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:

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 QPainter::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.

QBitmap

The QBitmap class provides monochrome (1-bit depth) pixmaps.

QIcon

The QIcon class provides scalable icons in different modes and states.

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.

QPicture

The QPicture class is a paint device that records and replays QPainter commands.

QPixmap

The QPixmap class is an off-screen image representation that can be used as a paint device.

QCursor

The QCursor class provides a mouse cursor with an arbitrary shape.

QKeySequence

The QKeySequence class encapsulates a key sequence as used by shortcuts.

QPalette

The QPalette class contains color groups for each widget state.

QOpenGLDebugMessage

The QOpenGLDebugMessage class wraps an OpenGL debug message.

QBrush

The QBrush class defines the fill pattern of shapes drawn by QPainter.

QGradient

The QGradient class is used in combination with QBrush to specify gradient fills.

QPainterPath

The QPainterPath class provides a container for painting operations, enabling graphical shapes to be constructed and reused.

QPen

The QPen class defines how a QPainter should draw lines and outlines of shapes.

QPolygon

The QPolygon class provides a vector of points using integer precision.

QPolygonF

The QPolygonF class provides a vector of points using floating point precision.

QRegion

The QRegion class specifies a clip region for a painter.

QFont

The QFont class specifies a font used for drawing text.

QFontInfo

The QFontInfo class provides general information about fonts.

QFontMetrics

The QFontMetrics class provides font metrics information.

QFontMetricsF

The QFontMetricsF class provides font metrics information.

QGlyphRun

The QGlyphRun class provides direct access to the internal glyphs in a font.

QRawFont

The QRawFont class provides access to a single physical instance of a font.

QStaticText

The QStaticText class enables optimized drawing of text when the text and its layout is updated rarely.

QTextCursor

The QTextCursor class offers an API to access and modify QTextDocuments.

QTextDocumentFragment

The QTextDocumentFragment class represents a piece of formatted text from a QTextDocument.

QTextBlockFormat

The QTextBlockFormat class provides formatting information for blocks of text in a QTextDocument.

QTextCharFormat

The QTextCharFormat class provides formatting information for characters in a QTextDocument.

QTextFormat

The QTextFormat class provides formatting information for a QTextDocument.

QTextFrameFormat

The QTextFrameFormat class provides formatting information for frames in a QTextDocument.

QTextImageFormat

The QTextImageFormat class provides formatting information for images in a QTextDocument.

QTextListFormat

The QTextListFormat class provides formatting information for lists in a QTextDocument.

QTextTableCellFormat

The QTextTableCellFormat class provides formatting information for table cells in a QTextDocument.

QTextTableFormat

The QTextTableFormat class provides formatting information for tables in a QTextDocument.

QNetworkCacheMetaData

The QNetworkCacheMetaData class provides cache information.

QHttpPart

The QHttpPart class holds a body part to be used inside a HTTP multipart MIME message.

QNetworkCookie

The QNetworkCookie class holds one network cookie.

QNetworkRequest

The QNetworkRequest class holds a request to be sent with QNetworkAccessManager.

QNetworkConfiguration

The QNetworkConfiguration class provides an abstraction of one or more access point configurations.

QDnsDomainNameRecord

The QDnsDomainNameRecord class stores information about a domain name record.

QDnsHostAddressRecord

The QDnsHostAddressRecord class stores information about a host address record.

QDnsMailExchangeRecord

The QDnsMailExchangeRecord class stores information about a DNS MX record.

QDnsServiceRecord

The QDnsServiceRecord class stores information about a DNS SRV record.

QDnsTextRecord

The QDnsTextRecord class stores information about a DNS TXT record.

QNetworkAddressEntry

The QNetworkAddressEntry class stores one IP address supported by a network interface, along with its associated netmask and broadcast address.

QNetworkInterface

The QNetworkInterface class provides a listing of the host's IP addresses and network interfaces.

QNetworkProxy

The QNetworkProxy class provides a network layer proxy.

QNetworkProxyQuery

The QNetworkProxyQuery class is used to query the proxy settings for a socket.

QSslCertificate

The QSslCertificate class provides a convenient API for an X509 certificate.

QSslCertificateExtension

The QSslCertificateExtension class provides an API for accessing the extensions of an X509 certificate.

QSslCipher

The QSslCipher class represents an SSL cryptographic cipher.

QSslConfiguration

The QSslConfiguration class holds the configuration and state of an SSL connection

QSslError

The QSslError class provides an SSL error.

QSslKey

The QSslKey class provides an interface for private and public keys.

QDebug

Output stream for debugging information

QDir

Access to directory structures and their contents

QFileInfo

System-independent file information

QProcessEnvironment

Holds the environment variables that can be passed to a program

QUrl

Convenient interface for working with URLs

QUrlQuery

Way to manipulate a key-value pairs in a URL's query

QPersistentModelIndex

Used to locate data in a data model

QVariant

Acts like a union for the most common Qt data types

QMimeType

Describes types of file or data, represented by a MIME type string

QBitArray

Array of bits

QByteArray

Array of bytes

QCache

Template class that provides a cache

QCollator

Compares strings according to a localized collation algorithm

QCollatorSortKey

Can be used to speed up string collation

QCommandLineOption

Defines a possible command-line option

QContiguousCache

Template class that provides a contiguous cache

QDateTime

Date and time functions

QHash

Template class that provides a hash-table-based dictionary

QMultiHash

Convenience QHash subclass that provides multi-valued hashes

QLinkedList

Template class that provides linked lists

QList

Template class that provides lists

QLocale

Converts between numbers and their string representations in various languages

QMap

Template class that provides a red-black-tree-based dictionary

QMultiMap

Convenience QMap subclass that provides multi-valued maps

QQueue

Generic container that provides a queue

QRegExp

Pattern matching using regular expressions

QRegularExpression

Pattern matching using regular expressions

QRegularExpressionMatch

The results of a matching a QRegularExpression against a string

QRegularExpressionMatchIterator

Iterator on the results of a global match of a QRegularExpression object against a string

QSet

Template class that provides a hash-table-based set

QStack

Template class that provides a stack

QString

Unicode character string

QStringList

List of strings

QTextBoundaryFinder

Way of finding Unicode text boundaries in a string

QVector

Template class that provides a dynamic array

Notes provided by the Qt Community

No notes