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Error: no matching function for call to 'QObject::connect

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  • H Offline
    H Offline
    hskoglund
    wrote on 2 Jan 2018, 16:20 last edited by
    #7

    Hi, maybe you forgot to inherit from QObject or QWidget, i.e. your mainwindowcontent.h should look something like this:

    #pragma once
    #include "qobject.h"
    
    class MainWindowContent : QObject
    {
         Q_OBJECT
    
    public:
        MainWindowContent();
    
    private slots:
        void handleButton(QLabel* label,QLineEdit *lineEdit);
    }
    
    1 Reply Last reply
    0
    • W WhatIf
      2 Jan 2018, 14:03
      private slots:
          void handleButton();
      
      QObject::connect(button, SIGNAL (clicked()),this, SLOT (handleButton()));
      
      void MainWindowContent::handleButton()
      {
          //label->setText(lineEdit->text());
      }
      

      error: no matching function for call to 'QObject::connect(QPushButton*&, const char*, MainWindowContent*, const char*)'
      QObject::connect(button, SIGNAL (clicked()),this, SLOT (handleButton()));

      I also tried

      QObject::connect(button, &QPushButton::clicked,this, std::bind(&MainWindowContent::handleButton,this,label,lineEdit));
      

      and got

      error: no matching function for call to 'QObject::connect(QPushButton*&, void (QAbstractButton::)(bool), MainWindowContent, std::_Bind_helper<false, void (MainWindowContent::)(QLabel, QLineEdit*), MainWindowContent*, QLabel*&, QLineEdit*&>::type)'
      QObject::connect(button, &QPushButton::clicked,this, std::bind(&MainWindowContent::handleButton,this,label,lineEdit));

      V Offline
      V Offline
      VRonin
      wrote on 2 Jan 2018, 16:26 last edited by
      #8

      @WhatIf said in error: no matching function for call to 'QObject::connect:

      I also tried
      QObject::connect(button, &QPushButton::clicked,this, std::bind(&MainWindowContent::handleButton,this,label,lineEdit));

      and got

      error: no matching function for call to 'QObject::connect(QPushButton*&, void (QAbstractButton::)(bool), MainWindowContent, std::_Bind_helper<false, void (MainWindowContent::)(QLabel, QLineEdit*), MainWindowContent*, QLabel*&, QLineEdit*&>::type)'
      QObject::connect(button, &QPushButton::clicked,this, std::bind(&MainWindowContent::handleButton,this,label,lineEdit));

      read the error. (MainWindowContent::)(QLabel, QLineEdit*) did you forget a * next to QLabel in handleButton?

      "La mort n'est rien, mais vivre vaincu et sans gloire, c'est mourir tous les jours"
      ~Napoleon Bonaparte

      On a crusade to banish setIndexWidget() from the holy land of Qt

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      0
      • W Offline
        W Offline
        WhatIf
        wrote on 2 Jan 2018, 19:42 last edited by
        #9

        I forgot to subclass QObject!

        Now the program compiles but the slot is never called. here is how the code looks at this point.

        signals:
        
        private slots:
            void handleButton(); //handles button clicks
        
        QObject::connect(button, SIGNAL (clicked()),this, SLOT (handleButton()));
        
        void MainWindowContent::handleButton()
        {
            qDebug() << "INSIDE SLOT!";
        }
        
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        • M Offline
          M Offline
          mrjj
          Lifetime Qt Champion
          wrote on 2 Jan 2018, 19:57 last edited by
          #10

          Hi
          try with
          qDebug () << "conn:" << QObject::connect(button, SIGNAL (clicked()),this, SLOT (handleButton()));
          and see it says
          conn: true

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          0
          • H Offline
            H Offline
            hskoglund
            wrote on 2 Jan 2018, 20:12 last edited by
            #11

            Hi, also to add to @mrjj, perhaps the slot is never called because the button isn't visible, you could try insert a show() just after you've created the button, say like this:

            QPushButton* button = new QPushButton("OK!");
            button->show();
            
            QObject::connect(button, SIGNAL (clicked()),this, SLOT (handleButton()));
            1 Reply Last reply
            1
            • W Offline
              W Offline
              WhatIf
              wrote on 2 Jan 2018, 21:37 last edited by
              #12
              qDebug () << "conn:" << QObject::connect(button, SIGNAL (clicked()),this, SLOT (handleButton()));
              

              shows conn: true

              using button->show(); doesn't change anything.

