Qt 4.8
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The QSharedDataPointer class represents a pointer to an implicitly shared object. More...
#include <qshareddata.h>
Public Types | |
typedef T * | pointer |
typedef T | Type |
This is the type of the shared data object. More... | |
Public Functions | |
const T * | constData () const |
Returns a const pointer to the shared data object. More... | |
T * | data () |
Returns a pointer to the shared data object. More... | |
const T * | data () const |
Returns a pointer to the shared data object. More... | |
void | 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 d pointer of this to the copy. More... | |
operator const T * () const | |
Returns a pointer to the shared data object. More... | |
operator T* () | |
Returns a pointer to the shared data object. More... | |
bool | operator! () const |
Returns true if the d pointer of this is null. More... | |
bool | operator!= (const QSharedDataPointer< T > &other) const |
Returns true if other and this do not have the same d pointer. More... | |
T & | operator* () |
Provides access to the shared data object's members. More... | |
const T & | operator* () const |
Provides const access to the shared data object's members. More... | |
T * | operator-> () |
Provides access to the shared data object's members. More... | |
const T * | operator-> () const |
Provides const access to the shared data object's members. More... | |
QSharedDataPointer< T > & | operator= (const QSharedDataPointer< T > &o) |
Sets the d pointer of this to the d pointer of other and increments the reference count of the shared data object. More... | |
QSharedDataPointer & | operator= (T *o) |
Sets the d pointer og this to sharedData and increments {sharedData}'s reference count. More... | |
bool | operator== (const QSharedDataPointer< T > &other) const |
Returns true if other and this have the same d pointer. More... | |
QSharedDataPointer () | |
Constructs a QSharedDataPointer initialized with a null d pointer. More... | |
QSharedDataPointer (T *data) | |
Constructs a QSharedDataPointer with d pointer set to sharedData and increments {sharedData}'s reference count. More... | |
QSharedDataPointer (const QSharedDataPointer< T > &o) | |
Sets the d pointer of this to the d pointer in other and increments the reference count of the shared data object. More... | |
void | swap (QSharedDataPointer &other) |
Swap this instance's shared data pointer with the shared data pointer in other. More... | |
~QSharedDataPointer () | |
Decrements the reference count of the shared data object. More... | |
Protected Functions | |
T * | clone () |
Private Functions | |
void | detach_helper () |
Properties | |
T * | d |
The QSharedDataPointer class represents a pointer to an implicitly shared object.
QSharedDataPointer<T> makes writing your own implicitly shared classes easy. QSharedDataPointer implements thread-safe reference counting, ensuring that adding QSharedDataPointers to your reentrant classes won't make them non-reentrant.
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 Shared Classes page for more information.
Suppose you want to make an Employee
class implicitly shared. The procedure is:
Define the class Employee
to have a single data member of type {QSharedDataPointer<EmployeeData>}
.
Define the EmployeeData
class derived from QSharedData to contain all the data members you would normally have put in the Employee
class.
To show this in practice, we review the source code for the implicitly shared Employee
class. In the header file we define the two classes Employee
and EmployeeData
.
In class Employee
, note the single data member, a {d pointer} of type {QSharedDataPointer<EmployeeData>}
. All accesses of employee data must go through the {d pointer's} {operator->()}
. For write accesses, {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
Employee
object don't affect any other Employee
objects that share the same EmployeeData
object.Class EmployeeData
inherits QSharedData, which provides the behind the scenes reference counter. EmployeeData
has a default constructor, a copy constructor, and a destructor. Normally, trivial implementations of these are all that is needed in the {data} class for an implicitly shared class.
Implementing the two constructors for class Employee
is also straightforward. Both create a new instance of EmployeeData
and assign it to the d pointer .
Note that class Employee
also has a trivial copy constructor defined, which is not strictly required in this case.
The copy constructor is not strictly required here, because class EmployeeData
is included in the same file as class Employee
(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 EmployeeData
in a separate file, which would not be included in employee.h
. Instead, we would just predeclare the private subclass EmployeeData
in {employee.h}
this way:
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 Employee
object is copied, assigned, or passed as a parameter. It decrements the reference count whenever an Employee
object is deleted or goes out of scope. The shared EmployeeData
object is deleted automatically if and when the reference count reaches 0.
In a non-const member function of Employee
, whenever the {d pointer} is dereferenced, QSharedDataPointer automatically calls detach() to ensure that the function operates on its own copy of the data.
Note that if detach() is called more than once in a member function due to multiple dereferences of the {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 Employee
constructor, which takes an employee ID and a name, both setId() and setName() are called, but they don't cause copy on write, because the reference count for the newly constructed EmployeeData
object has just been set to 1.
In Employee's
const member functions, dereferencing the {d pointer} does not cause detach() to be called.
