@russjohn834 said in error building a window application:

class ClinicalProtocolWindow : public QMainWindow

I wouldn't make ClinicalProtocolWindow a QMainWindow-based class unless you really need the toolbars and menus, which come with it.
QWidget or even QDialog is better, esp. since you have Qt::WA_DeleteOnClose set.
For example, create your dialog, fill in/change whatever you want to edit and return to your mainWindow again.
(I assume it's gonna be some patient form)

@russjohn834 said in error building a window application:

this->close();
clinwindow = new ClinicalProtocolWindow(pLabel, nullptr);
clinwindow -> setAttribute(Qt::WA_DeleteOnClose);
clinwindow -> show();

This could easily run into QApplication:quitOnLastWindowClosed when there are no other windows or parent widgets involved and then your app just exits without showing anything.

• https://doc.qt.io/qt-6/qguiapplication.html#quitOnLastWindowClosed-prop

Make sure you have a parent window or change the order of close() / hide() and show()

class ClinicalProtocolWindow;

and removed #include "clinicalprotocolwindow.h". Which will make it compilable.

@russjohn834 no line 33.

I modified the original .h file to show only what I thought would be relevant.
It looks like something silly I'm doing!
What is strange is , I have other windows works fine. The error is only when I added the new ClinicalProtocolWindow

#ifndef STAGEONEMAIN_H
#define STAGEONEMAIN_H

#include <QMainWindow>
#include "clinicalprotocolwindow.h"

and

#ifndef CLINICALPROTOCOLWINDOW_H
#define CLINICALPROTOCOLWINDOW_H

#include <QMainWindow>
#include "stageonemain.h"

Follow through in your head/on a piece of paper what happens. You will find that when

 ClinicalProtocolWindow *clinwindow;

in StageOneMain.h is reached ClinicalProtocolWindow class is not defined. Hence the error messages.

Mutual inclusion like this is often a sign a bad design. I suspect it is here. Why should both of these classes need to know al about each other just because you have two windows opening?

Find a way to get round this. Just having StageOneMain.h go class ClinicalProtocolWindow; like that may be enough to break to the dependency/error. Or as it currently stands ClinicalProtocolWindow has no need to include StageOneMain.h.

#ifndef STAGEONEMAIN_H
#define STAGEONEMAIN_H

#include <QMainWindow>

QT_BEGIN_NAMESPACE
namespace Ui { class StageOneMain; }
QT_END_NAMESPACE

class ClinicalProtocolWindow;

class StageOneMain : public QMainWindow
{
    Q_OBJECT

public:
     StageOneMain(QString, QWidget *parent = nullptr);
    ~StageOneMain();

private:
    Ui::StageOneMain *ui{};     

    ClinicalProtocolWindow *clinwindow{};
    QString pLabel;
};
#endif // STAGEONEMAIN_H