Important: Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

Connecting two windows



  • Hi all,

    I have a basic question:

    I need to connect a signal (QString) from a QMainWindow (MainWindow) to slot (QLabel::settext) from QMainWindow (StageOne). I did this:

    #include "stageone.h"
    #include "ui_stageone.h"
    #include "mainwindow.h"
    
    StageOne::StageOne(QWidget *parent) :
        QMainWindow(parent),
        ui(new Ui::StageOne)
    {
        ui->setupUi(this);
    
        connect(&MainWindow, &MainWindow::patientID, ui->label, &QLabel::setText);  
    
    }
    

    this gives an error says:

    stageone.cpp:11:14: error: 'MainWindow' does not refer to a value
    mainwindow.h:17:7: note: declared here
    

    What I'm doing wrong here?

    Thank you



  • @russjohn834 said in Connecting two windows:

    &MainWindow, &MainWindow::patientID

    Start with:

    • &MainWindow probably should be this
    • &MainWindow::patientID needs to be a signal which is emitted when something happens. Yours looks to me like a member variable?

    There will be further issues after these...

    However, I am wondering whether you do not have/want any signal at all, and this should be nothing to do with signals/slots. If all you want to do is set the text of that label in the constructor don't you just want:

    ui->label->setText("something");
    

    Depends what you are actually trying to achieve? Is this really to do with a signal happening or not?



  • @JonB Thank you.

    &MainWindow::patientID is a signal which is emitted as

    emit patientID(selection[0].data().toString());
    

    So I need to setText label in another window (StageOne). The text is selection[0].data().toString()



  • @JonB

    I tried this:

    connect(this, &MainWindow::patientID, ui->label, &QLabel::setText);
    

    this throws a different error:

    stageone.cpp:11:5: error: no matching member function for call to 'connect'
    qobject.h:228:43: note: candidate function [with Func1 = void (MainWindow::*)(QString), Func2 = void (QLabel::*)(const QString &)] not viable: no known conversion from 'StageOne *' to 'const typename QtPrivate::FunctionPointer<void (MainWindow::*)(QString)>::Object *' (aka 'const MainWindow *') for 1st argument
    qobject.h:208:36: note: candidate function not viable: no known conversion from 'void (MainWindow::*)(QString)' to 'const char *' for 2nd argument
    qobject.h:211:36: note: candidate function not viable: no known conversion from 'void (MainWindow::*)(QString)' to 'const QMetaMethod' for 2nd argument
    qobject.h:463:41: note: candidate function not viable: no known conversion from 'void (MainWindow::*)(QString)' to 'const char *' for 2nd argument
    qobject.h:269:13: note: candidate template ignored: requirement '!QtPrivate::FunctionPointer<void (QLabel::*)(const QString &)>::IsPointerToMemberFunction' was not satisfied [with Func1 = void (MainWindow::*)(QString), Func2 = void (QLabel::*)(const QString &)]
    qobject.h:308:13: note: candidate template ignored: requirement 'QtPrivate::FunctionPointer<void (QLabel::*)(const QString &)>::ArgumentCount == -1' was not satisfied [with Func1 = void (MainWindow::*)(QString), Func2 = void (QLabel::*)(const QString &)]
    qobject.h:260:13: note: candidate function template not viable: requires 3 arguments, but 4 were provided
    qobject.h:300:13: note: candidate function template not viable: requires 3 arguments, but 4 were provided
    


  • @russjohn834
    I would suggest that patientID is not at all a good name for a signal or a method! Something more like patientIdChanged would be usual and intelligible.

    The error message is complaining about your first parameter, &MainWindow. I don't know what you think that is. As I said, you probably mean this there.



  • @russjohn834
    In answer to your new messages, when you wrote earlier

    &MainWindow::patientID is a signal which is emitted as

    emit patientID(selection[0].data().toString());

    this is not good enough for people to help you, I would suggest. We need to see the definition in the .cpp file and the declaration in the .h file....


  • Lifetime Qt Champion

    @russjohn834 Please show the exact declaration of patientID...



