Previous topic
Next topic
Implicit Sharing¶
Many C++ classes in Qt use implicit data sharing to maximize resource usage and minimize copying. Implicitly shared classes are both safe and efficient when passed as arguments, because only a pointer to the data is passed around, and the data is copied only if and when a function writes to it, i.e., copy-on-write.
Overview¶
A shared class consists of a pointer to a shared data block that contains a reference count and the data.
When a shared object is created, it sets the reference count to 1. The reference count is incremented whenever a new object references the shared data, and decremented when the object dereferences the shared data. The shared data is deleted when the reference count becomes zero.
When dealing with shared objects, there are two ways of copying an object. We usually speak about deep and shallow copies. A deep copy implies duplicating an object. A shallow copy is a reference copy, i.e. just a pointer to a shared data block. Making a deep copy can be expensive in terms of memory and CPU. Making a shallow copy is very fast, because it only involves setting a pointer and incrementing the reference count.
Object assignment (with operator=()) for implicitly shared objects is implemented using shallow copies.
The benefit of sharing is that a program does not need to duplicate data unnecessarily, which results in lower memory use and less copying of data. Objects can easily be assigned, sent as function arguments, and returned from functions.
Implicit sharing mostly takes place behind the scenes; the programmer rarely needs to worry about it. However, Qt’s container iterators have different behavior than those from the STL. Read Implicit sharing iterator problem .
In multithreaded applications, implicit sharing takes place, as explained in Threads and Implicitly Shared Classes.
When implementing your own implicitly shared classes, use the QSharedData and QSharedDataPointer classes.
Implicit Sharing in Detail¶
Implicit sharing automatically detaches the object from a shared block if the object is about to change and the reference count is greater than one. (This is often called copy-on-write or value semantics.)
An implicitly shared class has control of its internal data. In any member functions that modify its data, it automatically detaches before modifying the data. Notice, however, the special case with container iterators; see Implicit sharing iterator problem .
The QPen class, which uses implicit sharing, detaches from the shared data in all member functions that change the internal data.
Code fragment:
def setStyle(self, style): detach() # detach from common data d.style = style # set the style member def detach(self): if (d.ref != 1) { ... // perform a deep copy
List of Classes¶
The classes listed below automatically detach from common data if an object is about to be changed. The programmer will not even notice that the objects are shared. Thus you should treat separate instances of them as separate objects. They will always behave as separate objects but with the added benefit of sharing data whenever possible. For this reason, you can pass instances of these classes as arguments to functions by value without concern for the copying overhead.
Example:
p1, = QPixmap() p1.load("image.bmp") p2 = p1 # p1 and p2 share data paint = QPainter() paint.begin(p2) # cuts p2 loose from p1 paint.drawText(0,50, "Hi") paint.end()
In this example, p1 and p2 share data until begin() is called for p2, because painting a pixmap will modify it.
Warning
Be careful with copying an implicitly shared container ( QMap , QList , etc.) while you use STL-style iterator . See Implicit sharing iterator problem .
|
The QDebug class provides an output stream for debugging information. |
|
The QDir class provides access to directory structures and their contents. |
|
The QFileInfo class provides system-independent file information. |
|
The QProcessEnvironment class holds the environment variables that can be passed to a program. |
|
Provides information about currently mounted storage and drives. |
|
The QUrl class provides a convenient interface for working with URLs. |
|
The QUrlQuery class provides a way to manipulate a key-value pairs in a URL’s query. |
|
The QPersistentModelIndex class is used to locate data in a data model. |
|
The QVariant class acts like a union for the most common Qt data types. |
|
The QMimeType class describes types of file or data, represented by a MIME type string. |
|
The QJsonArray class encapsulates a JSON array. |
|
The QJsonDocument class provides a way to read and write JSON documents. |
|
The QJsonObject class encapsulates a JSON object. |
|
The QJsonParseError class is used to report errors during JSON parsing. |
|
The QJsonValue class encapsulates a value in JSON. |
|
The QByteArray class provides an array of bytes. |
|
The QByteArrayList class provides a list of byte arrays. |
|
The QByteArrayView class provides a view on an array of bytes with a read-only subset of the QByteArray API. |
|
The QCollator class compares strings according to a localized collation algorithm. |
|
The QCollatorSortKey class can be used to speed up string collation. |
|
The QLocale class converts between numbers and their string representations in various languages. |
|
The QRegularExpression class provides pattern matching using regular expressions. |
|
The QRegularExpressionMatch class provides the results of a matching a QRegularExpression against a string. |
|
The QRegularExpressionMatchIterator class provides an iterator on the results of a global match of a QRegularExpression object against a string. |
|
The QString class provides a Unicode character string. |
|
The QStringList class provides a list of strings. |
|
The QTextBoundaryFinder class provides a way of finding Unicode text boundaries in a string. |
|
The QDateTime class provides date and time functions. |
|
The QBitArray class provides an array of bits. |
|
The QCache class is a template class that provides a cache. |
|
The QCommandLineOption class defines a possible command-line option. |
|
The QContiguousCache class is a template class that provides a contiguous cache. |
|
The QHash class is a template class that provides a hash-table-based dictionary. |
|
The QMultiHash class is a convenience QHash subclass that provides multi-valued hashes. |
|
The QList class is a template class that provides a dynamic array. |
|
The QMap class is a template class that provides an associative array. |
|
The QMultiMap class is a template class that provides an associative array with multiple equivalent keys. |
|
