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/****************************************************************************
**
** Copyright (C) 2013 Digia Plc and/or its subsidiary(-ies).
** Contact: http://www.qt-project.org/legal
**
** This file is part of the QtCore module of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:LGPL$
** Commercial License Usage
** Licensees holding valid commercial Qt licenses may use this file in
** accordance with the commercial license agreement provided with the
** Software or, alternatively, in accordance with the terms contained in
** a written agreement between you and Digia. For licensing terms and
** conditions see http://qt.digia.com/licensing. For further information
** use the contact form at http://qt.digia.com/contact-us.
**
** GNU Lesser General Public License Usage
** Alternatively, 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, Digia gives you certain additional
** rights. These rights are described in the Digia 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.
**
**
** $QT_END_LICENSE$
**
****************************************************************************/
#include <qshareddata.h>
QT_BEGIN_NAMESPACE
/*!
\class QSharedData
\brief The QSharedData class is a base class for shared data objects.
\reentrant
QSharedData is designed to be used with QSharedDataPointer or
QExplicitlySharedDataPointer to implement custom \l{implicitly
shared} or explicitly shared classes. QSharedData provides
\l{thread-safe} reference counting.
See QSharedDataPointer and QExplicitlySharedDataPointer 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 reference count 0.
\a other is ignored.
*/
/*!
\class QSharedDataPointer
\brief The QSharedDataPointer class represents a pointer to an implicitly shared object.
\since 4.0
\reentrant
QSharedDataPointer\<T\> makes writing your own \l {implicitly
shared} classes easy. QSharedDataPointer implements \l {thread-safe}
reference counting, ensuring that adding QSharedDataPointers to your
\l {reentrant} classes won't make them non-reentrant.
\l {Implicit sharing} is used by many Qt classes to combine the
speed and memory efficiency of pointers with the ease of use of
classes. See the \l{Shared Classes} page for more information.
\target Employee example
Suppose you want to make an \c Employee class implicitly shared. The
procedure is:
\list
\o Define the class \c Employee to have a single data member of
type \c {QSharedDataPointer<EmployeeData>}.
\o Define the \c EmployeeData class derived from \l QSharedData to
contain all the data members you would normally have put in the
\c Employee class.
\endlist
To show this in practice, we review the source code for the
implicitly shared \c Employee class. In the header file we define the
two classes \c Employee and \c EmployeeData.
\snippet doc/src/snippets/sharedemployee/employee.h 0
In class \c Employee, note the single data member, a \e {d pointer}
of type \c {QSharedDataPointer<EmployeeData>}. All accesses of
employee data must go through the \e {d pointer's} \c
{operator->()}. For write accesses, \c {operator->()} will
automatically call detach(), which creates a copy of the shared data
object if the shared data object's reference count is greater than
1. This ensures that writes to one \c Employee object don't affect
any other \c Employee objects that share the same \c EmployeeData
object.
Class \c EmployeeData inherits QSharedData, which provides the
\e{behind the scenes} reference counter. \c EmployeeData has a default
constructor, a copy constructor, and a destructor. Normally, trivial
implementations of these are all that is needed in the \e {data}
class for an implicitly shared class.
Implementing the two constructors for class \c Employee is also
straightforward. Both create a new instance of \c EmployeeData
and assign it to the \e{d pointer} .
\snippet doc/src/snippets/sharedemployee/employee.h 1
\codeline
\snippet doc/src/snippets/sharedemployee/employee.h 2
Note that class \c Employee also has a trivial copy constructor
defined, which is not strictly required in this case.
\snippet doc/src/snippets/sharedemployee/employee.h 7
The copy constructor is not strictly required here, because class \c
EmployeeData is included in the same file as class \c Employee
(\c{employee.h}). However, including the private subclass of
QSharedData in the same file as the public class containing the
QSharedDataPointer is not typical. Normally, the idea is to hide the
private subclass of QSharedData from the user by putting it in a
separate file which would not be included in the public file. In
this case, we would normally put class \c EmployeeData in a separate
file, which would \e{not} be included in \c{employee.h}. Instead, we
would just predeclare the private subclass \c EmployeeData in \c
{employee.h} this way:
\code
class EmployeeData;
\endcode
If we had done it that way here, the copy constructor shown would be
required. Since the copy constructor is trivial, you might as well
just always include it.
Behind the scenes, QSharedDataPointer automatically increments the
reference count whenever an \c Employee object is copied, assigned,
or passed as a parameter. It decrements the reference count whenever
an \c Employee object is deleted or goes out of scope. The shared
\c EmployeeData object is deleted automatically if and when the
reference count reaches 0.
In a non-const member function of \c Employee, whenever the \e {d
pointer} is dereferenced, QSharedDataPointer automatically calls
detach() to ensure that the function operates on its own copy of the
data.
