undefined reference to `Counter::valueChanged(int)' - qt

Question

i'm trying to learn qt.this is my first example that I'm practicing.but i have this error:C:\Qt2\Qt5.2.1\Tools\QtCreator\bin\recognize_signal_slot\main.cpp:19: error: undefined reference to `Counter::valueChanged(int)'
I don't know what I should do..someone told me you should put your class in header file.but I couldn't understand what he said.can anyone tell me step by step.thank you so much.
here is my code in main.cpp :
#include <QCoreApplication>
#include <QObject>
class Counter : public QObject
{
int m_value;
public:
int value() const { return m_value; }
public slots:
void setValue(int value);
signals:
void valueChanged(int newValue);
};
void Counter::setValue(int value)
{
if (value != m_value)
{
m_value = value;
emit valueChanged(value);
}
}
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
Counter d, b;
QObject::connect(&d, SIGNAL(valueChanged(int)),
&b, SLOT(setValue(int)));
d.setValue(12); // a.value() == 12, b.value() == 12
return a.exec();
}

Answer 1

[What's Wrong?]
You signals & slots are not invoked by Meta-Object-Compiler (MOC).
Suggested reading: Why Does Qt Use Moc for Signals and Slots?.
[Solution]
Step 1. Add Q_OBJECT macro to the QObject derivatives that use signals & slots.
class Counter : public QObject
{
Q_OBJECT // <-----HERE
int m_value;
public:
int value() const { return m_value; }
public slots:
void setValue(int value);
signals:
void valueChanged(int newValue);
};
Step 2. move your class declaration to counter.h and implementation to counter.cpp. Since MOC searches header files that contain Q_OBJECT, it's better to keep your QObject classes and main well separate, even for a small test project.
Step 3. Clean all ---> run qmake ---> rebuild (qmake will automatically call MOC to translate signals & slots syntax into compilable C++ code)