Implicit Sharing

Reference counting for fast copying.

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:

void QPen::setStyle(Qt::PenStyle style)
{
    detach();           // detach from common data
    d->style = style;   // set the style member
}

void QPen::detach()
{
    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:

QPixmap p1, p2;
p1.load("image.bmp");
p2 = p1;                        // p1 and p2 share data

QPainter paint;
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 , QVector , etc.) while you use STL-style iterator . See Implicit sharing iterator problem .

QDebug	The QDebug class provides an output stream for debugging information.
PySide2.QtCore.QDir	The QDir class provides access to directory structures and their contents.
PySide2.QtCore.QFileInfo	The QFileInfo class provides system-independent file information.
PySide2.QtCore.QProcessEnvironment	The QProcessEnvironment class holds the environment variables that can be passed to a program.
PySide2.QtCore.QStorageInfo	Provides information about currently mounted storage and drives.
PySide2.QtCore.QUrl	The QUrl class provides a convenient interface for working with URLs.
PySide2.QtCore.QUrlQuery	The QUrlQuery class provides a way to manipulate a key-value pairs in a URL’s query.
PySide2.QtCore.QPersistentModelIndex	The QPersistentModelIndex class is used to locate data in a data model.
PySide2.QtCore.QVariant	The QVariant class acts like a union for the most common Qt data types.
PySide2.QtCore.QMimeType	The QMimeType class describes types of file or data, represented by a MIME type string.
PySide2.QtCore.QJsonArray	The QJsonArray class encapsulates a JSON array.
PySide2.QtCore.QJsonDocument	The QJsonDocument class provides a way to read and write JSON documents.
PySide2.QtCore.QJsonObject	The QJsonObject class encapsulates a JSON object.
PySide2.QtCore.QJsonParseError	The QJsonParseError class is used to report errors during JSON parsing.
PySide2.QtCore.QJsonValue	The QJsonValue class encapsulates a value in JSON.
PySide2.QtCore.QByteArray	The QByteArray class provides an array of bytes.
QByteArrayList	The QByteArrayList class provides a list of byte arrays.
PySide2.QtCore.QCollator	The QCollator class compares strings according to a localized collation algorithm.
PySide2.QtCore.QCollatorSortKey	The QCollatorSortKey class can be used to speed up string collation.
PySide2.QtCore.QLocale	The QLocale class converts between numbers and their string representations in various languages.
PySide2.QtCore.QRegExp	The QRegExp class provides pattern matching using regular expressions.
PySide2.QtCore.QRegularExpression	The QRegularExpression class provides pattern matching using regular expressions.
PySide2.QtCore.QRegularExpressionMatch	The QRegularExpressionMatch class provides the results of a matching a QRegularExpression against a string.
PySide2.QtCore.QRegularExpressionMatchIterator	The QRegularExpressionMatchIterator class provides an iterator on the results of a global match of a QRegularExpression object against a string.
PySide2.QtCore.QString	The QString class provides a Unicode character string.
	The QStringBuilder class is a template class that provides a facility to build up QStrings and QByteArrays from smaller chunks.
PySide2.QtCore.QStringList	The QStringList class provides a list of strings.
PySide2.QtCore.QTextBoundaryFinder	The QTextBoundaryFinder class provides a way of finding Unicode text boundaries in a string.
PySide2.QtCore.QDateTime	The QDateTime class provides date and time functions.
PySide2.QtCore.QBitArray	The QBitArray class provides an array of bits.
QCache	The QCache class is a template class that provides a cache.
PySide2.QtCore.QCommandLineOption	The QCommandLineOption class defines a possible command-line option.
QContiguousCache	The QContiguousCache class is a template class that provides a contiguous cache.
QHash	The QHash class is a template class that provides a hash-table-based dictionary.
QMultiHash	The QMultiHash class is a convenience QHash subclass that provides multi-valued hashes.
QLinkedList	The QLinkedList class is a template class that provides linked lists.
QList	The QList class is a template class that provides lists.
QMap	The QMap class is a template class that provides a red-black-tree-based dictionary.
QMultiMap	The QMultiMap class is a convenience QMap subclass that provides multi-valued maps.
QQueue	The QQueue class is a generic container that provides a queue.
QSet	The QSet class is a template class that provides a hash-table-based set.
QStack	The QStack class is a template class that provides a stack.
QVector	The QVector class is a template class that provides a dynamic array.
PySide2.QtGui.QBitmap	The QBitmap class provides monochrome (1-bit depth) pixmaps.
PySide2.QtGui.QIcon	The QIcon class provides scalable icons in different modes and states.
PySide2.QtGui.QImage	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.
PySide2.QtGui.QPicture	The QPicture class is a paint device that records and replays QPainter commands.
PySide2.QtGui.QPixmap	The QPixmap class is an off-screen image representation that can be used as a paint device.
PySide2.QtGui.QCursor	The QCursor class provides a mouse cursor with an arbitrary shape.
PySide2.QtGui.QKeySequence	The QKeySequence class encapsulates a key sequence as used by shortcuts.
PySide2.QtGui.QPalette	The QPalette class contains color groups for each widget state.
PySide2.QtGui.QOpenGLDebugMessage	The QOpenGLDebugMessage class wraps an OpenGL debug message.
PySide2.QtGui.QBrush	The QBrush class defines the fill pattern of shapes drawn by QPainter.