\snippet doc/src/snippets/sharedemployee/employee.h 3
\codeline
\snippet doc/src/snippets/sharedemployee/employee.h 4
Note that if detach() is called more than once in a member function
due to multiple dereferences of the \e {d pointer}, detach() will
only create a copy of the shared data the first time it is called,
if at all, because on the second and subsequent calls of detach(),
the reference count will be 1 again.
But note that in the second \c Employee constructor, which takes an
employee ID and a name, both setId() and setName() are called, but
they don't cause \e{copy on write}, because the reference count for
the newly constructed \c EmployeeData object has just been set to 1.
In \c Employee's \e const member functions, dereferencing the \e {d
pointer} does \e not cause detach() to be called.
\snippet doc/src/snippets/sharedemployee/employee.h 5
\codeline
\snippet doc/src/snippets/sharedemployee/employee.h 6
Notice that there is no need to implement a copy constructor or an
assignment operator for the \c Employee class, because the copy
constructor and assignment operator provided by the C++ compiler
will do the \e{member by member} shallow copy required. The only
member to copy is the \e {d pointer}, which is a QSharedDataPointer,
whose \c {operator=()} just increments the reference count of the
shared \c EmployeeData object.
\target Implicit vs Explicit Sharing
\section1 Implicit vs Explicit Sharing
Implicit sharing might not be right for the \c Employee class.
Consider a simple example that creates two instances of the
implicitly shared \c Employee class.
\snippet doc/src/snippets/sharedemployee/main.cpp 0
After the second employee e2 is created and e1 is assigned to it,
both \c e1 and \c e2 refer to Albrecht Durer, employee 1001. Both \c
Employee objects point to the same instance of \c EmployeeData,
which has reference count 2. Then \c {e1.setName("Hans Holbein")} is
called to change the employee name, but because the reference count
is greater than 1, a \e{copy on write} is performed before the name
is changed. Now \c e1 and \c e2 point to different \c EmployeeData
objects. They have different names, but both have ID 1001, which is
probably not what you want. You can, of course, just continue with
\c {e1.setId(1002)}, if you really mean to create a second, unique
employee, but if you only want to change the employee's name
everywhere, consider using \l {QExplicitlySharedDataPointer}
{explicit sharing} in the \c Employee class instead of implicit
sharing.
If you declare the \e {d pointer} in the \c Employee class to be
\c {QExplicitlySharedDataPointer<EmployeeData>}, then explicit
sharing is used and \e{copy on write} operations are not performed
automatically (i.e. detach() is not called in non-const
functions). In that case, after \c {e1.setName("Hans Holbein")}, the
employee's name has been changed, but both e1 and e2 still refer to
the same instance of \c EmployeeData, so there is only one employee
with ID 1001.
In the member function documentation, \e{d pointer} always refers
to the internal pointer to the shared data object.
\sa QSharedData, QExplicitlySharedDataPointer, QScopedPointer, QSharedPointer
*/
/*! \typedef QSharedDataPointer::Type
This is the type of the shared data object. The \e{d pointer}
points to an object of this type.
*/
/*! \typedef QSharedDataPointer::pointer
\internal
*/
/*! \fn T& QSharedDataPointer::operator*()
Provides access to the shared data object's members.
This function calls detach().
*/
/*! \fn const T& QSharedDataPointer::operator*() const
Provides const access to the shared data object's members.
This function does \e not call detach().
*/
/*! \fn T* QSharedDataPointer::operator->()
Provides access to the shared data object's members.
This function calls detach().
*/
/*! \fn const T* QSharedDataPointer::operator->() const
Provides const access to the shared data object's members.
This function does \e not call detach().
*/
/*! \fn QSharedDataPointer::operator T*()
Returns a pointer to the shared data object.
This function calls detach().
\sa data(), constData()
*/
/*! \fn QSharedDataPointer::operator const T*() const
Returns a pointer to the shared data object.
This function does \e not call detach().
*/
/*! \fn T* QSharedDataPointer::data()
Returns a pointer to the shared data object.
This function calls detach().
\sa constData()
*/
/*! \fn const T* QSharedDataPointer::data() const
Returns a pointer to the shared data object.
This function does \e not call detach().
*/
/*! \fn const T* QSharedDataPointer::constData() const
Returns a const pointer to the shared data object.
This function does \e not call detach().
\sa data()
*/
/*! \fn void QSharedDataPointer::swap(QSharedDataPointer &other)
Swap this instance's shared data pointer with the shared
data pointer in \a other.
*/
/*! \fn bool QSharedDataPointer::operator==(const QSharedDataPointer<T>& other) const
Returns true if \a other and \e this have the same \e{d pointer}.
This function does \e not call detach().
*/
/*! \fn bool QSharedDataPointer::operator!=(const QSharedDataPointer<T>& other) const
Returns true if \a other and \e this do \e not have the same
\e{d pointer}. This function does \e not call detach().
