qshareddata.cpp

qt-x11-opensource-src-4.1.4.tar.gz源码

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#include <qshareddata.h>

/*! \class QSharedData
    \brief The QSharedData class is a base class for shared data objects.

    \reentrant
    \ingroup misc

    QSharedData is designed to be used together with
    QSharedDataPointer to implement custom \l{implicitly shared}
    classes. It provides thread-safe reference counting.

    See the QSharedDataPointer documentation for details.
*/

/*! \fn QSharedData::QSharedData()

    Constructs a QSharedData object with a reference count of 0.
*/

/*! \fn QSharedData::QSharedData(const QSharedData &other)

    Constructs a QSharedData object with a reference count of 0. (\a
    other is ignored.)
*/

/*! \class QSharedDataPointer
    \brief The QSharedDataPointer class provides a pointer to a shared data object.

    \reentrant
    \ingroup misc
    \mainclass

    QSharedDataPointer\<T\> makes it easier to write your own
    implicitly shared classes. It handles reference counting behind
    the scenes in a thread-safe manner, ensuring that classes that use
    it can be \l{reentrant}.

    Implicit sharing is used throughout Qt to combine the memory and
    speed efficiency of pointers with the ease of use of value types.
    See the \l{Shared Classes} page for more information.

    Let's suppose that you want to make an \c Employee class
    implicitly shared. The procedure is:

    \list
    \i Define the \c Employee class with a single data member variable of
       type QSharedDataPointer<\c{EmployeeData}>.
    \i Define an \c EmployeeData class that derives from \l QSharedData
       and that contains all the variables that you would normally
       put in \c Employee.
    \endlist

    To show how this works in practice, we will review the entire
    source code for an implicitly shared \c Employee class. Here's the
    header file that defines the \c Employee class:

    \include snippets/sharedemployee/employee.h

    All accesses to the data in the setter and getter functions are
    made through the QSharedDataPointer object \c d. For non-const
    functions, operator->() automatically calls detach(), ensuring
    that modifications to one \c Employee object don't affect other
    \c Employee objects.

    In this example, the \c EmployeeData type is a simple class with a
    default constructor and a copy constructor provided by C++. If
    member-per-member copy isn't sufficient for your own data type,
    you must implement your own copy constructor.

    Let's now see how to implement the \c Employee constructors:

    \quotefile snippets/sharedemployee/employee.cpp

    \skipto ::Employee()
    \printuntil }

    In the default constructor, we create an object of type
    \c EmployeeData and assign it to the \c d pointer using operator=().

    \skipto ::Employee(int
    \printuntil }

    In the constructor that takes an ID and an employee's name, we
    also create an object of type \c EmployeeData and assign it to the
    \c d pointer.

    In this example, we don't need to provide a copy constructor, an
    assignment operator, or a destructor for \c Employee. The default
    implementations provided by C++, which invoke QSharedDataPointer's
    copy constructor, assignment operator, or destructor, are
    sufficient. And this is true in general, i.e. for any QSharedData
    subclass which only stores values (or implicitly shared classes),
    such as int, double, QString, QStringList, QList\<QWidget*\>, and
    so on.

    Behind the scenes, QSharedDataPointer automatically increments or
    decrements the reference count of the shared data object pointed
    to by \c d, and deletes shared objects when the reference count
    reaches 0.

    \sa QSharedData
*/

/*! \fn T &QSharedDataPointer::operator*()

    Provides access to the shared object's members.

    This function does a detach().
*/

/*! \fn const T &QSharedDataPointer::operator*() const

    \overload

    This function does not call detach().
*/

/*! \fn T *QSharedDataPointer::operator->()

    Provides access to the shared object's members.

    This function does a detach().
*/

/*! \fn const T *QSharedDataPointer::operator->() const

    \overload

    This function does not call detach().
*/

/*! \fn QSharedDataPointer::operator T *()

    Returns a pointer to the shared object.

    This function does a detach().

    \sa data(), constData()
*/

/*! \fn QSharedDataPointer::operator const T *() const

    Returns a pointer to the shared object.

    This function does not call detach().
*/

/*! \fn T * QSharedDataPointer::data()

    Returns a pointer to the shared object.

    This function does a detach().

    \sa constData()
*/

/*! \fn const T * QSharedDataPointer::data() const

    \overload

    This function does not call detach().
*/

/*! \fn const T * QSharedDataPointer::constData() const

    Returns a const pointer to the shared object.

    This function does not call detach().

    \sa data()
*/

/*! \fn bool QSharedDataPointer::operator==(const QSharedDataPointer<T> &other) const

    Returns a true if the pointer to the shared object in \a other is equal to
    to the pointer to the shared data in this else returns false.

    This function does not call detach().
*/

/*! \fn bool QSharedDataPointer::operator!=(const QSharedDataPointer<T> &other) const

    Returns a true if the pointer to the shared object in \a other is not equal to
    to the pointer to the shared data in this else returns false.

    This function does not call detach().
*/

/*! \fn QSharedDataPointer::QSharedDataPointer()

    Constructs a QSharedDataPointer initialized with a null pointer.
*/

/*! \fn QSharedDataPointer::~QSharedDataPointer()

    Destroys the QSharedDataPointer.

    This function automatically decrements the reference count of the
    shared object and deletes the object if the reference count
    reaches 0.
*/

/*! \fn QSharedDataPointer::QSharedDataPointer(T *sharedData)

    Constructs a QSharedDataPointer that points to \a sharedData.

    This function automatically increments \a{sharedData}'s reference
    count.
*/

/*! \fn QSharedDataPointer::QSharedDataPointer(const QSharedDataPointer<T> &other)

    Constructs a copy of \a other.

    This function automatically increments the reference count of the
    shared data object pointed to by \a{other}.
*/

/*! \fn QSharedDataPointer<T> &QSharedDataPointer::operator=(const QSharedDataPointer<T> &other)

    Assigns \a other to this pointer.

    This function automatically increments the reference count of the
    shared data object pointed to by \a{other}, and decrements the
    reference count of the object previously pointed to by this
    QSharedDataPointer. If the reference count reaches 0, the shared
    data object is deleted.
*/

/*! \fn QSharedDataPointer &QSharedDataPointer::operator=(T *sharedData)

    \overload

    Sets this QSharedDataPointer to point to \a sharedData.

    This function automatically increments \a{sharedData}'s reference
    count, and decrements the reference count of the object
    previously pointed to by this QSharedDataPointer. If the
    reference count reaches 0, the shared data object is deleted.
*/

/*! \fn bool QSharedDataPointer::operator!() const

    Returns true if this pointer is null; otherwise returns false.
*/

/*! \fn void QSharedDataPointer::detach()

    If the shared data's reference count is greater than 1, creates a
    deep copy of the shared data.

    This function is automatically called by QSharedDataPointer when
    necessary. You should never need to call it yourself.
*/