Question

What is the difference between these two types of pointers? As far as I can read, QSharedPointer can handle situation well, so what is the need for QSharedDataPointer?

Answer 1

From Qt documentation
QSharedDataPointer

The QSharedDataPointer class
represents a pointer to an implicitly
shared object. QSharedDataPointer
makes writing your own implicitly
shared classes easy.
QSharedDataPointer implements
thread-safe reference counting,
ensuring that adding
QSharedDataPointers to your reentrant
classes won’t make them non-reentrant.
Implicit sharing is used by many Qt
classes to combine the speed and
memory efficiency of pointers with the
ease of use of classes. See the Shared
Classes page for more information.

Example usage –

 #include <QSharedData>
 #include <QString>

 class EmployeeData : public QSharedData
 {
   public:
     EmployeeData() : id(-1) { }
     EmployeeData(const EmployeeData &other)
         : QSharedData(other), id(other.id), name(other.name) { }
     ~EmployeeData() { }

For QSharedPointer

The QSharedPointer class holds a
strong reference to a shared pointer
The QSharedPointer is an automatic,
shared pointer in C++. It behaves
exactly like a normal pointer for
normal purposes, including respect for
constness. QSharedPointer will delete
the pointer it is holding when it goes
out of scope, provided no other
QSharedPointer objects are referencing
it.

>  QSharedPointer<MyObject> obj =
>          QSharedPointer<MyObject>(new MyObject);

So, the QSharedDataPointer is used to make creating implicititly shared classes. Whereas QSharedPointer is a reference counting Smart pointer that points to classes.

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EDIT

When reading Memory management in Qt?, I found this link http://blog.qt.io/blog/2009/08/25/count-with-me-how-many-smart-pointer-classes-does-qt-have/. A really excellent discussion of the different smart pointers Qt has (current API has 8).