  • @jsulm Hi,

    I declared patientID as:

    QT_BEGIN_NAMESPACE
    namespace Ui { class MainWindow; }
    QT_END_NAMESPACE
    
    class MainWindow : public QMainWindow
    {
        Q_OBJECT
    
    public:
        MainWindow(QWidget *parent = nullptr);
        ~MainWindow();
        StageTwoNew *stagetwonew;
        StageOne *stageone;
    
    signals:
    
        void patientID(QString);  ----------------> signal declaration
    
    
    public slots:
        void on_pushButton_New_clicked();
    
        void on_pushButton_Open_clicked();
    
        void parseDataEntry(const QString dataPath);
    
    private:
        Ui::MainWindow *ui;
    
    };
    #endif // MAINWINDOW_H
    

  • Lifetime Qt Champion

    @russjohn834 Looks good. Try a complete rebuild: delete build folder, run qmake and then build.



  • @jsulm

    I'm getting still the same error:

    stageone.cpp:11:5: error: no matching member function for call to 'connect'
    qobject.h:228:43: note: candidate function [with Func1 = void (MainWindow::*)(QString), Func2 = void (QLabel::*)(const QString &)] not viable: no known conversion from 'StageOne *' to 'const typename QtPrivate::FunctionPointer<void (MainWindow::*)(QString)>::Object *' (aka 'const MainWindow *') for 1st argument
    qobject.h:208:36: note: candidate function not viable: no known conversion from 'void (MainWindow::*)(QString)' to 'const char *' for 2nd argument
    qobject.h:211:36: note: candidate function not viable: no known conversion from 'void (MainWindow::*)(QString)' to 'const QMetaMethod' for 2nd argument
    qobject.h:463:41: note: candidate function not viable: no known conversion from 'void (MainWindow::*)(QString)' to 'const char *' for 2nd argument
    qobject.h:269:13: note: candidate template ignored: requirement '!QtPrivate::FunctionPointer<void (QLabel::*)(const QString &)>::IsPointerToMemberFunction' was not satisfied [with Func1 = void (MainWindow::*)(QString), Func2 = void (QLabel::*)(const QString &)]
    qobject.h:308:13: note: candidate template ignored: requirement 'QtPrivate::FunctionPointer<void (QLabel::*)(const QString &)>::ArgumentCount == -1' was not satisfied [with Func1 = void (MainWindow::*)(QString), Func2 = void (QLabel::*)(const QString &)]
    qobject.h:260:13: note: candidate function template not viable: requires 3 arguments, but 4 were provided
    qobject.h:300:13: note: candidate function template not viable: requires 3 arguments, but 4 were provided
    

    This is how I call the connect:

    #include "stageone.h"
    #include "ui_stageone.h"
    #include "mainwindow.h"
    
    StageOne::StageOne(QWidget *parent) :
        QMainWindow(parent),
        ui(new Ui::StageOne)
    {
        ui->setupUi(this);
    
        connect(this, &MainWindow::patientID, ui->label, &QLabel::setText);  //
    
    }
    
    

    Any idea?


  • Moderators

    @russjohn834

    do you happen to have a function body inside mainwindow.cpp ?

    somethinglike:
    void MainWindow::patientID(QString string)

    That's not allowed for signals


  • Moderators

    @russjohn834 said in Connecting two windows:

    connect(this, &MainWindow::patientID, ui->label, &QLabel::setText);

    Oh its much simpler

    You class is not called MainWindow, but StageOne

    connect(this, &StageOne::patientID, ui->label, &QLabel::setText); 
    

  • Lifetime Qt Champion

    @J-Hilk @russjohn834 I'm lost. The signal is declared in MainWindow, but connect is in StageOne.
    @russjohn834 What is it now?!



  • @jsulm @J-Hilk ,

    Still the same issue.

    I changed the name of signal from patientID to patientIdChanged.

    signal is Declared and emitted in MainWindow. Connection is in StageOne (with a QLabel).