              Basically, main.cpp calls mainwindow.cpp to get the central widget. mainwindow.cpp in turn calls mainwindowcontent.cpp to get the layout to set as centralwidget's.

              I'm passing "this" (connect 3rd argument) of mainwindowcontent.cpp to the connect statement.

              Will this make the connect work when mainwindowcontent.cpp returns the layout to mainwindow.cpp?

              1 Reply Last reply
              0
              • S Offline
                S Offline
                SGaist
                Lifetime Qt Champion
                wrote on 2 Jan 2018, 21:46 last edited by
                #13

                Hi,

                Did you check whether you have any warning at run time ?

                How do you know it's not getting called ? Did you put a breakpoint in your slot or a qDebug statement ?

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

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                0
                • W Offline
                  W Offline
                  WhatIf
                  wrote on 2 Jan 2018, 22:06 last edited by
                  #14

                  No warnings at run time.

                  I used both breakpoints and a qDebug statement.

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                  0
                  • S Offline
                    S Offline
                    SGaist
                    Lifetime Qt Champion
                    wrote on 2 Jan 2018, 22:07 last edited by
                    #15

                    Then a silly question: how are you ensuring that you are clicking on the correct button ?

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

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                    1
                    • W Offline
                      W Offline
                      WhatIf
                      wrote on 2 Jan 2018, 22:28 last edited by
                      #16

                      There is only one button

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                      0
                      • H Offline
                        H Offline
                        hskoglund
                        wrote on 2 Jan 2018, 23:40 last edited by
                        #17

                        Hi, just guessing, but how do you assign the grid layout into the centralwidget in your mainwindowp.cpp, is it something similar to this:

                        ...
                        setCentralWidget(new QWidget());
                        centralWidget()->setLayout(mainWindowContent->getPageWidgetsLayout());
                        ...
                        
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                        0
                        • P Offline
                          P Offline
                          Paul Colby
                          wrote on 2 Jan 2018, 23:42 last edited by
                          #18

                          What's the lifetime of your MainWindowContent instance?

                          It looks to me that MainWindowContent::getPageWidgetsLayout() could be entirely static (if you wanted it to be) except for the this in the connect call. So if, for example, your MainWindowContent instance is destroyed, and re-created later, the slot will have been automatically disconnected, and not reconnected.

                          Just something else to check...

                          Cheers.

                          1 Reply Last reply
                          1
                          • W Offline
                            W Offline
                            WhatIf
                            wrote on 3 Jan 2018, 00:38 last edited by
                            #19

                            main.cpp

                            #include "mainwindow.h"
                            #include <QApplication>
                            
                            int main(int argc, char *argv[])
                            {
                                QApplication a(argc, argv);
                                MainWindow w;
                            
                                w.buildPage(); 
                            
                                w.show();
                            
                                return a.exec();
                            }
                            

                            mainwindow.cpp

                            #include "mainwindow.h"
                            #include "ui_mainwindow.h"
                            
                            #include <QLabel>
                            #include <QHBoxLayout>
                            
                            MainWindow::MainWindow(QWidget *parent) :
                                QMainWindow(parent),
                                ui(new Ui::MainWindow)
                            {
                                ui->setupUi(this);
                            }
                            
                            MainWindow::~MainWindow()
                            {
                                delete ui;
                            }
                            
                            void MainWindow::buildPage()
                            {
                                MainWindowContent winContent;
                            
