Connecting two windows

[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