    StageOne::StageOne(QWidget *parent) :
        QMainWindow(parent),
        ui(new Ui::StageOne)
    {
        ui->setupUi(this);
    
        connect(this, &MainWindow::patientIdChanged, ui->label, &QLabel::setText);  //
    }
    

    the error says:

    stageone.cpp:11:5: error: no matching member function for call to 'connect'
    qobject.h:228:43: note: candidate function [with Func1 = void (MainWindow::*)(QString), Func2 = void (QLabel::*)(const QString &)] not viable: no known conversion from 'StageOne *' to 'const typename QtPrivate::FunctionPointer<void (MainWindow::*)(QString)>::Object *' (aka 'const MainWindow *') for 1st argument
    qobject.h:208:36: note: candidate function not viable: no known conversion from 'void (MainWindow::*)(QString)' to 'const char *' for 2nd argument
    qobject.h:211:36: note: candidate function not viable: no known conversion from 'void (MainWindow::*)(QString)' to 'const QMetaMethod' for 2nd argument
    qobject.h:463:41: note: candidate function not viable: no known conversion from 'void (MainWindow::*)(QString)' to 'const char *' for 2nd argument
    qobject.h:269:13: note: candidate template ignored: requirement '!QtPrivate::FunctionPointer<void (QLabel::*)(const QString &)>::IsPointerToMemberFunction' was not satisfied [with Func1 = void (MainWindow::*)(QString), Func2 = void (QLabel::*)(const QString &)]
    qobject.h:308:13: note: candidate template ignored: requirement 'QtPrivate::FunctionPointer<void (QLabel::*)(const QString &)>::ArgumentCount == -1' was not satisfied [with Func1 = void (MainWindow::*)(QString), Func2 = void (QLabel::*)(const QString &)]
    qobject.h:260:13: note: candidate function template not viable: requires 3 arguments, but 4 were provided
    qobject.h:300:13: note: candidate function template not viable: requires 3 arguments, but 4 were provided
    

  • Lifetime Qt Champion

    @russjohn834 Come on it is not about patientID!
    In the connect bellow this is NOT MainWindow, but StageOne!
    It simply can't work this way!

    StageOne::StageOne(QWidget *parent) :
        QMainWindow(parent),
        ui(new Ui::StageOne)
    {
        ui->setupUi(this);
    
        connect(this, &MainWindow::patientIdChanged, ui->label, &QLabel::setText);  // this != MaiNWindow*!
    }
    

    If you want to connect in StageOne then you need an instance of MainWindow:

    StageOne::StageOne(QWidget *parent) :
        QMainWindow(parent),
        ui(new Ui::StageOne)
    {
        ui->setupUi(this);
        MainWindow *mainWindow = new MainWindow(...);
        connect(mainWindow, &MainWindow::patientIdChanged, ui->label, &QLabel::setText);  // this != MaiNWindow*!
    }
    


  • @jsulm
    This is partly due to my bad... :(

    No declaration of StageOne was shown. I quickly read:

    StageOne::StageOne(QWidget *parent) :
        QMainWindow(parent)
    

    I read it too quickly, and thought StageOne was derived from QMainWindow. Comes from doing Python all the time instead of C++. So I misled OP by saying put in this where he should have mainWindow, my fault not his!



  • I'm a bit lost!

    I now get a different error

    mainwindow.h:25: error: C2143: syntax error: missing ';' before '*'
    mainwindow.h:25: error: C4430: missing type specifier - int assumed. Note: C++ does not support default-int
    mainwindow.h:25: error: C2238: unexpected token(s) preceding ';'
    

    this is my mainwindow.h

    #ifndef MAINWINDOW_H
    #define MAINWINDOW_H
    
    #include <QMainWindow>
    #include <QDebug>
    #include <QFile>
    #include <QDirIterator>
    #include <QXmlStreamReader>
    #include <QMessageBox>
    #include "stagetwonew.h"
    #include "stageone.h"
    
    QT_BEGIN_NAMESPACE
    namespace Ui { class MainWindow; }
    QT_END_NAMESPACE
    
    class MainWindow : public QMainWindow
    {
        Q_OBJECT
    
    public:
        MainWindow(QWidget *parent = nullptr);
        ~MainWindow();
        StageTwoNew *stagetwonew;
        StageOne *stageone;  ------------------>error points here
    
    signals:
    
        void patientIdChanged(QString);
    
    
    private slots:
    
        void on_pushButton_New_clicked();
    
        void on_pushButton_Open_clicked();
    
        void parseDataEntry(const QString dataPath);
    
    private:
        Ui::MainWindow *ui;
    
    };
    #endif // MAINWINDOW_H
    

    I tried complete rebuild, no luck

    Any ideas where I'm wrong?



