTextEdit->appendPlainText(*pItem)));
            // );

            This is invalid connect() call. Please check https://doc.qt.io/qt-5/signalsandslots.html And it is better to use Qt5 connect() syntax to detect such issues during compile time instead of run time.
            It should be

            connect(
                this->ui->list,
                SIGNAL(itemDoubleClicked(QListWidgetItem*)),
                this->ui->plainTextEdit,
                SLOT(appendPlainText(QListWidgetItem*)));
            

            But this will also not work as QPlainTextEdit does not have a slot appendPlainText which takes a QListWidgetItem* as parameter (use new Qt5 connect syntax and a lambda)...

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

            A 1 Reply Last reply
            5
            • A Anonymous_Banned275

              Please note - I am posting this to illustrate the problem, please please please - NO comments on my programming style!

              My primary goal is to duplicate XML "connect" using code.
              The attached code and its variations generally compiles - that is one goal done.

              HOWEVER - it crashes during run time and I SUSPECT the problem is
              after
              clicking button 11 my code builds new instance of scanner dialog and it has build-in timers . The "list" is , in theory, filed AFTER each item individual timer expire.
              BUT
              my test connect is executed immediately after the first timer expires and the "item" is changed .

              I am asking - is it possible I have a timing problem?

              Here is my current - under construction connect to copy the "list" lines.
              All it does - just compiles and cannot be tested , in existing code , due to this issue.

              #ifdef BYPASS
                  // interacts with pushing buitto 11
                  // keeps crashing
                  
                  connect(
                              this->ui->list,
                              //          SIGNAL(ui->list->currentTextChanged()),
                              SIGNAL(ui->list->itemDoubleClicked(*pItem)),
                              //           SIGNAL(DDD->ui->list->itemChanged(TEST_LABEL)),
                              //           SIGNAL(itemClicked(QListWidgetItem *)),
                              //              SIGNAL(itemChanged(TEST_LABEL)),
                              this->ui->plainTextEdit,
                              //           SLOT(DDD->ui->plainTextEdit->appendPlainText(TEST_LABEL))
                              //             SLOT(appendPlainText(TEST_LABEL))
                              SLOT(ui->plainTextEdit->appendPlainText(*pItem)));
                  //           );
                  
              #endif
                  
                  
              

              This is how it tracks when working - without the code above

              QDEBUG TRACE 
              QDEBUG TRACE TASK 
              		 END imoplement Device DiscoiveryDialog  
              
              QDEBUG TRACE TASK 
              		 find nearby BT devices 
              
              file      /media/f/QT/QT_PROJECT/TEST_PROJECT_14/mainwindow_cat_tab.cpp
              function  on_pushButton_11_clicked
              @line     236
              TEMPORARY EXIT 
              /home/f/build-TEST_PROJECT-Desktop-Debug/TEST_PROJECT exited with code 0
              

              And this is failing track

              QDEBUG TRACE 
              QDEBUG TRACE TASK 
              		 START imoplement Device DiscoveryDialog  
              
              QDEBUG TRACE TASK 
              		 find nearby BT devices 
              
              file      /media/f/QT/QT_PROJECT/TEST_PROJECT_14/mainwindow_cat_tab.cpp
              function  on_pushButton_11_clicked
              @line     206
              TEMPORARY EXIT 
              TRACE SCAN FLOW 
              			 START TOK TRACE SCAN FLOW CONSTRUCTOR 
              			DeviceDiscoveryDialog::DeviceDiscoveryDialog(QWidget *parent)
              
              
              TEMPORARY exit  @line 
              133
              The program has unexpectedly finished.
              /home/f/build-TEST_PROJECT-Desktop-Debug/TEST_PROJECT crashed
              

              Again - all I am asking - is it possible I have a timing problem?

              JonBJ Offline
              JonBJ Offline
              JonB
              wrote on last edited by JonB
              #17

              @AnneRanch
              As @jsulm has said. As long as you use the old-style SIGNAL & SLOT() macros, you are liable to have code which compiles OK but does not behave correctly at runtime. With the new style syntax, if the signal/slot code is wrong, you get told at compile-time. It might seem harder to get it right, but at least you know that when it does compile it will do what you expect when you run it; if it does not compile, you have to sort it out to be correct at that stage.

