error: variable 'QQmlComponent component' has initializer but incomplete type in Qt5 - qt

Question

i am playing with Exposing Attributes of C++ Types to QML in Qt5 based on this tutor http://qt-project.org/doc/qt-5.0/qtqml/qtqml-cppintegration-exposecppattributes.html. when i run it i got this error on my issues pane error: variable 'QQmlComponent component' has initializer but incomplete type not only i have this error i have also this error the signal i have created using Q_PROPERTY is not detected
C:\Users\Tekme\Documents\QtStuf\quick\QmlCpp\message.h:15: error: 'authorChanged' was not declared in this scope
emit authorChanged();
^
my code is
#ifndef MESSAGE_H
#define MESSAGE_H
#include <QObject>
class Message : public QObject
{
Q_OBJECT
Q_PROPERTY(QString author READ author WRITE setAuthor NOTIFY authorChanged)
public:
void setAuthor(const QString &a) {
if (a != m_author) {
m_author = a;
emit authorChanged();
}
}
QString author() const {
return m_author;
}
private:
QString m_author;
};
#endif
and in my main.cpp
#include "message.h"
#include <QApplication>
#include <QQmlEngine>
#include <QQmlContext>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
QQmlEngine engine;
Message msg;
engine.rootContext()->setContextProperty("msg",&msg);
QQmlComponent component(&engine, QUrl::fromLocalFile("main.qml"));
component.create();
return a.exec();
}

Answer 1

You are not including QQmlComponent header in your main.cpp:
#include <QQmlComponent>
You are also trying to emit a signal that you haven't declared yet. You should declare it in your message.h like this:
signals:
void authorChanged();
Check this example.

Answer 2

I believe you need to add:
signals:
void authorChanged();
to your class like this:
class Message : public QObject
{
Q_OBJECT
Q_PROPERTY(QString author READ author WRITE setAuthor NOTIFY authorChanged)
public:
void setAuthor(const QString &a) {
if (a != m_author) {
m_author = a;
emit authorChanged();
}
}
QString author() const {
return m_author;
}
signals:
void authorChanged();
private:
QString m_author;
};