Qt "private slots:" what is this?

Question

I understand how to use it, but the syntax of it bothers me. What is "private slots:" doing?

I have never seen something between the private keyword and the : in a class definition before. Is there some fancy C++ magic going on here?

And example here:

 #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);

 ...

Answer 1

Slots are a Qt-specific extension of C++. It only compiles after sending the code through Qt's preprocessor, the Meta-Object Compiler (moc). See http://doc.qt.io/qt-5/moc.html for documentation.

Edit: As Frank points out, moc is only required for linking. The extra keywords are #defined away with the standard preprocessor.