Qt文档阅读笔记-隐式共享（Implicit Sharing）深入研究（理论及实例）

Qt里面很多C++类都是采用隐式共享最大限度的提高资源利用率以及最新复制的原则。隐式共享数据作为参数时，不仅安全而且高效，因为传的是一个指针并且只有要修改这个数据时才会去拷贝，这里有个专业词汇copy-on-write在Qt官方提供的ppt中经常会出现这个词，说的就是隐式共享。

共享类（这个应该是指value type的类，学过Qt的都知道，Qt类可以分为两类，一个是identity type和value type）由一个共享数据块及指向这个块的计数器组成。

当共享对象被创建，这个块的计数器的值会被设置为1，并且当有新的对象指向共享数据库，那么他的计数器就会+1，当对象不指向这个内存块时，计数器会减少，当计数器为0时，这个共享块将会被删除。

当要对共享数据进行处理时，Qt有2种方式进行拷贝，也就是常说的深拷贝与浅拷贝。这里不再解释这2个拷贝了，这个隐式共享就和浅拷贝差不多，只不过是多了给增加了个引用计数器。

=这个操作一般是浅拷贝。

Qt的容器iterator和STL的不同，主要也是围绕隐式共享。如下代码：

#include <QVector>
#include <QDebug>int main(int argc, char *argv[])
{QVector<int> a, b;a.resize(100000);QVector<int>::iterator i = a.begin();b = a;*i = 4;a[0] = 5;b.clear();int j = *i;qDebug() << "OVER";}

下面最关键的来了，演示下上面所说的理论

这里可以看到当*i = 4还没执行的时候a中RefCount为2，也就是a，b都引用到了这个隐式内存中，所以为2。而这个i是直接指向了目前a最开始的那个内存。

下面看下一张图：

这里可以看到*i=4改变后，因为改变的是共享内存，所有a和b里面的数据都被改了。

最关键的地方来了，大家要注意，Qt最强大的机制之一copy-on-write来了。

这里a[0]=5,写及拷贝。a重新进行构造，现在RefCount又变成1了。

这个程序想表示的就是这个。也就是Qt很重要的运行机制。

还有给要提的就是这个i。

当b被clear后这个i就有问题，int j = i;这种操作很不安全。

当开发者需要写自己的隐式共享类时，使用QSharedData和QSharedDataPointer类。

当需要改变数据的时候，数据将会从隐式共享分离。

如下面这个QPen类，内部是这样处理的

void QPen::setStyle(Qt::PenStyle style){detach();           // detach from common datad->style = style;   // set the style member}void QPen::detach(){if (d->ref != 1) {...             // perform a deep copy}}

当setStyle被调用后，这个set为设置，所以改变了，就会调用detach()，当d-ref不为1时，就会把所有数据拷贝一份。

下面是一些隐式共享的类，这里要记住，QTL的容器如果用STL风格去操作，需要考虑下隐式共享方面的问题。

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 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
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

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Qt文档阅读笔记-隐式共享（Implicit Sharing）深入研究（理论及实例）

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