隐式共享

Qt的许多类使用隐式数据共享, 最大限度提高资源使用率, 减少复制. 函数传参时, 隐式共享只传递数据指针, 既安全又高效, 并且只有函数往变量写入数据时, 数据才会复制, 即., 写时复制.

概述

共享类包含指向共享数据库的指针, 这个数据块包含引用计数和数据.

创建共享对象时, 引用计数设置为1. 有新对象引用共享数据时, 引用计数增加. 当对象取消引用共享数据时, 引用计数减少. 引用计数为0时, 删除共享数据.

处理共享对象时, 有两种方法复制对象. 我们通常讨论深拷贝和浅拷贝. 深拷贝表示复制对象. 浅拷贝是一种引用复制, 即. 指向共享数据的指针. 对于内存和CPU而言, 深拷贝代价可能很高. 浅拷贝非常快, 因为它只涉及指针复制和引用计数.

隐式共享对象的对象分配采用浅拷贝.

隐式共享的好处是程序不需要进行不必要的数据复制, 这样可以降低内存的使用率, 减少数据复制. 对象容器分配, 作为函数参数传递, 作为函数返回值.

隐式共享发生在后台; 程序员不需要担心这个事情. 然而, Qt的容器迭代器与STL的不同. 阅读 Implicit sharing iterator problem.

在多线程应用程序中, 也会隐式共享, 参见 Threads and Implicitly Shared Classes.

使用 QSharedData 和 QSharedDataPointer 自定义隐式共享类.

如果对象即将改变, 且引用计数大于1, 则隐式共享自动将对象与共享块分离. (这通常被称为 写时复制 或 值语义.)

隐式共享类控制内部数据. 在任何修改数据的成员函数中, 它必须在修改数据之前自动分离数据. 注意: 隐式对象的容器迭代器必须考虑; 参见 Implicit sharing iterator problem.

QPen 使用隐式共享, 所有的成员函数在改变内部数据之前会分离共享数据.

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
      }
  }

类列表

下表列出的类在修改对象时, 自动分离公共数据. 开发人员甚至不会注意到这些对象是共享的. 因此, 你应该将它们的单独实例视为独立对象. 除非共享数据, 它们始终表现的跟独立对象一样. 因此, 你可以将这些类作为函数参数, 不必担心复制开销.

示例:


  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();

上述示例中, p1 和 p2 在调用 QPainter::begin() 之前共享数据, 绘制修改图片.

警告: 使用STL风格迭代器时, 复制共享容器必须小心 (QMap, QVector等). 参见 Implicit sharing iterator problem.

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
QStorageInfo	Provides information about currently mounted storage and drives
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
QJsonArray	Encapsulates a JSON array
QJsonDocument	Way to read and write JSON documents
QJsonParseError	Used to report errors during JSON parsing
QJsonObject	Encapsulates a JSON object
QJsonValue	Encapsulates a value in JSON
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
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
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
QDBusPendingCall	Refers to one pending asynchronous call
QDBusUnixFileDescriptor	Holds one Unix file descriptor
QBitmap	Monochrome (1-bit depth) pixmaps
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
QPicture	Paint device that records and replays QPainter commands
QPixmap	Off-screen image representation that can be used as a paint device
QCursor	Mouse cursor with an arbitrary shape
QKeySequence	Encapsulates a key sequence as used by shortcuts
QPalette	Contains color groups for each widget state
QOpenGLDebugMessage	Wraps an OpenGL debug message
QBrush	Defines the fill pattern of shapes drawn by QPainter
QGradient	Used in combination with QBrush to specify gradient fills
QPainterPath	Container for painting operations, enabling graphical shapes to be constructed and reused
QPen	Defines how a QPainter should draw lines and outlines of shapes
QPolygon	Vector of points using integer precision
QPolygonF	Vector of points using floating point precision
QRegion	Specifies a clip region for a painter
QFont	Specifies 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
QRawFont	Access to a single physical instance of a font
QStaticText	Enables optimized drawing of text when the text and its layout is updated rarely
QTextCursor	Offers an API to access and modify QTextDocuments
QTextDocumentFragment	Represents a piece of formatted text from a QTextDocument
QTextBlockFormat	Formatting information for blocks of text in a QTextDocument
QTextCharFormat	Formatting information for characters in 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
QNetworkCacheMetaData	Cache information
QHttpPart	Holds a body part to be used inside a HTTP multipart MIME message
QNetworkCookie	Holds one network cookie
QNetworkRequest	Holds a request to be sent with QNetworkAccessManager
QNetworkConfiguration	Abstraction of one or more access point configurations
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
QHostAddress	IP address
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
QNetworkProxy	Network layer proxy
QNetworkProxyQuery	Used to query the proxy settings for a socket
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

目录

隐式共享

概述

隐式共享的细节

类列表