*/
/*! \fn QSharedDataPointer::QSharedDataPointer()
Constructs a QSharedDataPointer initialized with a null \e{d pointer}.
*/
/*! \fn QSharedDataPointer::~QSharedDataPointer()
Decrements the reference count of the shared data object.
If the reference count becomes 0, the shared data object
is deleted. \e This is then destroyed.
*/
/*! \fn QSharedDataPointer::QSharedDataPointer(T* sharedData)
Constructs a QSharedDataPointer with \e{d pointer} set to
\a sharedData and increments \a{sharedData}'s reference count.
*/
/*! \fn QSharedDataPointer::QSharedDataPointer(const QSharedDataPointer<T>& other)
Sets the \e{d pointer} of \e this to the \e{d pointer} in
\a other and increments the reference count of the shared
data object.
*/
/*! \fn QSharedDataPointer<T>& QSharedDataPointer::operator=(const QSharedDataPointer<T>& other)
Sets the \e{d pointer} of \e this to the \e{d pointer} of
\a other and increments the reference count of the shared
data object. The reference count of the old shared data
object of \e this is decremented. If the reference count
of the old shared data object becomes 0, the old shared
data object is deleted.
*/
/*! \fn QSharedDataPointer& QSharedDataPointer::operator=(T* sharedData)
Sets the \e{d pointer} og \e this to \a sharedData and increments
\a{sharedData}'s reference count. The reference count of the old
shared data object of \e this is decremented. If the reference
count of the old shared data object becomes 0, the old shared data
object is deleted.
*/
/*! \fn bool QSharedDataPointer::operator!() const
Returns true if the \e{d pointer} of \e this is null.
*/
/*! \fn void QSharedDataPointer::detach()
If the shared data object's reference count is greater than 1, this
function creates a deep copy of the shared data object and sets the
\e{d pointer} of \e this to the copy.
This function is called automatically by non-const member
functions of QSharedDataPointer if \e{copy on write} is
required. You don't need to call it yourself.
*/
/*! \fn T *QSharedDataPointer::clone()
\since 4.5
Creates and returns a deep copy of the current data. This function
is called by detach() when the reference count is greater than 1 in
order to create the new copy. This function uses the \e {operator
new} and calls the copy constructor of the type T.
This function is provided so that you may support "virtual copy
constructors" for your own types. In order to so, you should declare
a template-specialization of this function for your own type, like
the example below:
\code
template<>
EmployeeData *QSharedDataPointer<EmployeeData>::clone()
{
return d->clone();
}
\endcode
In the example above, the template specialization for the clone()
function calls the \e {EmployeeData::clone()} virtual function. A
class derived from EmployeeData could override that function and
return the proper polymorphic type.
*/
/*!
\class QExplicitlySharedDataPointer
\brief The QExplicitlySharedDataPointer class represents a pointer to an explicitly shared object.
\since 4.4
\reentrant
QExplicitlySharedDataPointer\<T\> makes writing your own explicitly
shared classes easy. QExplicitlySharedDataPointer implements
\l {thread-safe} reference counting, ensuring that adding
QExplicitlySharedDataPointers to your \l {reentrant} classes won't
make them non-reentrant.
Except for one big difference, QExplicitlySharedDataPointer is just
like QSharedDataPointer. The big difference is that member functions
of QExplicitlySharedDataPointer \e{do not} do the automatic
\e{copy on write} operation (detach()) that non-const members of
QSharedDataPointer do before allowing the shared data object to be
modified. There is a detach() function available, but if you really
want to detach(), you have to call it yourself. This means that
QExplicitlySharedDataPointers behave like regular C++ pointers,
except that by doing reference counting and not deleting the shared
data object until the reference count is 0, they avoid the dangling
pointer problem.
It is instructive to compare QExplicitlySharedDataPointer with
QSharedDataPointer by way of an example. Consider the \l {Employee
example} in QSharedDataPointer, modified to use explicit sharing as
explained in the discussion \l {Implicit vs Explicit Sharing}.
Note that if you use this class but find you are calling detach() a
lot, you probably should be using QSharedDataPointer instead.
In the member function documentation, \e{d pointer} always refers
to the internal pointer to the shared data object.
\sa QSharedData, QSharedDataPointer
*/
/*! \fn T& QExplicitlySharedDataPointer::operator*() const
Provides access to the shared data object's members.
*/
/*! \fn T* QExplicitlySharedDataPointer::operator->()
Provides access to the shared data object's members.
*/
/*! \fn const T* QExplicitlySharedDataPointer::operator->() const
Provides const access to the shared data object's members.
*/
/*! \fn T* QExplicitlySharedDataPointer::data() const
Returns a pointer to the shared data object.
*/
/*! \fn const T* QExplicitlySharedDataPointer::constData() const
Returns a const pointer to the shared data object.
