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

QDir

QFileInfo

QProcessEnvironment

QStorageInfo

QUrl

QUrlQuery

QPersistentModelIndex

QVariant

QMimeType

QJsonArray

QJsonDocument

QJsonObject

QJsonParseError

QJsonValue

QBitArray

QByteArray

QByteArrayList

QCache

QCollator

QCollatorSortKey

QCommandLineOption

QContiguousCache

QDateTime

QHash

QMultiHash

QLinkedList

QList

QLocale

QMap

QMultiMap

QQueue

QRegExp

QRegularExpression

QRegularExpressionMatch

QRegularExpressionMatchIterator

QSet

QStack

QString

QStringBuilder

QStringList

QTextBoundaryFinder

QVector

QBitmap

QIcon

QImage

QPicture

QPixmap

QCursor

QKeySequence

QPalette

QOpenGLDebugMessage

QBrush

QGradient

QPainterPath

QPen

QPolygon

QPolygonF

QRegion

QFont

QFontInfo

QFontMetrics

QFontMetricsF

QGlyphRun

QRawFont

QStaticText

QTextCursor

QTextDocumentFragment

QTextFormat

QTextCharFormat

QTextBlockFormat

QTextListFormat

QTextFrameFormat

QTextTableFormat

QTextImageFormat

QTextTableCellFormat

QNetworkCacheMetaData

QHttpPart

QNetworkCookie

QNetworkRequest

QNetworkConfiguration

QDnsDomainNameRecord

QDnsHostAddressRecord

QDnsMailExchangeRecord

QDnsServiceRecord

QDnsTextRecord

QHostAddress

QNetworkAddressEntry

QNetworkInterface

QNetworkProxy

QNetworkProxyQuery

QSslCertificate

QSslCertificateExtension

QSslCipher

QSslConfiguration

QSslDiffieHellmanParameters

QSslError

QSslKey

QSslPreSharedKeyAuthenticator

QGLColormap

QSqlField

QSqlQuery

QSqlRecord