path: root/src/corelib/kernel/qpointer.cpp

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/*!
    \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<T>, 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<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 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<T> &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<T>& QPointer::operator=(const QPointer<T> &p)

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

/*!
    \fn QPointer<T> & 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\<T\> to a function where a T*
    is required.
*/

/*!
    \fn bool operator==(const T *o, const QPointer<T> &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<T> &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<T> &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<T> &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<T> &p1, const QPointer<T> &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<T> &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<T> &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<T> &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<T> &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<T> &p1, const QPointer<T> &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.
*/