              1 Reply Last reply
              0
              • jsulmJ jsulm

                @AnneRanch said in Please help with lambda sytax:

                connect(
                this->ui->list,
                // SIGNAL(ui->list->currentTextChanged()),
                SIGNAL(ui->list->itemDoubleClicked(*pItem)),
                // SIGNAL(DDD->ui->list->itemChanged(TEST_LABEL)),
                // SIGNAL(itemClicked(QListWidgetItem *)),
                // SIGNAL(itemChanged(TEST_LABEL)),
                this->ui->plainTextEdit,
                // SLOT(DDD->ui->plainTextEdit->appendPlainText(TEST_LABEL))
                // SLOT(appendPlainText(TEST_LABEL))
                SLOT(ui->plainTextEdit->appendPlainText(*pItem)));
                // );

                This is invalid connect() call. Please check https://doc.qt.io/qt-5/signalsandslots.html And it is better to use Qt5 connect() syntax to detect such issues during compile time instead of run time.
                It should be

                connect(
                    this->ui->list,
                    SIGNAL(itemDoubleClicked(QListWidgetItem*)),
                    this->ui->plainTextEdit,
                    SLOT(appendPlainText(QListWidgetItem*)));
                

                But this will also not work as QPlainTextEdit does not have a slot appendPlainText which takes a QListWidgetItem* as parameter (use new Qt5 connect syntax and a lambda)...

                A Offline
                A Offline
                Anonymous_Banned275
                wrote on last edited by
                #18

                @jsulm said in Please help with lambda sytax:

                But this will also not work as QPlainTextEdit does not have a slot appendPlainText which takes a QListWidgetItem* as parameter (use new Qt5 connect syntax and a lambda)...

                I do appreciate the post.
                It may seems strange, but at this point I am NOT after "it" working.
                I just need to figure out WHY adding this "connect" , in any form or format , crashes the program.
                After I figure out the crasher I will work on form / format/ style.
                Of course I could verify the "connect" using separate test program.

                Pl45m4P 1 Reply Last reply
                0
                • A Anonymous_Banned275

                  Please note - I am posting this to illustrate the problem, please please please - NO comments on my programming style!

                  My primary goal is to duplicate XML "connect" using code.
                  The attached code and its variations generally compiles - that is one goal done.

                  HOWEVER - it crashes during run time and I SUSPECT the problem is
                  after
                  clicking button 11 my code builds new instance of scanner dialog and it has build-in timers . The "list" is , in theory, filed AFTER each item individual timer expire.
                  BUT
                  my test connect is executed immediately after the first timer expires and the "item" is changed .

                  I am asking - is it possible I have a timing problem?

                  Here is my current - under construction connect to copy the "list" lines.
                  All it does - just compiles and cannot be tested , in existing code , due to this issue.

                  #ifdef BYPASS
                      // interacts with pushing buitto 11
                      // keeps crashing
                      
                      connect(
                                  this->ui->list,
                                  //          SIGNAL(ui->list->currentTextChanged()),
                                  SIGNAL(ui->list->itemDoubleClicked(*pItem)),
                                  //           SIGNAL(DDD->ui->list->itemChanged(TEST_LABEL)),
                                  //           SIGNAL(itemClicked(QListWidgetItem *)),
                                  //              SIGNAL(itemChanged(TEST_LABEL)),
                                  this->ui->plainTextEdit,
                                  //           SLOT(DDD->ui->plainTextEdit->appendPlainText(TEST_LABEL))
                                  //             SLOT(appendPlainText(TEST_LABEL))
                                  SLOT(ui->plainTextEdit->appendPlainText(*pItem)));
                      //           );
                      
                  #endif
                      
                      
                  

                  This is how it tracks when working - without the code above

                  QDEBUG TRACE 
                  QDEBUG TRACE TASK 
                  		 END imoplement Device DiscoiveryDialog  
                  
                  QDEBUG TRACE TASK 
                  		 find nearby BT devices 
                  
                  file      /media/f/QT/QT_PROJECT/TEST_PROJECT_14/mainwindow_cat_tab.cpp
                  function  on_pushButton_11_clicked
                  @line     236
                  TEMPORARY EXIT 
                  /home/f/build-TEST_PROJECT-Desktop-Debug/TEST_PROJECT exited with code 0
                  

                  And this is failing track

                  QDEBUG TRACE 
                  QDEBUG TRACE TASK 
                  		 START imoplement Device DiscoveryDialog  
                  
                  QDEBUG TRACE TASK 
                  		 find nearby BT devices 
                  
                  file      /media/f/QT/QT_PROJECT/TEST_PROJECT_14/mainwindow_cat_tab.cpp
                  function  on_pushButton_11_clicked
                  @line     206
                  TEMPORARY EXIT 
                  TRACE SCAN FLOW 
                  			 START TOK TRACE SCAN FLOW CONSTRUCTOR 
                  			DeviceDiscoveryDialog::DeviceDiscoveryDialog(QWidget *parent)
                  
                  
                  TEMPORARY exit  @line 
                  133
                  The program has unexpectedly finished.
                  /home/f/build-TEST_PROJECT-Desktop-Debug/TEST_PROJECT crashed
                  

                  Again - all I am asking - is it possible I have a timing problem?