The QQueue class is a generic container that provides a queue. |
|
The QSet class is a template class that provides a hash-table-based set. |
|
The QStack class is a template class that provides a stack. |
|
The QLowEnergyAdvertisingData class represents the data to be broadcast during Bluetooth Low Energy advertising. |
|
The QLowEnergyAdvertisingParameters class represents the parameters used for Bluetooth Low Energy advertising. |
|
The QLowEnergyCharacteristicData class is used to set up GATT service data. |
|
The QLowEnergyConnectionParameters class is used when requesting or reporting an update of the parameters of a Bluetooth LE connection. |
|
The QLowEnergyDescriptorData class is used to create GATT service data. |
|
The QLowEnergyServiceData class is used to set up GATT service data. |
|
The QDBusPendingCall class refers to one pending asynchronous call. |
|
The QDBusUnixFileDescriptor class holds one Unix file descriptor. |
|
The QBitmap class provides monochrome (1-bit depth) pixmaps. |
|
The QIcon class provides scalable icons in different modes and states. |
|
The QImage class provides a hardware-independent image representation that allows direct access to the pixel data, and can be used as a paint device. |
|
The QPicture class is a paint device that records and replays QPainter commands. |
|
The QPixmap class is an off-screen image representation that can be used as a paint device. |
|
The QCursor class provides a mouse cursor with an arbitrary shape. |
|
The QKeySequence class encapsulates a key sequence as used by shortcuts. |
|
The QPalette class contains color groups for each widget state. |
|
The QBrush class defines the fill pattern of shapes drawn by QPainter. |
|
The QGradient class is used in combination with QBrush to specify gradient fills. |
|
The QPageRanges class represents a collection of page ranges. |
|
The QPainterPath class provides a container for painting operations, enabling graphical shapes to be constructed and reused. |
|
The QPen class defines how a QPainter should draw lines and outlines of shapes. |
|
The QPolygon class provides a list of points using integer precision. |
|
The QPolygonF class provides a list of points using floating point precision. |
|
The QRegion class specifies a clip region for a painter. |
|
The QFont class specifies a query for a font used for drawing text. |
|
The QFontInfo class provides general information about fonts. |
|
The QFontMetrics class provides font metrics information. |
|
The QFontMetricsF class provides font metrics information. |
|
The QGlyphRun class provides direct access to the internal glyphs in a font. |
|
The QRawFont class provides access to a single physical instance of a font. |
|
The QStaticText class enables optimized drawing of text when the text and its layout is updated rarely. |
|
The QTextCursor class offers an API to access and modify QTextDocuments. |
|
The QTextDocumentFragment class represents a piece of formatted text from a QTextDocument. |
|
The QTextFormat class provides formatting information for a QTextDocument. |
|
The QTextCharFormat class provides formatting information for characters in a QTextDocument. |
|
The QTextBlockFormat class provides formatting information for blocks of text in a QTextDocument. |
|
The QTextListFormat class provides formatting information for lists in a QTextDocument. |
|
The QTextFrameFormat class provides formatting information for frames in a QTextDocument. |
|
The QTextTableFormat class provides formatting information for tables in a QTextDocument. |
|
The QTextImageFormat class provides formatting information for images in a QTextDocument. |
|
The QTextTableCellFormat class provides formatting information for table cells in a QTextDocument. |
|
The QNetworkCacheMetaData class provides cache information. |
|
The QHttp2Configuration class controls HTTP/2 parameters and settings. |
|
The QHttpPart class holds a body part to be used inside a HTTP multipart MIME message. |
|
The QNetworkCookie class holds one network cookie. |
|
The QNetworkRequest class holds a request to be sent with QNetworkAccessManager. |
|
The QDnsDomainNameRecord class stores information about a domain name record. |
|
The QDnsHostAddressRecord class stores information about a host address record. |
|
The QDnsMailExchangeRecord class stores information about a DNS MX record. |
|
The QDnsServiceRecord class stores information about a DNS SRV record. |
|
The QDnsTextRecord class stores information about a DNS TXT record. |
|
The QHostAddress class provides an IP address. |
|
The QNetworkAddressEntry class stores one IP address supported by a network interface, along with its associated netmask and broadcast address. |
|
The QNetworkInterface class provides a listing of the host’s IP addresses and network interfaces. |
|
The QNetworkProxy class provides a network layer proxy. |
|
The QNetworkProxyQuery class is used to query the proxy settings for a socket. |
|
The QSslCertificate class provides a convenient API for an X509 certificate. |
|
The QSslCertificateExtension class provides an API for accessing the extensions of an X509 certificate. |
|
The QSslCipher class represents an SSL cryptographic cipher. |
|
The QSslConfiguration class holds the configuration and state of an SSL connection. |
|
The QSslDiffieHellmanParameters class provides an interface for Diffie-Hellman parameters for servers. |
|
The QSslError class provides an SSL error. |
|
The QSslKey class provides an interface for private and public keys. |
|
The QSslPreSharedKeyAuthenticator class provides authentication data for pre shared keys (PSK) ciphersuites. |
|
The QOpenGLDebugMessage class wraps an OpenGL debug message. |
|
The QGeoAreaMonitorInfo class describes the parameters of an area or region to be monitored for proximity. |
|
The QGeoPositionInfo class contains information gathered on a global position, direction and velocity at a particular point in time. |
|
The QGeoSatelliteInfo class contains basic information about a satellite. |
|
The QSqlField class manipulates the fields in SQL database tables and views. |
|
The QSqlQuery class provides a means of executing and manipulating SQL statements. |
|
The QSqlRecord class encapsulates a database record. |
© 2022 The Qt Company Ltd. Documentation contributions included herein are the copyrights of their respective owners. The documentation provided herein is licensed under the terms of the GNU Free Documentation License version 1.3 as published by the Free Software Foundation. Qt and respective logos are trademarks of The Qt Company Ltd. in Finland and/or other countries worldwide. All other trademarks are property of their respective owners.