[ fromfile: memberselect.xml id: memberselect ]

There are two forms of the member selection operator:

They look the same, but differ in the following ways.

When it is defined for a class, the unary operator->() should return a pointer to an object that has a member whose name is memberName.

An object that implements operator-> is typically called a smart pointer. Smart pointers are so called because they can be programmed to be “smarter” than ordinary pointers. For example, a QPointer is automatically set to 0 when the referenced object is destroyed. Examples of smart pointers include

All of these smart pointers are template classes.

Example 19.12 shows the class definition of QPointer.

[ . . . . ]
template <class T>
class QPointer
{
    QObject *o;
public:
    inline QPointer() : o(0) {}
    inline QPointer(T *p) : o(p)
        { QMetaObject::addGuard(&o); }
    inline QPointer(const QPointer<T> &p) : o(p.o)
        { QMetaObject::addGuard(&o); }
    inline ~QPointer()
        { QMetaObject::removeGuard(&o); }
    inline QPointer<T> &operator=(const QPointer<T> &p)
        { if (this != &p) QMetaObject::changeGuard(&o, p.o); return *this; }
    inline QPointer<T> &operator=(T* p)
        { if (o != p) QMetaObject::changeGuard(&o, p); return *this; }

    inline bool isNull() const
        { return !o; }

    inline T* operator->() const
        { return static_cast<T*>(const_cast<QObject*>(o)); }
[ . . . . ]

The operators makes use of two ANSI-style typecasts: (Section 19.7) static_cast and const_cast. const_cast is required because the function is const, and it must return a non-const pointer. RTTI is not necessary here because the template ensures that the actual pointer stored was a T*. Therefore, static_cast can be used instead of dynamic_cast.

A QPointer<T> is said to be a guarded pointer to a QObject of type T. By guarded we mean that the QPointer is automatically set to 0 if the object that it is pointing to is destroyed. Here is a code fragment that demonstrates how a QPointer can be used.

  [. . .]
  QPointer<QIntValidator> val = new QIntValidator(someParent);
  val->setRange(20, 60);
  [. . .]

In the second line of code, val-> returns a pointer to the newly allocated object, and it is used to access the setRange() member function.