Q_declare_private()

[marf]

I'm just looking at some example QT code, and I notice that someties in the header file there will be something like so:

class MyClassPrivate;
 
class MyClass : public QObject
{
  public:
    MyClass( QObject *parent = 0 );
    ...other non relevant stuff...
  protected:
    MyClassPrivate *const ptr;
    MyClass( MyClassPrivate &the_ptr, QObject *parent = 0 );
  private:
    Q_DECLARE_PRIVATE( MyClass );
};

class MyClassPrivate;

class MyClass : public QObject
{
  public:
    MyClass( QObject *parent = 0 );
    ...other non relevant stuff...
  protected:
    MyClassPrivate *const ptr;
    MyClass( MyClassPrivate &the_ptr, QObject *parent = 0 );
  private:
    Q_DECLARE_PRIVATE( MyClass );
};

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I'm just curious what does this "macro" do? Does it offer the ability to have a private version of the class? Why do they want a separate Protected Constructor? Can somebody give me an example why it is useful?

Thanks,

Michael