QPointer Class

The QPointer class is a template class that provides guarded pointers to QObject. More...

Header:	#include <QPointer>
qmake:	QT += core

• List of all members, including inherited members

Public Functions

	QPointer(T *p)
	QPointer()
	~QPointer()
void	clear()
T *	data() const
bool	isNull() const
void	swap(QPointer<T> &other)
T *	operator T *() const
T &	operator*() const
T *	operator->() const
QPointer<T> &	operator=(T *p)

Related Non-Members

bool	operator!=(const T *o, const QPointer<T> &p)
bool	operator!=(const QPointer<T> &p, const T *o)
bool	operator!=(T *o, const QPointer<T> &p)
bool	operator!=(const QPointer<T> &p, T *o)
bool	operator!=(const QPointer<T> &p1, const QPointer<T> &p2)
bool	operator==(const T *o, const QPointer<T> &p)
bool	operator==(const QPointer<T> &p, const T *o)
bool	operator==(T *o, const QPointer<T> &p)
bool	operator==(const QPointer<T> &p, T *o)
bool	operator==(const QPointer<T> &p1, const QPointer<T> &p2)

Detailed Description

A guarded pointer, QPointer<T>, behaves like a normal C++ pointer T *, except that it is automatically cleared when the referenced object is destroyed (unlike normal C++ pointers, which become "dangling pointers" in such cases). T must be a subclass of QObject.

Guarded pointers are useful whenever you need to store a pointer to a QObject that is owned by someone else, and therefore might be destroyed while you still hold a reference to it. You can safely test the pointer for validity.

Note that Qt 5 introduces a slight change in behavior when using QPointer.

• When using QPointer on a QWidget (or a subclass of QWidget), previously the QPointer would be cleared by the QWidget destructor. Now, the QPointer is cleared by the QObject destructor (since this is when QWeakPointer objects are cleared). Any QPointers tracking a widget will NOT be cleared before the QWidget destructor destroys the children for the widget being tracked.

Qt also provides QSharedPointer, an implementation of a reference-counted shared pointer object, which can be used to maintain a collection of references to an individual pointer.

Example:

    QPointer<QLabel> label = new QLabel;
    label->setText("&Status:");
    ...
    if (label)
        label->show();

If the QLabel is deleted in the meantime, the label variable will hold nullptr instead of an invalid address, and the last line will never be executed.

The functions and operators available with a QPointer are the same as those available with a normal unguarded pointer, except the pointer arithmetic operators (+, -, ++, and --), which are normally used only with arrays of objects.

Use QPointers like normal pointers and you will not need to read this class documentation.

For creating guarded pointers, you can construct or assign to them from a T* or from another guarded pointer of the same type. You can compare them with each other using operator==() and operator!=(), or test for nullptr with isNull(). You can dereference them using either the *x or the x->member notation.

A guarded pointer will automatically cast to a T *, so you can freely mix guarded and unguarded pointers. This means that if you have a QPointer<QWidget>, you can pass it to a function that requires a QWidget *. For this reason, it is of little value to declare functions to take a QPointer as a parameter; just use normal pointers. Use a QPointer when you are storing a pointer over time.

Note that class T must inherit QObject, or a compilation or link error will result.

See also QSharedPointer, QObject, and QObjectCleanupHandler.

Member Function Documentation

QPointer::QPointer(T *p)

Constructs a guarded pointer that points to the same object that p points to.

QPointer::QPointer()

Constructs a guarded pointer with value nullptr.

See also isNull().

QPointer::~QPointer()

Destroys the guarded pointer. Just like a normal pointer, destroying a guarded pointer does not destroy the object being pointed to.

void QPointer::clear()

Clears this QPointer object.

This function was introduced in Qt 5.0.

See also isNull().

T *QPointer::data() const

Returns the pointer to the object being guarded.

This function was introduced in Qt 4.4.

bool QPointer::isNull() const

Returns true if the referenced object has been destroyed or if there is no referenced object; otherwise returns false.

void QPointer::swap(QPointer<T> &other)

Swaps the contents of this QPointer with the contents of other. This operation is very fast and never fails.

This function was introduced in Qt 5.6.

T *QPointer::operator T *() const

Cast operator; implements pointer semantics. Because of this function you can pass a QPointer<T> to a function where a T* is required.

T &QPointer::operator*() const

Dereference operator; implements pointer semantics. Just use this operator as you would with a normal C++ pointer.

T *QPointer::operator->() const

Overloaded arrow operator; implements pointer semantics. Just use this operator as you would with a normal C++ pointer.

QPointer<T> &QPointer::operator=(T *p)

Assignment operator. This guarded pointer will now point to the same object that p points to.