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

                  @AnneRanch said in Please help with lambda sytax:

                  function on_pushButton_11_clicked
                  @line 206

                  Can you show your on_pushButton_11_clicked? The crash is there at line 206.

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

                  A 1 Reply Last reply
                  1
                  • A Anonymous_Banned275

                    @jsulm said in Please help with lambda sytax:

                    But this will also not work as QPlainTextEdit does not have a slot appendPlainText which takes a QListWidgetItem* as parameter (use new Qt5 connect syntax and a lambda)...

                    I do appreciate the post.
                    It may seems strange, but at this point I am NOT after "it" working.
                    I just need to figure out WHY adding this "connect" , in any form or format , crashes the program.
                    After I figure out the crasher I will work on form / format/ style.
                    Of course I could verify the "connect" using separate test program.

                    Pl45m4P Offline
                    Pl45m4P Offline
                    Pl45m4
                    wrote on last edited by Pl45m4
                    #20

                    @AnneRanch

                    Back to lambdas:

                    You said, you are struggling to understand lambda connections correctly.
                    It's not as complicated as it might look.

                    • You have your sender (first argument)

                      connect(ui->list,.....)
                      
                    • You have your sender's class + signal

                       connect(ui->list, &QListWidget::itemDoubleClicked,.....)
                      
                    • Then you can define a scope for your lambda

                      connect(ui->list, &QListWidget::itemDoubleClicked, this,......)
                      
                    • the last part consists of the capture, the function or slot arguments and the actual code itself, using the passed and captured values

                      • Capture

                        • "[ ]": None of the variables from current class / scope are captured and can be used inside lambda "code"
                        • " [=]": Captures everything (by value!)
                        • "[this] ": (current instance) or any combination of variables (e.g. [x, a]), to pick variables you need explicitely.
                        • "[&]" or "[ &someVar ]": Captures by reference. Changes made inside lambda will affect and change the captured variable outside the lambda as well ( Thx, @JonB )
                      • Signal / Slot arguments after capture inside ( .... ), e.g. if signal sends a QListWidgetItem, you need a (QListWidgetItem *item) here.

                      • lambda code inside { .... }

                        • do whatever you like and what you would put in your slot here.

                    One big advantage using lambda connections is, that you can connect signals to slots (or "code"), that would be incompatible using a "direct" connection because of any reason (e.g. parameter mismatch).

                    Simple example:

                    // signal
                    void signal(QString s);
                    // slot
                    void onSignalFired(int i);
                    

                    A direct connection of these two won't work neither using the SIGNAL(...) / SLOT( ... ) syntax
                    (-> crash / error at runtime),
                    nor using the function based connection &Class::signal... &Class::onSignalFired
                    ( -> wont compile).

                    With lambda, you can make it work:

                    connect(this, &Class::signal, [] (QString s) {  onSignalFired( s.toInt() ); } ); 
                    

                    Instead of calling the slot, you could also skip the slot and do whatever you want to do with this string inside the lambda directly, e.g. send your number to a QLineEdit ("A" would lead to a 65, simple ASCII).

                    @AnneRanch said in Please help with lambda sytax:

                    Here is my current - under construction connect to copy the "list" lines.
                    All it does - just compiles and cannot be tested , in existing code , due to this issue.

                    ... and this is exactly where you can make use of lambdas ( as @jsulm already said ).
                    Pass your QListWidgetItem from signal to your lambda, get the text from your item using text() (Qt Doc), then pass the text to QPlainTextEdit's appendPlainText(QString).


                    If debugging is the process of removing software bugs, then programming must be the process of putting them in.

                    ~E. W. Dijkstra

                    JonBJ 1 Reply Last reply
                    4
                    • Pl45m4P Pl45m4

                      @AnneRanch

                      Back to lambdas:

                      You said, you are struggling to understand lambda connections correctly.
                      It's not as complicated as it might look.

                      • You have your sender (first argument)

                        connect(ui->list,.....)
                        
                      • You have your sender's class + signal

                         connect(ui->list, &QListWidget::itemDoubleClicked,.....)
                        
                      • Then you can define a scope for your lambda

                        connect(ui->list, &QListWidget::itemDoubleClicked, this,......)
                        