  • @russjohn834

    #include "stagetwonew.h"
    #include "stageone.h"
    ...
        StageTwoNew *stagetwonew;
        StageOne *stageone;  ------------------>error points here
    

    Prove to us/yourself that those two header files declare StageTwoNew & StageOne as classes? (Having said that, I'd more have expected "undeclared", but at least check what I've said, even if I did mis-lead you earlier :) )


  • Lifetime Qt Champion

    @JonB said in Connecting two windows:

    I read it too quickly, and thought StageOne was derived from QMainWindow

    It apparently is derived from QMainWindow, but this does not have anything to do with your MainWindow.
    Does StageOne include mainwindow.h? If so you have circular dependency.



  • @jsulm , @JonB

    @jsulm said in Connecting two windows:

    Does StageOne include mainwindow.h? If so you have circular dependency.

    Yes. StageOne inlcude mainwindow.h. Any suggestion, How do I go about with circular dependency in this scenario?

    Or is there a better way to do this (connect between two forms (QMainWindow))?



  • @russjohn834 could you please state your requirement again?
    How are the window(s) suppose to work together?
    What data you want from what window updating what widget in what other window?



  • @Pablo-J-Rogina Hi,

    There are two forms.
    MainWindow and StageOne (Both inherits QMainWindow).

    MainWindow has a signal patientIdChanged(QString), which is emitted upon clicking a button, I did this:

    void MainWindow::on_pushButton_Open_clicked()
    {
    
        QModelIndexList selection=ui->tableWidget->selectionModel()->selectedRows(0);
    
        this->hide();
        stageone = new StageOne(this);
        stageone->show();
        
      emit patientIdChanged(selection[0].data().toString());
        
        
    }
    

    And in StageOne I have a QLabel, its setText should get signal (Qstring) value from MainwWndow and show. That's the requirement.

    I tried to do this in StageOne:

     MainWindow *mainWindow= new MainWindow(this);
    connect(mainWindow, &MainWindow::patientIdChanged, ui->label, &QLabel::setText);
    
    

    But this is not working



  • @russjohn834 said in Connecting two windows:

    Ok, it looks like you're overcomplicating things here.

    So you want a string (some selection in MainWindow widget) passed to StageOne window, right?
    What about having the StageOne constructor receiving such string? Pseudo-code:

    void MainWindow::on_pushButton_Open_clicked()
    {
         QModelIndexList selection=ui->tableWidget->selectionModel()->selectedRows(0);
     
         this->hide();
         stageone = new StageOne(selection[0].data().toString(), this);
         stageone->show();
    ...
    

    and obviusly StageOne class need a change in constructor:

    StageOne::StageOne(Qstring someString, QWidget *parent) :
        QMainWindow(parent),
        ui(new Ui::StageOne)
    {
    ...
    

    I don't see the need to deal with signals here



  • @Pablo-J-Rogina Thanks a lot that's exactly I was trying for.



  • Thank you @JonB, @jsulm , @J-Hilk and @Pablo-J-Rogina for your feedback



  • Hi everybody.

    Let's have a look to the code @russjohn834 gave us :

    class MainWindow : public QMainWindow
    {
        Q_OBJECT
    
    public:
        MainWindow(QWidget *parent = nullptr);
        ~MainWindow();
        StageTwoNew *stagetwonew;
        StageOne *stageone; 
    
    signals:
    
        void patientID(QString);  ----------------> signal declaration
    
    
    public slots:
        void on_pushButton_New_clicked();
    
        void on_pushButton_Open_clicked();
    
        void parseDataEntry(const QString dataPath);
    
    private:
        Ui::MainWindow *ui;
    
    };
    #endif // MAINWINDOW_H
    

    Here we can see that the MainWindow owns an object of the class StageOne.
    So we can assume that the StageOne instance is built with specifying the parent as the MainWindow, certainly with code that looks like this :

    MainWindow::MainWindow() : QMainWindow() 
        : stageone(new StageOne(this))
    {
    }
    

    If i am right, the parent of the stageone object is the MainWindow from which the signal patientID is emitted.

