Q_Object in class inherited by QwtPlot - Undefined Reference

Hi to all,

I have some very strange problems.

I would like to add the Q_OBJECT macro to a class called Plot, inherited by QwtPlot.

I did everything in the right way and if I replace QwtPlot by QObject it works.

If I inherit by QwtPlot I get an "Undefined reference" error by the linker, it complains about missing QMetaObject const QwtPlot::staticMetaObject ...

The .pro file is:

Code:

---

#-------------------------------------------------
#
# Project created by QtCreator 2017-03-08T16:00:42
#
#-------------------------------------------------
 
QT       += core gui
 
greaterThan(QT_MAJOR_VERSION, 4): QT += widgets
 
TARGET = Test
TEMPLATE = app
 
 
SOURCES += main.cpp\
        mainwindow.cpp \
    plot.cpp
 
HEADERS  += mainwindow.h \
    plot.h
 
FORMS    += mainwindow.ui
 
INCLUDEPATH += C:\Qwt-6.1.3\include
LIBS += -LC:\Qwt-6.1.3\lib\ -lqwt

---

The plot.h is:

Code:

---

#ifndef PLOT_H
#define PLOT_H
 
#include <QObject>
#include <qwt_plot.h>
 
class Plot: public QwtPlot
{
    Q_OBJECT
public:
    Plot(QWidget *parent);
};
 
#endif // PLOT_H

---

Funny thing is that they do it in pretty much the same way in the examples of Qwt.

Does anyone know how to solve this problem, please? :(
Thanks a lot.

Quote:

---

LIBS += -LC:\Qwt-6.1.3\lib\ -lqwt

---

Are you trying to link a debug mode app with a release mode Qwt or vice-versa? When you derive from QObject, qmake will link everything to the right Qt libraries depending on whether you are building in debug or release mode, but I am not sure what happens in the case of linking to Qwt. It may depend on whether you have built Qwt with the right options.

After many hours I found out that I need to append

Code:

---

DEFINES += QWT_DLL

---

in my project file.

I don't know why I have to do this but it works.

Thanks. :)