Notice that there is no need to implement a copy constructor or an assignment operator for the Employee
class, because the copy constructor and assignment operator provided by the C++ compiler will do the member by member shallow copy required. The only member to copy is the {d pointer}, which is a QSharedDataPointer, whose {operator=()}
just increments the reference count of the shared EmployeeData
object.
Implicit sharing might not be right for the Employee
class. Consider a simple example that creates two instances of the implicitly shared Employee
class.
After the second employee e2 is created and e1 is assigned to it, both e1
and e2
refer to Albrecht Durer, employee 1001. Both Employee
objects point to the same instance of EmployeeData
, which has reference count 2. Then {e1.setName
("Hans Holbein")} is called to change the employee name, but because the reference count is greater than 1, a copy on write is performed before the name is changed. Now e1
and e2
point to different 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 {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 QExplicitlySharedDataPointer {explicit sharing} in the Employee
class instead of implicit sharing.
If you declare the {d pointer} in the Employee
class to be {QExplicitlySharedDataPointer<EmployeeData>}
, then explicit sharing is used and copy on write operations are not performed automatically (i.e. detach() is not called in non-const functions). In that case, after {e1.setName
("Hans Holbein")}, the employee's name has been changed, but both e1 and e2 still refer to the same instance of EmployeeData
, so there is only one employee with ID 1001.
In the member function documentation, d pointer always refers to the internal pointer to the shared data object.
Definition at line 54 of file qshareddata.h.
QSharedDataPointer< T >::pointer |
Definition at line 73 of file qshareddata.h.
QSharedDataPointer< T >::Type |
This is the type of the shared data object.
The d pointer points to an object of this type.
Definition at line 72 of file qshareddata.h.
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inline |
Constructs a QSharedDataPointer initialized with a null d pointer.
Definition at line 89 of file qshareddata.h.
Referenced by QSharedDataPointer< QSslConfigurationPrivate >::operator=(), and QSharedDataPointer< QSslConfigurationPrivate >::~QSharedDataPointer().
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inline |
Decrements the reference count of the shared data object.
If the reference count becomes 0, the shared data object is deleted. This is then destroyed.
Definition at line 90 of file qshareddata.h.
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explicit |
Constructs a QSharedDataPointer with d pointer set to sharedData and increments {sharedData}'s reference count.
Definition at line 221 of file qshareddata.h.
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protected |
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 {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:
In the example above, the template specialization for the clone() function calls the {EmployeeData::clone()} virtual function. A class derived from EmployeeData could override that function and return the proper polymorphic type.
Definition at line 225 of file qshareddata.h.
Referenced by QSharedDataPointer< QSslConfigurationPrivate >::detach_helper(), QExplicitlySharedDataPointer< QSslConfigurationPrivate >::detach_helper(), QSharedDataPointer< QSslConfigurationPrivate >::swap(), and QExplicitlySharedDataPointer< QSslConfigurationPrivate >::swap().
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inline |
Returns a const pointer to the shared data object.
This function does not call detach().
Definition at line 84 of file qshareddata.h.
Referenced by QDir::absoluteFilePath(), QDir::absolutePath(), QDir::canonicalPath(), QDir::cd(), QDir::count(), QFileInfo::d_func(), QDir::dirName(), QDir::entryInfoList(), QDir::entryList(), QDir::filePath(), QDir::filter(), QDir::isReadable(), QOCIResult::lastInsertId(), QFileInfo::makeAbsolute(), QDir::makeAbsolute(), QDir::mkdir(), QDir::mkpath(), QDir::nameFilters(), QDir::nameFiltersFromString(), QDir::operator==(), QDir::operator[](), QDir::path(), QDir::rmdir(), QDir::rmpath(), QSslConfigurationPrivate::setDefaultConfiguration(), QFileInfo::setFile(), and QDir::sorting().
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Returns a pointer to the shared data object.
This function calls detach().
Definition at line 82 of file qshareddata.h.
Referenced by QVolatileImage::constImageRef(), QDir::nameFiltersFromString(), QDir::refresh(), QDir::setFilter(), QDir::setNameFilters(), QDir::setSorting(), QExplicitlySharedDataPointer< QSslConfigurationPrivate >::~QExplicitlySharedDataPointer(), and QSharedDataPointer< QSslConfigurationPrivate >::~QSharedDataPointer().
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inline |
Returns a pointer to the shared data object.
This function does not call detach().
Definition at line 83 of file qshareddata.h.
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inline |
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 d pointer of this to the copy.
This function is called automatically by non-const member functions of QSharedDataPointer if copy on write is required. You don't need to call it yourself.
Definition at line 75 of file qshareddata.h.
Referenced by QSharedDataPointer< QSslConfigurationPrivate >::data(), QFileInfo::detach(), QProcessEnvironment::insert(), QSharedDataPointer< T >::operator T*(), QSharedDataPointer< QSslConfigurationPrivate >::operator*(), QSharedDataPointer< QSslConfigurationPrivate >::operator->(), QNetworkProxyPrivate::operator==(), QProcessEnvironment::remove(), QUnixSocketMessage::setBytes(), and QUnixSocketMessage::setRights().
