Qt5 error: variable QQmlComponent component has initializer but incomplete

Question

i have one problem with the example of Qt site, the message error compile is:

this is error:

error: variable 'QQmlComponent component' has initializer but incomplete type
     QQmlComponent component(&engine, QUrl::fromLocalFile("qml/untitled3/main.qml"));
                            ^

My code is similar at this post and with the correction in this post but not work, i am new using qt 5.1... any idea?.

I try add the library #include <QQmlComponent> or #include <QtQml/QQmlContext> but the message error is same.

the path from my main.qml is correct: qml/untitled3/main.qml

Please help me.

untitled3.pro

# Add more folders to ship with the application, here
folder_01.source = qml/untitled3
folder_01.target = qml
DEPLOYMENTFOLDERS = folder_01

# Additional import path used to resolve QML modules in Creator's code model
QML_IMPORT_PATH =

# If your application uses the Qt Mobility libraries, uncomment the following
# lines and add the respective components to the MOBILITY variable.
# CONFIG += mobility
# MOBILITY +=

# The .cpp file which was generated for your project. Feel free to hack it.
SOURCES += main.cpp

# Installation path
# target.path =

# Please do not modify the following two lines. Required for deployment.
include(qtquick2applicationviewer/qtquick2applicationviewer.pri)
qtcAddDeployment()

HEADERS += \
    message.h

message.h

#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;
    }
signals:
    void authorChanged();
private:
    QString m_author;
};
#endif

main.cpp

#include <QtGui/QGuiApplication>
#include <QtQml/QQmlContext>
#include <QQmlEngine>
#include <QQmlComponent>
#include "message.h"


int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);

    QQmlEngine *engine = new QQmlEngine;
    Message msg;

    engine->rootContext()->setContextProperty("msg", &msg);
    QQmlComponent component(&engine, QUrl::fromLocalFile("qml/untitled3/main.qml"));
    component.create();



    return app.exec();
}