PySide2.QtGui.QGradient	The QGradient class is used in combination with QBrush to specify gradient fills.
PySide2.QtGui.QPainterPath	The QPainterPath class provides a container for painting operations, enabling graphical shapes to be constructed and reused.
PySide2.QtGui.QPen	The QPen class defines how a QPainter should draw lines and outlines of shapes.
PySide2.QtGui.QPolygon	The QPolygon class provides a vector of points using integer precision.
PySide2.QtGui.QPolygonF	The QPolygonF class provides a vector of points using floating point precision.
PySide2.QtGui.QRegion	The QRegion class specifies a clip region for a painter.
PySide2.QtGui.QFont	The QFont class specifies a query for a font used for drawing text.
PySide2.QtGui.QFontInfo	The QFontInfo class provides general information about fonts.
PySide2.QtGui.QFontMetrics	The QFontMetrics class provides font metrics information.
PySide2.QtGui.QFontMetricsF	The QFontMetricsF class provides font metrics information.
QGlyphRun	The QGlyphRun class provides direct access to the internal glyphs in a font.
PySide2.QtGui.QRawFont	The QRawFont class provides access to a single physical instance of a font.
PySide2.QtGui.QStaticText	The QStaticText class enables optimized drawing of text when the text and its layout is updated rarely.
PySide2.QtGui.QTextCursor	The QTextCursor class offers an API to access and modify QTextDocuments.
PySide2.QtGui.QTextDocumentFragment	The QTextDocumentFragment class represents a piece of formatted text from a QTextDocument.
PySide2.QtGui.QTextFormat	The QTextFormat class provides formatting information for a QTextDocument.
PySide2.QtGui.QTextCharFormat	The QTextCharFormat class provides formatting information for characters in a QTextDocument.
PySide2.QtGui.QTextBlockFormat	The QTextBlockFormat class provides formatting information for blocks of text in a QTextDocument.
PySide2.QtGui.QTextListFormat	The QTextListFormat class provides formatting information for lists in a QTextDocument.
PySide2.QtGui.QTextFrameFormat	The QTextFrameFormat class provides formatting information for frames in a QTextDocument.
PySide2.QtGui.QTextTableFormat	The QTextTableFormat class provides formatting information for tables in a QTextDocument.
PySide2.QtGui.QTextImageFormat	The QTextImageFormat class provides formatting information for images in a QTextDocument.
PySide2.QtGui.QTextTableCellFormat	The QTextTableCellFormat class provides formatting information for table cells in a QTextDocument.
PySide2.QtNetwork.QNetworkCacheMetaData	The QNetworkCacheMetaData class provides cache information.
QHttp2Configuration	The QHttp2Configuration class controls HTTP/2 parameters and settings.
PySide2.QtNetwork.QHttpPart	The QHttpPart class holds a body part to be used inside a HTTP multipart MIME message.
PySide2.QtNetwork.QNetworkCookie	The QNetworkCookie class holds one network cookie.
PySide2.QtNetwork.QNetworkRequest	The QNetworkRequest class holds a request to be sent with QNetworkAccessManager.
PySide2.QtNetwork.QNetworkConfiguration	The QNetworkConfiguration class provides an abstraction of one or more access point configurations.
PySide2.QtNetwork.QDnsDomainNameRecord	The QDnsDomainNameRecord class stores information about a domain name record.
PySide2.QtNetwork.QDnsHostAddressRecord	The QDnsHostAddressRecord class stores information about a host address record.
PySide2.QtNetwork.QDnsMailExchangeRecord	The QDnsMailExchangeRecord class stores information about a DNS MX record.
PySide2.QtNetwork.QDnsServiceRecord	The QDnsServiceRecord class stores information about a DNS SRV record.
PySide2.QtNetwork.QDnsTextRecord	The QDnsTextRecord class stores information about a DNS TXT record.
PySide2.QtNetwork.QHostAddress	The QHostAddress class provides an IP address.
PySide2.QtNetwork.QNetworkAddressEntry	The QNetworkAddressEntry class stores one IP address supported by a network interface, along with its associated netmask and broadcast address.
PySide2.QtNetwork.QNetworkInterface	The QNetworkInterface class provides a listing of the host’s IP addresses and network interfaces.
PySide2.QtNetwork.QNetworkProxy	The QNetworkProxy class provides a network layer proxy.
PySide2.QtNetwork.QNetworkProxyQuery	The QNetworkProxyQuery class is used to query the proxy settings for a socket.
PySide2.QtNetwork.QSslCertificate	The QSslCertificate class provides a convenient API for an X509 certificate.
PySide2.QtNetwork.QSslCertificateExtension	The QSslCertificateExtension class provides an API for accessing the extensions of an X509 certificate.
PySide2.QtNetwork.QSslCipher	The QSslCipher class represents an SSL cryptographic cipher.
PySide2.QtNetwork.QSslConfiguration	The QSslConfiguration class holds the configuration and state of an SSL connection.
PySide2.QtNetwork.QSslDiffieHellmanParameters	The QSslDiffieHellmanParameters class provides an interface for Diffie-Hellman parameters for servers.
PySide2.QtNetwork.QSslError	The QSslError class provides an SSL error.
PySide2.QtNetwork.QSslKey	The QSslKey class provides an interface for private and public keys.
PySide2.QtNetwork.QSslPreSharedKeyAuthenticator	The QSslPreSharedKeyAuthenticator class provides authentication data for pre shared keys (PSK) ciphersuites.
PySide2.QtOpenGL.QGLColormap	The QGLColormap class is used for installing custom colormaps into a QGLWidget.
PySide2.QtSql.QSqlField	The QSqlField class manipulates the fields in SQL database tables and views.
PySide2.QtSql.QSqlQuery	The QSqlQuery class provides a means of executing and manipulating SQL statements.
PySide2.QtSql.QSqlRecord	The QSqlRecord class encapsulates a database record.