/**************************************************************************** ** ** Copyright (C) 2012 Nokia Corporation and/or its subsidiary(-ies). ** Contact: http://www.qt-project.org/ ** ** This file is part of the QtCore module of the Qt Toolkit. ** ** $QT_BEGIN_LICENSE:LGPL$ ** GNU Lesser General Public License Usage ** This file may be used under the terms of the GNU Lesser General Public ** License version 2.1 as published by the Free Software Foundation and ** appearing in the file LICENSE.LGPL included in the packaging of this ** file. Please review the following information to ensure the GNU Lesser ** General Public License version 2.1 requirements will be met: ** http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html. ** ** In addition, as a special exception, Nokia gives you certain additional ** rights. These rights are described in the Nokia Qt LGPL Exception ** version 1.1, included in the file LGPL_EXCEPTION.txt in this package. ** ** GNU General Public License Usage ** Alternatively, this file may be used under the terms of the GNU General ** Public License version 3.0 as published by the Free Software Foundation ** and appearing in the file LICENSE.GPL included in the packaging of this ** file. Please review the following information to ensure the GNU General ** Public License version 3.0 requirements will be met: ** http://www.gnu.org/copyleft/gpl.html. ** ** Other Usage ** Alternatively, this file may be used in accordance with the terms and ** conditions contained in a signed written agreement between you and Nokia. ** ** ** ** ** ** ** $QT_END_LICENSE$ ** ****************************************************************************/ /*! \class QPointer \brief The QPointer class is a template class that provides guarded pointers to QObject. \ingroup objectmodel \obsolete Use QWeakPointer instead. A guarded pointer, QPointer, behaves like a normal C++ pointer \c{T *}, except that it is automatically set to 0 when the referenced object is destroyed (unlike normal C++ pointers, which become "dangling pointers" in such cases). \c 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 two slight changes in behavior when using QPointer. \list \i 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 QWeakPointers are cleared). Any QPointers tracking a widget will \b NOT be cleared before the QWidget destructor destroys the children for the widget being tracked. \i When constructing a QSharedPointer to take ownership of an object after a QPointer is already tracking the object. Previously, the shared pointer construction would not be affected by the QPointer, but now that QPointer is implemented using QWeakPoiner, constructing the QSharedPointer will cause an \c abort(). \endlist 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: \snippet doc/src/snippets/pointer/pointer.cpp 0 \dots \snippet doc/src/snippets/pointer/pointer.cpp 1 \snippet doc/src/snippets/pointer/pointer.cpp 2 If the QLabel is deleted in the meantime, the \c label variable will hold 0 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 (\c{+}, \c{-}, \c{++}, and \c{--}), 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 0 with isNull(). You can dereference them using either the \c *x or the \c x->member notation. A guarded pointer will automatically cast to a \c T *, so you can freely mix guarded and unguarded pointers. This means that if you have a QPointer, 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 \c T must inherit QObject, or a compilation or link error will result. \sa QSharedPointer, QObject, QObjectCleanupHandler */ /*! \fn QPointer::QPointer() Constructs a 0 guarded pointer. \sa isNull() */ /*! \fn QPointer::QPointer(T* p) Constructs a guarded pointer that points to same object that \a p points to. */ /*! \fn QPointer::QPointer(const QPointer &p) Copies one guarded pointer from another. The constructed guarded pointer points to the same object that \a p points to (which may be 0). */ /*! \fn QPointer::~QPointer() Destroys the guarded pointer. Just like a normal pointer, destroying a guarded pointer does \e not destroy the object being pointed to. */ /*! \fn QPointer& QPointer::operator=(const QPointer &p) Assignment operator. This guarded pointer will now point to the same object that \a p points to. */ /*! \fn QPointer & QPointer::operator=(T* p) Assignment operator. This guarded pointer will now point to the same object that \a p points to. */ /*! \fn T* QPointer::data() const \since 4.4 Returns the pointer to the object being guarded. */ /*! \fn bool QPointer::isNull() const Returns \c true if the referenced object has been destroyed or if there is no referenced object; otherwise returns false. */ /*! \fn T* QPointer::operator->() const Overloaded arrow operator; implements pointer semantics. Just use this operator as you would with a normal C++ pointer. */ /*! \fn T& QPointer::operator*() const Dereference operator; implements pointer semantics. Just use this operator as you would with a normal C++ pointer. */ /*! \fn QPointer::operator T*() const Cast operator; implements pointer semantics. Because of this function you can pass a QPointer\ to a function where a T* is required. */ /*! \fn bool operator==(const T *o, const QPointer &p) \relates QPointer Equality operator. Returns true if \a o and the guarded pointer \a p are pointing to the same object, otherwise returns false. */ /*! \fn bool operator==(const QPointer &p, const T *o) \relates QPointer Equality operator. Returns true if \a o and the guarded pointer \a p are pointing to the same object, otherwise returns false. */ /*! \fn bool operator==(T *o, const QPointer &p) \relates QPointer Equality operator. Returns true if \a o and the guarded pointer \a p are pointing to the same object, otherwise returns false. */ /*! \fn bool operator==(const QPointer &p, T *o) \relates QPointer Equality operator. Returns true if \a o and the guarded pointer \a p are pointing to the same object, otherwise returns false. */ /*! \fn bool operator==(const QPointer &p1, const QPointer &p2) \relates QPointer Equality operator. Returns true if the guarded pointers \a p1 and \a p2 are pointing to the same object, otherwise returns false. */ /*! \fn bool operator!=(const T *o, const QPointer &p) \relates QPointer Inequality operator. Returns true if \a o and the guarded pointer \a p are not pointing to the same object, otherwise returns false. */ /*! \fn bool operator!=(const QPointer &p, const T *o) \relates QPointer Inequality operator. Returns true if \a o and the guarded pointer \a p are not pointing to the same object, otherwise returns false. */ /*! \fn bool operator!=(T *o, const QPointer &p) \relates QPointer Inequality operator. Returns true if \a o and the guarded pointer \a p are not pointing to the same object, otherwise returns false. */ /*! \fn bool operator!=(const QPointer &p, T *o) \relates QPointer Inequality operator. Returns true if \a o and the guarded pointer \a p are not pointing to the same object, otherwise returns false. */ /*! \fn bool operator!=(const QPointer &p1, const QPointer &p2) \relates QPointer Inequality operator. Returns true if the guarded pointers \a p1 and \a p2 are not pointing to the same object, otherwise returns false. */