Implicit Sharing | Qt Core 6.4.1

- Qt 6.4
- Qt Core
- Implicit Sharing
- Qt 6.4.1 Reference Documentation

# 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, QList, etc.) while you use STL-style iterator. See Implicit sharing iterator problem.

QBitmap | Monochrome (1-bit depth) pixmaps
QBrush | Defines the fill pattern of shapes drawn by QPainter
QByteArrayList | List of byte arrays
QByteArrayView | View on an array of bytes with a read-only subset of the QByteArray API
QCache | Template class that provides a cache
QCommandLineOption | Defines a possible command-line option
QContiguousCache | Template class that provides a contiguous cache
QDBusUnixFileDescriptor | Holds one Unix file descriptor
QDebug | Output stream for debugging information
QGlyphRun | Direct access to the internal glyphs in a font
QGradient | Used in combination with QBrush to specify gradient fills
QList | Template class that provides a dynamic array
QLocale | Converts between numbers and their string representations in various languages
QLowEnergyAdvertisingData | Represents the data to be broadcast during Bluetooth Low Energy advertising
QLowEnergyAdvertisingParameters | Represents the parameters used for Bluetooth Low Energy advertising
QLowEnergyCharacteristicData | Used to set up GATT service data
QLowEnergyConnectionParameters | Used when requesting or reporting an update of the parameters of a Bluetooth LE connection
QLowEnergyDescriptorData | Used to create GATT service data
QLowEnergyServiceData | Used to set up GATT service data
QMap | Template class that provides an associative array
QMultiMap | Template class that provides an associative array with multiple equivalent keys
QNetworkAddressEntry | Stores one IP address supported by a network interface, along with its associated netmask and broadcast address
QNetworkInterface | Listing of the host's IP addresses and network interfaces
QPageRanges | Represents a collection of page ranges
QPalette | Contains color groups for each widget state
QPen | Defines how a QPainter should draw lines and outlines of shapes
QPixmap | Off-screen image representation that can be used as a paint device
QQueue | Generic container that provides a queue
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
QSslCertificateExtension | API for accessing the extensions of an X509 certificate
QSslCipher | Represents an SSL cryptographic cipher
QSslPreSharedKeyAuthenticator | Authentication data for pre shared keys (PSK) ciphersuites
QStack | Template class that provides a stack
QStorageInfo | Provides information about currently mounted storage and drives
QStringList | List of strings