                      • the last part consists of the capture, the function or slot arguments and the actual code itself, using the passed and captured values

                        • Capture

                          • "[ ]": None of the variables from current class / scope are captured and can be used inside lambda "code"
                          • " [=]": Captures everything (by value!)
                          • "[this] ": (current instance) or any combination of variables (e.g. [x, a]), to pick variables you need explicitely.
                          • "[&]" or "[ &someVar ]": Captures by reference. Changes made inside lambda will affect and change the captured variable outside the lambda as well ( Thx, @JonB )
                        • Signal / Slot arguments after capture inside ( .... ), e.g. if signal sends a QListWidgetItem, you need a (QListWidgetItem *item) here.

                        • lambda code inside { .... }

                          • do whatever you like and what you would put in your slot here.

                      One big advantage using lambda connections is, that you can connect signals to slots (or "code"), that would be incompatible using a "direct" connection because of any reason (e.g. parameter mismatch).

                      Simple example:

                      // signal
                      void signal(QString s);
                      // slot
                      void onSignalFired(int i);
                      

                      A direct connection of these two won't work neither using the SIGNAL(...) / SLOT( ... ) syntax
                      (-> crash / error at runtime),
                      nor using the function based connection &Class::signal... &Class::onSignalFired
                      ( -> wont compile).

                      With lambda, you can make it work:

                      connect(this, &Class::signal, [] (QString s) {  onSignalFired( s.toInt() ); } ); 
                      

                      Instead of calling the slot, you could also skip the slot and do whatever you want to do with this string inside the lambda directly, e.g. send your number to a QLineEdit ("A" would lead to a 65, simple ASCII).

                      @AnneRanch said in Please help with lambda sytax:

                      Here is my current - under construction connect to copy the "list" lines.
                      All it does - just compiles and cannot be tested , in existing code , due to this issue.

                      ... and this is exactly where you can make use of lambdas ( as @jsulm already said ).
                      Pass your QListWidgetItem from signal to your lambda, get the text from your item using text() (Qt Doc), then pass the text to QPlainTextEdit's appendPlainText(QString).

                      JonBJ Offline
                      JonBJ Offline
                      JonB
                      wrote on last edited by JonB
                      #21

                      @Pl45m4
                      Since you've laid this out so neatly, may I suggest you add [&] to your Capture section, and note how it differs from [=]? Because some day someone's going to need it... :)

                      1 Reply Last reply
                      1
                      • jsulmJ jsulm

                        @AnneRanch said in Please help with lambda sytax:

                        function on_pushButton_11_clicked
                        @line 206

                        Can you show your on_pushButton_11_clicked? The crash is there at line 206.

                        A Offline
                        A Offline
                        Anonymous_Banned275
                        wrote on last edited by
                        #22

                        @jsulm said in Please help with lambda sytax:

                        @AnneRanch said in Please help with lambda sytax:

                        function on_pushButton_11_clicked
                        @line 206

                        Can you show your on_pushButton_11_clicked? The crash is there at line 206.

                        There it is.

                        I have left in all the trace / debug stuff .
                        And highlighted the line 206

                        code// scan for nearby BT devices
                        
                        void MainWindow_CAT_TAB::on_pushButton_11_clicked()
                        {
                        #ifdef TRACE
                            qDebug() << "QDEBUG TRACE ";
                            qDebug() << "QDEBUG TRACE TASK \n\t\t void MainWindow_CAT_TAB::on_pushButton_11_clicked()  \n";
                             qDebug() << "QDEBUG TRACE TASK \n\t\t find nearby BT devices \n";
                            qDebug() << "file     " << __FILE__;
                            qDebug() << "function "<<__FUNCTION__;
                            qDebug() << "@line    " << __LINE__;
                            qDebug()<<"TEMPORARY EXIT ";
                            //exit(99);
                        #endif
                        //   ui->listWidget_5->(" addItem(on_pushButton_11_clicked()");
                        //     ui->listWidget_5->(addItem(on_pushButton_11_clicked());
                                     ui->listWidget_5->addItem(" addItem(on_pushButton_11_clicked()");
                                     ui->listWidget_5->addItem(" addItem(on_pushButton_11_clicked()");
                                     ui->listWidget_5->addItem("information_main.count())");
                        
                        
                        #ifdef TRACE
                            qDebug() << "QDEBUG TRACE ";
                            qDebug() << "QDEBUG TRACE TASK \n\t\t START imoplement Device DiscoveryDialog  \n";
                             qDebug() << "QDEBUG TRACE TASK \n\t\t find nearby BT devices \n";
                            qDebug() << "file     " << __FILE__;
                            qDebug() << "function "<<__FUNCTION__;
                            **qDebug() << "@line    " << __LINE__;**
                            qDebug()<<"TEMPORARY EXIT ";
                            //exit(99);
                        #endif
                        
