QScopedPointer

The QScopedPointer class stores a pointer to a dynamically allocated object, and deletes it upon destruction. More...

 #include <QScopedPointer>

Inherited by: QScopedArrayPointer.

Note: All functions in this class are reentrant.

This class was introduced in Qt 4.6.

Public Functions

QScopedPointer ( T * p = 0 )
~QScopedPointer ()
T * data () const
bool isNull () const
void reset ( T * other = 0 )
void swap ( QScopedPointer<T, Cleanup> & other )
T * take ()
operator bool () const
bool operator! () const
T & operator* () const
T * operator-> () const

Detailed Description

The QScopedPointer class stores a pointer to a dynamically allocated object, and deletes it upon destruction.

Managing heap allocated objects manually is hard and error prone, with the common result that code leaks memory and is hard to maintain. QScopedPointer is a small utility class that heavily simplifies this by assigning stack-based memory ownership to heap allocations, more generally called resource acquisition is initialization(RAII).

QScopedPointer guarantees that the object pointed to will get deleted when the current scope disappears.

Consider this function which does heap allocations, and have various exit points:

 void myFunction(bool useSubClass)
 {
     MyClass *p = useSubClass ? new MyClass() : new MySubClass;
     QIODevice *device = handsOverOwnership();

     if (m_value > 3) {
         delete p;
         delete device;
         return;
     }

     try {
         process(device);
     }
     catch (...) {
         delete p;
         delete device;
         throw;
     }

     delete p;
     delete device;
 }

It's encumbered by the manual delete calls. With QScopedPointer, the code can be simplified to:

 void myFunction(bool useSubClass)
 {
     // assuming that MyClass has a virtual destructor
     QScopedPointer<MyClass> p(useSubClass ? new MyClass() : new MySubClass);
     QScopedPointer<QIODevice> device(handsOverOwnership());

     if (m_value > 3)
         return;

     process(device);
 }

The code the compiler generates for QScopedPointer is the same as when writing it manually. Code that makes use of delete are candidates for QScopedPointer usage (and if not, possibly another type of smart pointer such as QSharedPointer). QScopedPointer intentionally has no copy constructor or assignment operator, such that ownership and lifetime is clearly communicated.

The const qualification on a regular C++ pointer can also be expressed with a QScopedPointer:

     const QWidget *const p = new QWidget();
     // is equivalent to:
     const QScopedPointer<const QWidget> p(new QWidget());

     QWidget *const p = new QWidget();
     // is equivalent to:
     const QScopedPointer<QWidget> p(new QWidget());

     const QWidget *p = new QWidget();
     // is equivalent to:
     QScopedPointer<const QWidget> p(new QWidget());

Custom cleanup handlers

Arrays as well as pointers that have been allocated with malloc must not be deleted using delete. QScopedPointer's second template parameter can be used for custom cleanup handlers.

The following custom cleanup handlers exist:

  • QScopedPointerDeleter - the default, deletes the pointer using delete
  • QScopedPointerArrayDeleter - deletes the pointer using delete []. Use this handler for pointers that were allocated with new [].
  • QScopedPointerPodDeleter - deletes the pointer using free(). Use this handler for pointers that were allocated with malloc().

You can pass your own classes as handlers, provided that they have a public static function void cleanup(T *pointer).

 // this QScopedPointer deletes its data using the delete[] operator:
 QScopedPointer<int, QScopedPointerArrayDeleter<int> > arrayPointer(new int[42]);

 // this QScopedPointer frees its data using free():
 QScopedPointer<int, QScopedPointerPodDeleter> podPointer(reinterpret_cast<int *>(malloc(42)));

 // this struct calls "myCustomDeallocator" to delete the pointer
 struct ScopedPointerCustomDeleter
 {
     static inline void cleanup(MyCustomClass *pointer)
     {
         myCustomDeallocator(pointer);
     }
 };

 // QScopedPointer using a custom deleter:
 QScopedPointer<MyCustomClass, ScopedPointerCustomDeleter> customPointer(new MyCustomClass);

Forward Declared Pointers

Classes that are forward declared can be used within QScopedPointer, as long as the destructor of the forward declared class is available whenever a QScopedPointer needs to clean up.

Concretely, this means that all classes containing a QScopedPointer that points to a forward declared class must have non-inline constructors, destructors and assignment operators:

 class MyPrivateClass; // forward declare MyPrivateClass

 class MyClass
 {
 private:
     QScopedPointer<MyPrivateClass> privatePtr; // QScopedPointer to forward declared class

 public:
     MyClass(); // OK
     inline ~MyClass() {} // VIOLATION - Destructor must not be inline

 private:
     Q_DISABLE_COPY(MyClass) // OK - copy constructor and assignment operators
                              // are now disabled, so the compiler won't implicitely
                              // generate them.
 };

Otherwise, the compiler output a warning about not being able to destruct MyPrivateClass.

See also QSharedPointer.

Member Function Documentation

QScopedPointer::QScopedPointer ( T * p = 0 )

Constructs this QScopedPointer instance and sets its pointer to p.

QScopedPointer::~QScopedPointer ()

Destroys this QScopedPointer object. Delete the object its pointer points to.

T * QScopedPointer::data () const

Returns the value of the pointer referenced by this object. QScopedPointer still owns the object pointed to.

bool QScopedPointer::isNull () const

Returns true if this object is holding a pointer that is null.

void QScopedPointer::reset ( T * other = 0 )

Deletes the existing object it is pointing to if any, and sets its pointer to other. QScopedPointer now owns other and will delete it in its destructor.

void QScopedPointer::swap ( QScopedPointer<T, Cleanup> & other )

Swap this pointer with other.

T * QScopedPointer::take ()

Returns the value of the pointer referenced by this object. The pointer of this QScopedPointer object will be reset to null.

Callers of this function take ownership of the pointer.

QScopedPointer::operator bool () const

Returns true if this object is not null. This function is suitable for use in if-constructs, like:

 if (scopedPointer) {
     ...
 }

See also isNull().

bool QScopedPointer::operator! () const

Returns true if the pointer referenced by this object is null, otherwise returns false.

See also isNull().

T & QScopedPointer::operator* () const

Provides access to the scoped pointer's object.

If the contained pointer is null, behavior is undefined.

See also isNull().

T * QScopedPointer::operator-> () const

Provides access to the scoped pointer's object.

If the contained pointer is null, behavior is undefined.

See also isNull().

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