    You can then write things like this :

    StageOne::StageOne(QWidget *parent) :
        QMainWindow(parent), // HERE the pointed parent is the MainWindow object
        ui(new Ui::StageOne)
    {
        ui->setupUi(this);
        
        // We connect the MainWindow::patientID signal emitted from the parent object
        // to the QLabel::setText slot
        connect(parent, &MainWindow::patientID, ui->label, &QLabel::setText);  
    }
    


  • @jhx76 said in Connecting two windows:

    If i am right, the parent of the stageone object is the MainWindow from which the signal patientID is emitted.

    it looks like there's no need to have signals at all.



  • That's right :)

    was just answering to the original question :

    @russjohn834

    What I'm doing wrong here?

    have a nice day


  • Lifetime Qt Champion

    @russjohn834 Just a tip: to avoid circular dependencies use forward declarations:

    // a.h
    class B; // This is forward declaration for class B, no need to include b.h here
    class A
    {
        private:
            B *b;
    };
    
    // b.h
    #include "a.h"
    ...
    

    Forward declarations work only for pointers - in the above example it would not work if b would not be a pointer.



  • @jsulm Thank you for your suggestion.
    Just to further clarify myself the use of forward declaration:

    Two form classes MainWindow and StageOne

    StageOne needs to connect with a signal from MainWindow

    I did this in StageOne

    class MainWindow;  // ---------> forward declaration for Mainwindow
    
    namespace Ui {
    class StageOne;
    }
    
    class StageOne : public QMainWindow
    {
        Q_OBJECT
    
    public:
        explicit StageOne(QString, QWidget *parent = nullptr);
        ~StageOne();
    
    
    private:
        Ui::StageOne *ui;
    
        MainWindow *window;
    
    };
    

    I'm trying to connect as follows in StageOne:

    connect(window,&MainWindow::patientIdChanged, ui->label, &QLabel::setText);
    
    

    But this gives an error says:

    stageone.cpp:12:21: error: incomplete type 'MainWindow' named in nested name specifier
    stageone.h:10:7: note: forward declaration of 'MainWindow'
    
    

    in mainwindow, i did include #include "stageone.h"

    what mistake I'm doing here?

    Thank you


  • Lifetime Qt Champion

    @russjohn834 You need to include mainwindow.h header in stageone.cpp



  • @jsulm if I include mainwindow.h header in stageone.cpp, I get follwoing error:

    mainwindow.h:25: error: C2143: syntax error: missing ';' before '*'
    mainwindow.h:25: error: C4430: missing type specifier - int assumed. Note: C++ does not support default-int
    mainwindow.h:25: error: C2238: unexpected token(s) preceding ';'
    

    Which points here:

    #include <QMainWindow>
    #include "stagetwonew.h"
    #include "stageone.h"
    
    QT_BEGIN_NAMESPACE
    namespace Ui { class MainWindow; }
    QT_END_NAMESPACE
    
    class MainWindow : public QMainWindow
    {
        Q_OBJECT
    
    public:
        MainWindow(QWidget *parent = nullptr);
        ~MainWindow();
        StageTwoNew *stagetwonew;
        StageOne *stageone;  //--------------------> points here
    
    signals:
    
        void patientIdChanged(QString);
    
    
    private slots:
    
        void on_pushButton_New_clicked();
    
        void on_pushButton_Open_clicked();
    
        void parseDataEntry(const QString dataPath);
    
    private:
        Ui::MainWindow *ui;
    
    };
    #endif // MAINWINDOW_H
    

  • Lifetime Qt Champion

    @russjohn834 Are you sure you don't include mainwindow.h in stageone.h?
    Please show both header files.