\sa data()
*/
/*! \fn void QExplicitlySharedDataPointer::swap(QExplicitlySharedDataPointer &other)
Swap this instance's explicitly shared data pointer with
the explicitly shared data pointer in \a other.
*/
/*! \fn bool QExplicitlySharedDataPointer::operator==(const QExplicitlySharedDataPointer<T>& other) const
Returns true if \a other and \e this have the same \e{d pointer}.
*/
/*! \fn bool QExplicitlySharedDataPointer::operator==(const T* ptr) const
Returns true if the \e{d pointer} of \e this is \a ptr.
*/
/*! \fn bool QExplicitlySharedDataPointer::operator!=(const QExplicitlySharedDataPointer<T>& other) const
Returns true if \a other and \e this do \e not have the same
\e{d pointer}.
*/
/*! \fn bool QExplicitlySharedDataPointer::operator!=(const T* ptr) const
Returns true if the \e{d pointer} of \e this is \e not \a ptr.
*/
/*! \fn QExplicitlySharedDataPointer::QExplicitlySharedDataPointer()
Constructs a QExplicitlySharedDataPointer initialized with a null
\e{d pointer}.
*/
/*! \fn QExplicitlySharedDataPointer::~QExplicitlySharedDataPointer()
Decrements the reference count of the shared data object.
If the reference count becomes 0, the shared data object
is deleted. \e This is then destroyed.
*/
/*! \fn QExplicitlySharedDataPointer::QExplicitlySharedDataPointer(T* sharedData)
Constructs a QExplicitlySharedDataPointer with \e{d pointer}
set to \a sharedData and increments \a{sharedData}'s reference
count.
*/
/*! \fn QExplicitlySharedDataPointer::QExplicitlySharedDataPointer(const QExplicitlySharedDataPointer<T>& other)
This standard copy constructor sets the \e {d pointer} of \e this to
the \e {d pointer} in \a other and increments the reference count of
the shared data object.
*/
/*! \fn QExplicitlySharedDataPointer::QExplicitlySharedDataPointer(const QExplicitlySharedDataPointer<X>& other)
This copy constructor is different in that it allows \a other to be
a different type of explicitly shared data pointer but one that has
a compatible shared data object. It performs a static cast of the
\e{d pointer} in \a other and sets the \e {d pointer} of \e this to
the converted \e{d pointer}. It increments the reference count of
the shared data object.
*/
/*! \fn QExplicitlySharedDataPointer<T>& QExplicitlySharedDataPointer::operator=(const QExplicitlySharedDataPointer<T>& other)
Sets the \e{d pointer} of \e this to the \e{d pointer} of
\a other and increments the reference count of the shared
data object. The reference count of the old shared data
object of \e this is decremented. If the reference count
of the old shared data object becomes 0, the old shared
data object is deleted.
*/
/*! \fn QExplicitlySharedDataPointer& QExplicitlySharedDataPointer::operator=(T* sharedData)
Sets the \e{d pointer} of \e this to \a sharedData and
increments \a{sharedData}'s reference count. The reference
count of the old shared data object of \e this is decremented.
If the reference count of the old shared data object becomes
0, the old shared data object is deleted.
*/
/*! \fn void QExplicitlySharedDataPointer::reset()
Resets \e this to be null. i.e., this function sets the
\e{d pointer} of \e this to 0, but first it decrements
the reference count of the shared data object and deletes
the shared data object if the reference count became 0.
*/
/*! \fn QExplicitlySharedDataPointer::operator bool () const
Returns true if the \e{d pointer} of \e this is \e not null.
*/
/*! \fn bool QExplicitlySharedDataPointer::operator!() const
Returns true if the \e{d pointer} of \e this is null.
*/
/*! \fn void QExplicitlySharedDataPointer::detach()
If the shared data object's reference count is greater than 1, this
function creates a deep copy of the shared data object and sets the
\e{d pointer} of \e this to the copy.
Because QExplicitlySharedDataPointer does not do the automatic
\e{copy on write} operations that members of QSharedDataPointer do,
detach() is \e not called automatically anywhere in the member
functions of this class. If you find that you are calling detach()
everywhere in your code, consider using QSharedDataPointer instead.
*/
/*! \fn T *QExplicitlySharedDataPointer::clone()
\since 4.5
Creates and returns a deep copy of the current data. This function
is called by detach() when the reference count is greater than 1 in
order to create the new copy. This function uses the \e {operator
new} and calls the copy constructor of the type T.
See QSharedDataPointer::clone() for an explanation of how to use it.
*/
/*!
\typedef QExplicitlySharedDataPointer::Type
This is the type of the shared data object. The \e{d pointer}
points to an object of this type.
*/
/*! \typedef QExplicitlySharedDataPointer::pointer
\internal
*/
QT_END_NAMESPACE
|