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.

QBitArray	Array of bits
QBitmap	Monochrome (1-bit depth) pixmaps
QBrush	Defines the fill pattern of shapes drawn by QPainter
QByteArray	Array of bytes
QByteArrayList	List of byte arrays
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
QCursor	Mouse cursor with an arbitrary shape
QDBusPendingCall	Refers to one pending asynchronous call
QDBusUnixFileDescriptor	Holds one Unix file descriptor
QDateTime	Date and time functions
QDebug	Output stream for debugging information
QDir	Access to directory structures and their contents
QDnsDomainNameRecord	Stores information about a domain name record
QDnsHostAddressRecord	Stores information about a host address record
QDnsMailExchangeRecord	Stores information about a DNS MX record
QDnsServiceRecord	Stores information about a DNS SRV record
QDnsTextRecord	Stores information about a DNS TXT record
QFileInfo	System-independent file information
QFont	Specifies a query for a font used for drawing text
QFontInfo	General information about fonts
QFontMetrics	Font metrics information
QFontMetricsF	Font metrics information
QGlyphRun	Direct access to the internal glyphs in a font
QGradient	Used in combination with QBrush to specify gradient fills
QHash	Template class that provides a hash-table-based dictionary
QHostAddress	IP address
QHttp2Configuration	Controls HTTP/2 parameters and settings
QHttpPart	Holds a body part to be used inside a HTTP multipart MIME message
QIcon	Scalable icons in different modes and states
QImage	Hardware-independent image representation that allows direct access to the pixel data, and can be used as a paint device
QJsonArray	Encapsulates a JSON array
QJsonDocument	Way to read and write JSON documents
QJsonObject	Encapsulates a JSON object
QJsonParseError	Used to report errors during JSON parsing
QJsonValue	Encapsulates a value in JSON
QKeySequence	Encapsulates a key sequence as used by shortcuts
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
QMimeType	Describes types of file or data, represented by a MIME type string
QMultiHash	Convenience QHash subclass that provides multi-valued hashes
QMultiMap	Convenience QMap subclass that provides multi-valued maps
QNetworkAddressEntry	Stores one IP address supported by a network interface, along with its associated netmask and broadcast address
QNetworkCacheMetaData	Cache information
QNetworkCookie	Holds one network cookie
QNetworkInterface	Listing of the host's IP addresses and network interfaces
QNetworkProxy	Network layer proxy
QNetworkProxyQuery	Used to query the proxy settings for a socket
QNetworkRequest	Holds a request to be sent with QNetworkAccessManager
QOpenGLDebugMessage	Wraps an OpenGL debug message
QPainterPath	Container for painting operations, enabling graphical shapes to be constructed and reused
QPalette	Contains color groups for each widget state
QPen	Defines how a QPainter should draw lines and outlines of shapes
QPersistentModelIndex	Used to locate data in a data model
QPicture	Paint device that records and replays QPainter commands
QPixmap	Off-screen image representation that can be used as a paint device
QPolygon	Vector of points using integer precision
QPolygonF	Vector of points using floating point precision
QProcessEnvironment	Holds the environment variables that can be passed to a program
QQueue	Generic container that provides a queue
QRawFont	Access to a single physical instance of a font
QRegExp	Pattern matching using regular expressions
QRegion	Specifies a clip region for a painter
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
QSslCertificate	Convenient API for an X509 certificate
QSslCertificateExtension	API for accessing the extensions of an X509 certificate
QSslCipher	Represents an SSL cryptographic cipher
QSslConfiguration	Holds the configuration and state of an SSL connection
QSslDiffieHellmanParameters	Interface for Diffie-Hellman parameters for servers
QSslError	SSL error
QSslKey	Interface for private and public keys
QSslPreSharedKeyAuthenticator	Authentication data for pre shared keys (PSK) ciphersuites
QStack	Template class that provides a stack
QStaticText	Enables optimized drawing of text when the text and its layout is updated rarely
QStorageInfo	Provides information about currently mounted storage and drives
QString	Unicode character string
QStringList	List of strings
QTextBlockFormat	Formatting information for blocks of text in a QTextDocument
QTextBoundaryFinder	Way of finding Unicode text boundaries in a string
QTextCharFormat	Formatting information for characters in a QTextDocument
QTextCursor	Offers an API to access and modify QTextDocuments
QTextDocumentFragment	Represents a piece of formatted text from a QTextDocument
QTextFormat	Formatting information for a QTextDocument
QTextFrameFormat	Formatting information for frames in a QTextDocument
QTextImageFormat	Formatting information for images in a QTextDocument
QTextListFormat	Formatting information for lists in a QTextDocument
QTextTableCellFormat	Formatting information for table cells in a QTextDocument
QTextTableFormat	Formatting information for tables in a QTextDocument
QUrl	Convenient interface for working with URLs
QUrlQuery	Way to manipulate a key-value pairs in a URL's query
QVariant	Acts like a union for the most common Qt data types
QVector	Template class that provides a dynamic array