  • @jsulm

    I tried with without and with mainwindow.h in stageone.h. I'm a bit confused, you mentioned this earlier:
    @jsulm said in Connecting two windows:

    @russjohn834 You need to include mainwindow.h header in stageone.cpp



  • here are two header files:

    mainwindow.h

    #ifndef MAINWINDOW_H
    #define MAINWINDOW_H
    
    #include <QMainWindow>
    #include <QDebug>
    #include <QFile>
    #include <QDirIterator>
    #include <QXmlStreamReader>
    #include <QMessageBox>
    #include "stagetwonew.h"
    #include "stageone.h"
    
    QT_BEGIN_NAMESPACE
    namespace Ui { class MainWindow; }
    QT_END_NAMESPACE
    
    class MainWindow : public QMainWindow
    {
        Q_OBJECT
    
    public:
        MainWindow(QWidget *parent = nullptr);
        ~MainWindow();
        StageTwoNew *stagetwonew;
        StageOne *stageone;
    
    signals:
    
        void patientIdChanged(QString);
    
    
    private slots:
    
        void on_pushButton_New_clicked();
    
        void on_pushButton_Open_clicked();
    
        void parseDataEntry(const QString dataPath);
    
    private:
        Ui::MainWindow *ui;
    
    };
    #endif // MAINWINDOW_H
    

    and stageone.h:

    #ifndef STAGEONE_H
    #define STAGEONE_H
    
    #include <QMainWindow>
    //#include "mainwindow.h"
    #include "stagetwonew.h"
    
    
    
    class MainWindow;
    namespace Ui {
    class StageOne;
    }
    
    class StageOne : public QMainWindow
    {
        Q_OBJECT
    
    public:
        explicit StageOne(QString, QWidget *parent = nullptr);
        ~StageOne();
    
    
    private:
        Ui::StageOne *ui;
        MainWindow *window;
    };
    
    #endif // STAGEONE_H
    

    I do this at stageone:

    StageOne::StageOne(QString someLabel, QWidget *parent) :
        QMainWindow(parent),
        ui(new Ui::StageOne)
    {
        ui->setupUi(this);
    
    //    MainWindow *mainWindow= new MainWindow(this);
        connect(window,&MainWindow::patientIdChanged, ui->label, &QLabel::setText);  //
    
    }
    

    this gives an error says:

    stageone.cpp:12:21: error: incomplete type 'MainWindow' named in nested name specifier
    stageone.h:10:7: note: forward declaration of 'MainWindow'
    

  • Lifetime Qt Champion

    @russjohn834 said in Connecting two windows:

    #include "stagetwonew.h"

    Does this one include mainwindow.h?

    "stageone.cpp:12:21: error: incomplete type 'MainWindow' named in nested name specifier" - yes, because you did not include mainwindow.h in stageone.cpp
    You really messed up your includes...



  • @jsulm I have a doubt

    @jsulm said in Connecting two windows:

    @russjohn834 Just a tip: to avoid circular dependencies use forward declarations:

    // a.h
    class B; // This is forward declaration for class B, no need to include b.h here
    class A
    {
        private:
            B *b;
    };
    
    // b.h
    #include "a.h"
    ...
    

    in the above example, even if I forward declare class B , I may not able to use any methods in class B unless I include b.h?

    So I was thinking how this is useful in a case , for ex:

    class B contains a signal which needs to be connected in class A?


  • Lifetime Qt Champion

    Hi,

    In that case, the real question is: do you really need these classes to know each other ?


  • Lifetime Qt Champion

    @russjohn834 said in Connecting two windows:

    I may not able to use any methods in class B unless I include b.h?

    You can, in b.cpp if you include the header file there as I already said.
    Forward declaration only tells the compiler that B is a class, nothing more. This information is enough to declare a pointer to B (as pointer have always same size). But as soon as you want to access members/methods of B you need to include the header file, then compiler knows exactly what B is and which members/methods it has. See https://pvigier.github.io/2018/02/09/dependency-graph.html
    But as @SGaist said you should seriously rethink your design: are you sure both classes need to know each other? Usually it is enough that one class knows something about the other one. In Qt you can use signals/slots to implement loosely coupling.



  • Thank you @jsulm , @SGaist for your feedback


Log in to reply