                        // instatiate Devievce Discovery Dialog as in btscanner example
                        DeviceDiscoveryDialog *DDD = new DeviceDiscoveryDialog();
                        
                        DDD->startScan();  // takes time 2 seconds for each device 
                        
                        DDD->show();
                        
                        #ifdef BYPASS
                        // test add connect here
                        connect(
                                    DDD->ui->list,
                         //           SIGNAL(DDD->ui->list->itemChanged(TEST_LABEL)),
                                      SIGNAL(itemChanged(TEST_LABEL)),
                                    DDD->ui->plainTextEdit,
                         //           SLOT(DDD->ui->plainTextEdit->appendPlainText(TEST_LABEL))
                                    SLOT(appendPlainText(TEST_LABEL))
                                );
                        #endif
                        
                        
                        #ifdef TRACE
                            qDebug() << "QDEBUG TRACE ";
                            qDebug() << "QDEBUG TRACE TASK \n\t\t END imoplement Device DiscoiveryDialog  \n";
                             qDebug() << "QDEBUG TRACE TASK \n\t\t find nearby BT devices \n";
                            qDebug() << "file     " << __FILE__;
                            qDebug() << "function "<<__FUNCTION__;
                            qDebug() << "@line    " << __LINE__;
                            qDebug()<<"TEMPORARY EXIT ";
                            //exit(99);
                        #endif
                        
                        
                        }_text
                        
                        1 Reply Last reply
                        -1
                        • Christian EhrlicherC Offline
                          Christian EhrlicherC Offline
                          Christian Ehrlicher
                          Lifetime Qt Champion
                          wrote on last edited by
                          #23

                          @AnneRanch said in Please help with lambda sytax:

                          SIGNAL(itemChanged(TEST_LABEL)),

                          And you really want to tell us that you read at least one sentence from what we're telling you here??

                          Qt Online Installer direct download: https://download.qt.io/official_releases/online_installers/
                          Visit the Qt Academy at https://academy.qt.io/catalog

                          A 1 Reply Last reply
                          0
                          • Christian EhrlicherC Christian Ehrlicher

                            @AnneRanch said in Please help with lambda sytax:

                            SIGNAL(itemChanged(TEST_LABEL)),

                            And you really want to tell us that you read at least one sentence from what we're telling you here??

                            A Offline
                            A Offline
                            Anonymous_Banned275
                            wrote on last edited by
                            #24

                            Partial success.
                            Using both old and new "connect" style throws "segmentation fault" at either one.

                            It is my understanding that "connect" code placement is not critical - however my crash occurs exactly on "connect" which I have placed in a secondary dialog constructor. My main window dialog is primary.

                            RTFM implies that passing null pointer may be the problem...

                            A 1 Reply Last reply
                            0
                            • A Anonymous_Banned275

                              Partial success.
                              Using both old and new "connect" style throws "segmentation fault" at either one.

                              It is my understanding that "connect" code placement is not critical - however my crash occurs exactly on "connect" which I have placed in a secondary dialog constructor. My main window dialog is primary.

                              RTFM implies that passing null pointer may be the problem...

                              A Offline
                              A Offline
                              Anonymous_Banned275
                              wrote on last edited by
                              #25

                              @AnneRanch DONE!
                              Ready to tackle lambda .
                              Moral of the story - stay on original issue - it was my shallow knowledge of "connect" , my wrong syntax and my implementation of "connect" - applied in wrong place.

                              Help from everybody assisting is very much appreciated.

                              62318e6e-6ee2-4e8f-b993-51ed61c92d1b-image.png

                              Pablo J. RoginaP 1 Reply Last reply
                              1
                              • A Anonymous_Banned275

                                @AnneRanch DONE!
                                Ready to tackle lambda .
                                Moral of the story - stay on original issue - it was my shallow knowledge of "connect" , my wrong syntax and my implementation of "connect" - applied in wrong place.

                                Help from everybody assisting is very much appreciated.

                                62318e6e-6ee2-4e8f-b993-51ed61c92d1b-image.png

                                Pablo J. RoginaP Offline
                                Pablo J. RoginaP Offline
                                Pablo J. Rogina
                                wrote on last edited by
                                #26

                                @AnneRanch said in Please help with lambda sytax:

                                @AnneRanch DONE!

                                great, so please don't forget to mark your post as solved!

                                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
                                Don't ask support requests via chat/PM. Please use the forum so others can benefit from the solution in the future

                                1 Reply Last reply
                                0

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