qpointer.qdoc

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// Copyright (C) 2016 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
 
/*!
    \class QPointer
    \inmodule QtCore
    \brief The QPointer class is a template class that provides guarded pointers to QObject.
 
    \ingroup objectmodel
 
    \compares equality
    \compareswith equality QPointer<X> X* std::nullptr_t
    Where X and T are compatible types, which means that they are either the same (except
    for their cv-qualifiers), or one is a base type of the other.
    \endcompareswith
 
    A guarded pointer, QPointer<T>, behaves like a normal C++
    pointer \c{T *}, except that it is automatically cleared when the
    referenced object is destroyed (unlike normal C++ pointers, which
    become "dangling pointers" in such cases). The template parameter
    \a T specifies the QObject-derived type being pointed to; \a 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.
 
    \list
 
    \li 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 \b NOT be cleared before the
    QWidget destructor destroys the children for the widget being tracked.
 
    \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 pointer/pointer.cpp 0
    \dots
    \snippet pointer/pointer.cpp 1
    \snippet pointer/pointer.cpp 2
 
    If the QLabel is deleted in the meantime, the \c 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 (\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 \nullptr 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<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 \c T must inherit QObject, or a compilation or
    link error will result.
 
    \sa QSharedPointer, QObject, QObjectCleanupHandler
*/
 
/*!
    \fn template <class T> QPointer<T>::QPointer()
    \fn template <class T> QPointer<T>::QPointer(std::nullptr_t)
 
    Constructs a guarded pointer with value \nullptr.
 
    \sa isNull()
*/
 
/*!
    \fn template <class T> QPointer<T>::QPointer(T* p)
 
    Constructs a guarded pointer that points to the same object that \a p
    points to.
*/
 
/*!
    \fn template <class T> QPointer<T>::~QPointer()
 
    Destroys the guarded pointer. Just like a normal pointer,
    destroying a guarded pointer does \e not destroy the object being
    pointed to.
*/
 
/*!
    \fn template <class T> template <typename X, QPointer<T>::if_convertible<X> = true> QPointer<T>::QPointer(QPointer<X> &&other)
    \fn template <class T> template <typename X, QPointer<T>::if_convertible<X> = true> QPointer<T>::QPointer(const QPointer<X> &other)
    \since 6.6
 
    Conversion constructor. Constructs a new QPointer by moving or copying from
    \a other.
 
    The moved-from QPointer is reset to nullptr.
 
    \note These constructors participate in overload resolution only if \c{X*}
    is convertible to \c{T*}.
*/
 
/*!
    \fn template <class T> template <typename X, QPointer<T>::if_convertible<X> = true> QPointer<T> &QPointer<T>::operator=(const QPointer<X> &other)
    \since 6.6
 
    Conversion assignment operator. Makes this guarded pointer guard the
    same object guarded by \a other.
 
    \constraints \c{X*} is convertible to \c{T*}.
*/
 
/*!
    \fn template <class T> template <typename X, QPointer<T>::if_convertible<X> = true> &QPointer<T>::operator=(QPointer<X> &&other)
    \since 6.6.1
 
    Conversion move-assignment operator. Makes this guarded pointer guard the
    same object guarded by \a other and resets \a other to nullptr.
 
    \constraints \c{X*} is convertible to \c{T*}.
*/
 
/*!
    \fn template <class T> void QPointer<T>::swap(QPointer &other)
    \since 5.6
    \memberswap{pointer}
*/
 
/*!
    \fn template <class T> QPointer<T> & QPointer<T>::operator=(T* p)
 
    Assignment operator. This guarded pointer will now point to the
    same object that \a p points to.
*/
 
/*!
    \fn template <class T> T* QPointer<T>::data() const
    \since 4.4
 
    Returns the pointer to the object being guarded.
*/
 
/*!
    \fn template <class T> T* QPointer<T>::get() const
    \since 6.0
 
    Same as data(). This function is provided for STL compatibility.
*/
 
/*!
    \fn template <class T> bool QPointer<T>::isNull() const
 
    Returns \c true if the referenced object has been destroyed or if
    there is no referenced object; otherwise returns \c false.
*/
 
/*!
    \fn template <class T> void QPointer<T>::clear()
    \since 5.0
 
    Clears this QPointer object.
 
