Question

I was wondering if someone can explain this macro to me.

#define Q_DECLARE_PRIVATE(Class) \

inline Class##Private* d_func() { return reinterpret_cast<Class##Private
*>(d_ptr); } \

inline const Class##Private* d_func() const { return reinterpret_cast<const
Class##Private *>(d_ptr); } \

friend class Class##Private;

I have an application that uses QT and it crashes with an access violation exception on the last line from the following snippet.

class Q_GUI_EXPORT QWidget : public QObject, public QPaintDevice
{
    Q_OBJECT
    Q_DECLARE_PRIVATE(QWidget)

(full source here)

and what can cause a null pointer operation in on the last line?

Answer 1

The compiler can explain it best of all:

g++ -E foo.cc

will put foo.cc to stdout after passing through the pre-processor. It lets you see what the compiler sees which (especially with token pasting) is usually less than obvious.