programm:
```c++
#include <QCoreApplication>
#include <QObject>

class Counter : public QObject
{
  Q_OBJECT

public:
  Counter() { m_value = 0; }

  int value() const { return m_value; }

public slots:
  void setValue(int value);

signals:
  void valueChanged(int vewValue);

private:
  int m_value;
};

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 ca, cb;
  QObject::connect(&ca, &Counter::valueChanged, &cb, &Counter::setValue);

  return a.exec();
}

```

error:
```
$ make

g++ -Wl,-O1 -o New4 main.o   /usr/lib64/libQt5Core.so -lpthread
/usr/lib/gcc/x86_64-pc-linux-gnu/9.3.0/../../../../x86_64-pc-linux-gnu/bin/ld: main.o: in function `Counter::setValue(int)':
main.cpp:(.text+0x14): undefined reference to `Counter::valueChanged(int)'
/usr/lib/gcc/x86_64-pc-linux-gnu/9.3.0/../../../../x86_64-pc-linux-gnu/bin/ld: main.o: in function `main':
main.cpp:(.text.startup+0x58): undefined reference to `vtable for Counter'
/usr/lib/gcc/x86_64-pc-linux-gnu/9.3.0/../../../../x86_64-pc-linux-gnu/bin/ld: main.cpp:(.text.startup+0x90): undefined reference to `Counter::valueChanged(int)'
/usr/lib/gcc/x86_64-pc-linux-gnu/9.3.0/../../../../x86_64-pc-linux-gnu/bin/ld: main.cpp:(.text.startup+0x103): undefined reference to `Counter::staticMetaObject'
collect2: error: ld returned 1 exit status
mke: *** [Makefile:222: New4] Error 1

$
```