Linker errors creating QLabel with doubleclick event - c++

Question

I'm creating a QLabel subclass which adds the DoubleClickEvent to it. I have created the following, but I'm getting some strange linker errors, maybe someone can point out what I've done wrong?
//Header
#ifndef IMAGE_LABEL_H
#define IMAGE_LABEL_H
#include <QLabel>
#include <QMouseEvent>
class image_label : public QLabel
{
Q_OBJECT
public:
image_label(QWidget* parent = 0);
~image_label();
signals:
void doubleClicked();
protected:
void mouseDoubleClickEvent(QMouseEvent * e);
};
#endif
//CPP
#include "image_label.h"
#include <QMouseEvent>
image_label::image_label(QWidget* parent) : QLabel(parent)
{
}
image_label::~image_label()
{
}
void image_label::mouseDoubleClickEvent(QMouseEvent* e)
{
if (e->button() == Qt::LeftButton)
{
emit doubleClicked();
QLabel::mouseDoubleClickEvent(e);
}
}
I get the following linker errors when I compile:
image_label.obj : error LNK2001: unresolved external symbol "public: virtual struct QMetaObject const * __thiscall image_label::metaObject(void)const " (?metaObject#image_label##UBEPBUQMetaObject##XZ)
image_label.obj : error LNK2001: unresolved external symbol "public: virtual void * __thiscall image_label::qt_metacast(char const *)" (?qt_metacast#image_label##UAEPAXPBD#Z)
image_label.obj : error LNK2001: unresolved external symbol "public: virtual int __thiscall image_label::qt_metacall(enum QMetaObject::Call,int,void * *)" (?qt_metacall#image_label##UAEHW4Call#QMetaObject##HPAPAX#Z)
image_label.obj : error LNK2019: unresolved external symbol "protected: void __thiscall image_label::doubleClicked(void)" (?doubleClicked#image_label##IAEXXZ) referenced in function "protected: virtual void __thiscall image_label::mouseDoubleClickEvent(class QMouseEvent *)" (?mouseDoubleClickEvent#image_label##MAEXPAVQMouseEvent###Z)
Can anybody help why I get these errors?

Answer 1

You must run the MOC preprocessor on the file image_label.h. This generates a file moc_image_label.cppthat you must include in the build. The error message indicates that you have not done this. (The symbols image_label::metaObject etc. that are mentioned in the error message are defined in moc_image_label.cpp.)