                                ui->centralWidget->setLayout(winContent.getPageWidgetsLayout());
                            }
                            

                            mainwindowcontent.cpp

                            #include "mainwindowcontent.h"
                            
                            #include <QDebug>
                            
                            MainWindowContent::MainWindowContent(QObject *parent) : QObject(parent)
                            {
                            
                            }
                            
                            QGridLayout* MainWindowContent::getPageWidgetsLayout()
                            {
                                QGridLayout *gridLayout  =new QGridLayout();
                            
                                QLabel *label = new QLabel("");
                            
                                QLineEdit *lineEdit = new QLineEdit("Enter Text!");
                            
                                QPushButton *button = new QPushButton("OK!");
                            
                                QObject::connect(button, SIGNAL (clicked()),this, SLOT (handleButton()));
                            
                                gridLayout->addWidget(label,0,0);
                                gridLayout->addWidget(lineEdit,1,1);
                                gridLayout->addWidget(button,2,2);
                               
                            
                                return gridLayout;
                            }
                            
                            void MainWindowContent::handleButton()
                            {
                                qDebug() << "INSIDE SLOT!";
                            }
                            
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                            • P Offline
                              P Offline
                              Paul Colby
                              wrote on 3 Jan 2018, 00:46 last edited by Paul Colby 1 Mar 2018, 00:47
                              #20

                              The problem is with the winContent lifetime.

                              void MainWindow::buildPage()
                              {
                                  MainWindowContent winContent;
                              
                                  ui->centralWidget->setLayout(winContent.getPageWidgetsLayout());
                                  // at this point, winContent is destroyed, and automatically disconnected.
                              }
                              

                              You probably want to do something more like:

                              void MainWindow::buildPage()
                              {
                                  MainWindowContent * winContent = new MainWindowContent(this);
                              
                                  ui->centralWidget->setLayout(winContent->getPageWidgetsLayout());
                              }
                              

                              Or move the slot to the MainWindow class.

                              Cheers.

                              1 Reply Last reply
                              4
                              • W Offline
                                W Offline
                                WhatIf
                                wrote on 3 Jan 2018, 22:20 last edited by
                                #21

                                Thank you very much for all the help!

                                The last problem I have is how to pass the label and lineedit to the slot. I don't want to make them global variables. if I create a custom signal with the label and lineedit to match the slot, I'm confused about where to emit the signal.

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                                0
                                • S Offline
                                  S Offline
                                  SGaist
                                  Lifetime Qt Champion
                                  wrote on 3 Jan 2018, 23:12 last edited by
                                  #22

                                  Before that, why do you need the MainWindowContent. It looks like a convoluted mean to create the central widget content.

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

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                                  1
                                  • W Offline
                                    W Offline
                                    WhatIf
                                    wrote on 4 Jan 2018, 01:26 last edited by
                                    #23

                                    I want to keep the logic of how the program progresses from page to page in MainWindow. But I want other classes to generate the pages to be displayed. Basically I want to be able to manage the program without having a single .cpp file that is a thousand lines or so.

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                                    • S Offline
                                      S Offline
                                      SGaist
                                      Lifetime Qt Champion
                                      wrote on 4 Jan 2018, 07:06 last edited by
                                      #24

                                      Then why not make them full QWidget derivative ?

                                      That's what is usually done, cut your application in logical pieces and then build as many QWidget based classes as needed to keep the code clean, simple and tidy.

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

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                                      1
                                      • W Offline
                                        W Offline
                                        WhatIf
                                        wrote on 4 Jan 2018, 10:12 last edited by
                                        #25

                                        Can you give an example please? Or direct me to any tutorial on the topic?

                                        1 Reply Last reply
                                        0
                                        • S Offline
                                          S Offline
                                          SGaist
                                          Lifetime Qt Champion
                                          wrote on 4 Jan 2018, 20:15 last edited by
                                          #26

                                          The scribble example is a simple example to start with.

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

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