Qt pure virtual function error

Question

I'm tring to create an interface with Qt to oblige any subclass to implements two main methods set and get title. But when I try to compile it I get a weird error message that says something about qt_check_for_QOBJECT_macro and staticMetaObject. In mainwindow.cpp I have to cast any page to the interface so I can rely on getter and setter methods. I don't see any other way to do that.

this is my code:

//IPage.h
#ifndef IPAGE_H
#define IPAGE_H

#include <QString>

class IPage
{
public:
 virtual QString title()=0;
 virtual void setTitle(QString t)=0;
};

#endif // IPAGE_H


//buildings.h:
#ifndef BUILDINGS_H
#define BUILDINGS_H

#include "IPage.h"
#include <QDialog>

class Buildings : public IPage, public QDialog
{
   Q_OBJECT
 private:
   QString m_title;
 //stuff...
};
#endif
//buildings.cpp
//stuff...

void Buildings::setTitle(QString t)
{
   m_title = t;
   setWindowTitle(t);
}

QString Buildings::title()
{
   return m_title;
}

//mainwindow.cpp:
QMdiSubWindow *MainWindow::findChild(const QString &title)
{
    foreach (QMdiSubWindow *window, mdiArea->subWindowList()) {
        IPage *child = qobject_cast<IPage *>(window->widget()); /*line 178*/
        if (child->title() == title)
            return window;
    }
    return 0;
}

and I get this error message when I compile my code:

In file included from c:\QtSDK\Desktop\Qt\4.8.1\mingw\include/QtCore/qabstractanimation.h:45,
             from c:\QtSDK\Desktop\Qt\4.8.1\mingw\include/QtCore/QtCore:3,
             from c:\QtSDK\Desktop\Qt\4.8.1\mingw\include\QtGui/QtGui:3,
             from mainwindow.cpp:1:
c:\QtSDK\Desktop\Qt\4.8.1\mingw\include/QtCore/qobject.h: In function 'T qobject_cast(QObject*) [with T = IPage*]':
mainwindow.cpp:178:   instantiated from here
c:\QtSDK\Desktop\Qt\4.8.1\mingw\include/QtCore/qobject.h:378: error: 'class IPage' has no member named 'qt_check_for_QOBJECT_macro'
c:\QtSDK\Desktop\Qt\4.8.1\mingw\include/QtCore/qobject.h:380: error: 'class IPage' has no member named 'staticMetaObject'
mingw32-make.exe[1]: Leaving directory `D:/Dropbox/Programmi/Qt/Scadenziario'
mingw32-make.exe[1]: *** [build/o/mainwindow.o] Error 1
mingw32-make.exe: *** [debug] Error 2
01:23:26: The process "C:\QtSDK\mingw\bin\mingw32-make.exe" exited with code 2.
Error while building project scadenziario (target: Desktop)
When executing build step 'Make'

I can't understand error message. I tried to google it, but I can't find any useful informations. Any help will be appreciated, thanks in advance.

Answer 1

The meta object compiler requires that the first class you inherit from be the QObject-derived class.

So you should change:

class Buildings : public IPage, public QDialog

to:

class Buildings : public QDialog, public IPage

Answer 2

When you use qobject_cast<T *>, T has to inherit from QObject. In your case, T = IPage but IPage does not inherit QObject. That's why you get the error.