Connecting two windows

russjohn834

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

jhx76

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);  
}

Pablo J. Rogina

@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.

jhx76

That's right :)

was just answering to the original question :

@russjohn834

What I'm doing wrong here?

have a nice day

jsulm

@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.

russjohn834

@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

jsulm

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

russjohn834

@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

jsulm

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

russjohn834

@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

russjohn834

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'

jsulm

@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...

russjohn834

@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?

SGaist

Hi,

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

jsulm

@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.

russjohn834

Thank you @jsulm , @SGaist for your feedback