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 takes place behind the scenes; the programmer does not need to worry about it. Even 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 total control of its internal data. In any member functions that modify its data, it automatically detaches before modifying the data.

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: Do not copy an implicitly shared container (QMap, QVector, etc.) while you are iterating over it using an non-const STL-style iterator.

QBitArray

Array of bits

QBitmap

Monochrome (1-bit depth) pixmaps

QBrush

Defines the fill pattern of shapes drawn by QPainter

QByteArray

Array of bytes

QCache

Template class that provides a cache

QContiguousCache

Template class that provides a contiguous cache

QCursor

Mouse cursor with an arbitrary shape

QDir

Access to directory structures and their contents

QFileInfo

System-independent file information

QFont

Specifies a font used for drawing text

QFontInfo

General information about fonts

QFontMetrics

Font metrics information

QFontMetricsF

Font metrics information

QGLColormap

Used for installing custom colormaps into a QGLWidget

QGradient

Used in combination with QBrush to specify gradient fills

QHash

Template class that provides a hash-table-based dictionary

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

QKeySequence

Encapsulates a key sequence as used by shortcuts

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 skip-list-based dictionary

QMultiHash

Convenience QHash subclass that provides multi-valued hashes

QMultiMap

Convenience QMap subclass that provides multi-valued maps

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

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

QQueue

Generic container that provides a queue

QRegExp

Pattern matching using regular expressions

QRegion

Specifies a clip region for a painter

QSet

Template class that provides a hash-table-based set

QSqlField

Manipulates the fields in SQL database tables and views

QSqlQuery

Means of executing and manipulating SQL statements

QSqlRecord

Encapsulates a database record

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

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

QUrl

Convenient interface for working with URLs

QVariant

Acts like a union for the most common Qt data types

QVector

Template class that provides a dynamic array

QX11Info

Information about the X display configuration

Notes provided by the Qt Community

No notes