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Definition at line 231 of file qshareddata.h.
Referenced by QSharedDataPointer< QSslConfigurationPrivate >::detach(), QExplicitlySharedDataPointer< QSslConfigurationPrivate >::detach(), QSharedDataPointer< QSslConfigurationPrivate >::swap(), and QExplicitlySharedDataPointer< QSslConfigurationPrivate >::swap().
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Returns a pointer to the shared data object.
This function does not call detach().
Definition at line 81 of file qshareddata.h.
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Returns a pointer to the shared data object.
This function calls detach().
Definition at line 80 of file qshareddata.h.
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Returns true if other and this do not have the same d pointer.
This function does not call detach().
Definition at line 87 of file qshareddata.h.
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inline |
Provides access to the shared data object's members.
This function calls detach().
Definition at line 76 of file qshareddata.h.
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inline |
Provides const access to the shared data object's members.
This function does not call detach().
Definition at line 77 of file qshareddata.h.
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Provides access to the shared data object's members.
This function calls detach().
Definition at line 78 of file qshareddata.h.
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inline |
Provides const access to the shared data object's members.
This function does not call detach().
Definition at line 79 of file qshareddata.h.
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Sets the d pointer of this to the d pointer of other and increments the reference count of the shared data object.
The reference count of the old shared data object of this is decremented. If the reference count of the old shared data object becomes 0, the old shared data object is deleted.
Definition at line 94 of file qshareddata.h.
Referenced by QSharedDataPointer< QSslConfigurationPrivate >::operator=(), and QExplicitlySharedDataPointer< QSslConfigurationPrivate >::operator=().
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Sets the d pointer og this to sharedData and increments {sharedData}'s reference count.
The reference count of the old shared data object of this is decremented. If the reference count of the old shared data object becomes 0, the old shared data object is deleted.
Definition at line 105 of file qshareddata.h.
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Returns true if other and this have the same d pointer.
This function does not call detach().
Definition at line 86 of file qshareddata.h.
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Definition at line 133 of file qshareddata.h.
Referenced by QSharedDataPointer< QSslConfigurationPrivate >::clone(), QExplicitlySharedDataPointer< QSslConfigurationPrivate >::clone(), QSharedDataPointer< QSslConfigurationPrivate >::constData(), QExplicitlySharedDataPointer< QSslConfigurationPrivate >::constData(), QSharedDataPointer< QSslConfigurationPrivate >::data(), QExplicitlySharedDataPointer< QSslConfigurationPrivate >::data(), QSharedDataPointer< QSslConfigurationPrivate >::detach(), QExplicitlySharedDataPointer< QSslConfigurationPrivate >::detach(), QSharedDataPointer< QSslConfigurationPrivate >::detach_helper(), QExplicitlySharedDataPointer< QSslConfigurationPrivate >::detach_helper(), QExplicitlySharedDataPointer< QSslConfigurationPrivate >::operator bool(), QSharedDataPointer< T >::operator const T *(), QSharedDataPointer< T >::operator T*(), QSharedDataPointer< QSslConfigurationPrivate >::operator!(), QExplicitlySharedDataPointer< QSslConfigurationPrivate >::operator!(), QSharedDataPointer< QSslConfigurationPrivate >::operator!=(), QExplicitlySharedDataPointer< QSslConfigurationPrivate >::operator!=(), QSharedDataPointer< QSslConfigurationPrivate >::operator*(), QExplicitlySharedDataPointer< QSslConfigurationPrivate >::operator*(), QSharedDataPointer< QSslConfigurationPrivate >::operator->(), QExplicitlySharedDataPointer< QSslConfigurationPrivate >::operator->(), QSharedDataPointer< QSslConfigurationPrivate >::operator=(), QExplicitlySharedDataPointer< QSslConfigurationPrivate >::operator=(), QSharedDataPointer< QSslConfigurationPrivate >::operator==(), QExplicitlySharedDataPointer< QSslConfigurationPrivate >::operator==(), QExplicitlySharedDataPointer< QSslConfigurationPrivate >::QExplicitlySharedDataPointer(), QSharedDataPointer< QSslConfigurationPrivate >::QSharedDataPointer(), QExplicitlySharedDataPointer< QSslConfigurationPrivate >::reset(), QSharedDataPointer< QSslConfigurationPrivate >::swap(), QExplicitlySharedDataPointer< QSslConfigurationPrivate >::swap(), QExplicitlySharedDataPointer< QSslConfigurationPrivate >::~QExplicitlySharedDataPointer(), and QSharedDataPointer< QSslConfigurationPrivate >::~QSharedDataPointer().