    \sa isNull()
*/
 
/*!
    \fn template <class T> T* QPointer<T>::operator->() const
 
    Overloaded arrow operator; implements pointer semantics. Just use
    this operator as you would with a normal C++ pointer.
*/
 
/*!
    \fn template <class T> T& QPointer<T>::operator*() const
 
    Dereference operator; implements pointer semantics. Just use this
    operator as you would with a normal C++ pointer.
*/
 
/*!
    \fn template <class T> QPointer<T>::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.
*/
 
/*!
    \fn template <typename T> template<typename X> bool QPointer<T>::operator==(X* const &lhs, const QPointer<T> &rhs)
 
    Equality operator. Returns \c true if \a lhs and the guarded
    pointer \a rhs are pointing to the same object, otherwise
    returns \c false.
 
*/
/*!
    \fn template <typename T> template<typename X> bool QPointer<T>::operator==(const QPointer<T> &lhs, X* const &rhs)
 
    Equality operator. Returns \c true if \a rhs and the guarded
    pointer \a lhs are pointing to the same object, otherwise
    returns \c false.
 
*/
/*!
    \fn template <typename T> template<typename X> bool QPointer<T>::operator==(const QPointer<T> &lhs, const QPointer<X> &rhs)
 
    Equality operator. Returns \c true if the guarded pointers \a lhs and \a rhs
    are pointing to the same object, otherwise
    returns \c false.
 
*/
/*!
    \fn template <typename T> bool QPointer<T>::operator==(std::nullptr_t const &lhs, const QPointer<T> &rhs)
 
    Equality operator. Returns \c true if the pointer guarded by \a rhs
    is \nullptr, otherwise
    returns \c false.
*/
/*!
    \fn template <typename T> bool QPointer<T>::operator==(const QPointer<T> &lhs, std::nullptr_t const &rhs)
 
    Equality operator. Returns \c true if the pointer guarded by \a lhs
    is \nullptr, otherwise
    returns \c false.
*/
 
/*!
    \fn template <typename T> template<typename X> bool QPointer<T>::operator!=(const QPointer<T> &lhs, X* const &rhs)
 
    Inequality operator. Returns \c true if \a rhs and the guarded
    pointer \a lhs are not pointing to the same object, otherwise
    returns \c false.
*/
/*!
    \fn template <typename T> template<typename X> bool QPointer<T>::operator!=(X* const &lhs, const QPointer<T> &rhs)
 
    Inequality operator. Returns \c true if \a lhs and the guarded
    pointer \a rhs are not pointing to the same object, otherwise
    returns \c false.
*/
/*!
    \fn template <typename T> template<typename X> bool QPointer<T>::operator!=(const QPointer<T> &lhs, const QPointer<X> &rhs)
 
    Inequality operator. Returns \c true if  the guarded pointers \a lhs and
    \a rhs are not pointing to the same object, otherwise
    returns \c false.
*/
/*!
    \fn template <typename T> bool QPointer<T>::operator!=(std::nullptr_t const &lhs, const QPointer<T> &rhs)
 
    Inequality operator. Returns \c true if the pointer guarded by \a rhs is
    a valid (ie not \nullptr) pointer, otherwise
    returns \c false.
*/
/*!
    \fn template <typename T> bool QPointer<T>::operator!=(const QPointer<T> &lhs, std::nullptr_t const &rhs)
 
    Inequality operator. Returns \c true if the pointer guarded by \a lhs is
    a valid (ie not \nullptr) pointer, otherwise
    returns \c false.
*/
 
/*!
    \fn template <typename T> QPointer<T> qPointerFromVariant(const QVariant &variant)
 
    \internal
 
    Returns a guarded pointer that points to the same object that
    \a variant holds.
*/