QT的隐式共享(Implicit Sharing)

        为了最大化使用系统资源，更少的内存拷贝，QT中的很多类使用了隐式数据共享。当传递参数时，使用隐式共享类既安全又高效，因为只传递对象指针。当对象中的数据被修改时，对象才会被拷贝。也就是我们常说的“写拷贝”(copy-on-write)。

 

概述：

        共享类由一个指向共享数据块的指针，此共享数据块包含对象的引用计数和对象的数据。

        当一个共享对象被创建时，它的引用计数设置为1。当一个新的对象引用了此对象，那么引用计数就会加1。反之，如果一个对象不再引用此对象，引用计数就会减1。而引用计数为0时，共享对象会被删除。

        有两种方法拷贝共享对象，我们通常叫做深拷贝和浅拷贝。深拷贝会复制对象，而浅拷贝是引用拷贝，只复制共享数据块的指针。深拷贝会占用较多内存和CPU资源，因为它仅仅设置指针的值，增加引用计数。

        隐式数据对象赋值(操作符=())操作使用浅拷贝。

        共享的优点，在不必要的情况下，程序不需要复制数据，从而减少内存使用和数据拷贝。对象能被简单的赋值，作为函数参数传递，并从函数返回。

        隐式数据的特性对于用户是透明的。程序员不需要去担心这些特性。甚至在多线的应用中，隐式共享的具体细节，在“ Threads and Implicitly Shared Classes”中有详细解释。

        如果想自定义隐式共享类，可以使用QSharedData和QSharedDataPointer。

 

隐式共享的具体细节：

        如果对象即将改变，并且引用计数大于1，隐式共享对象会自动从共享数据块中分离。（通常叫做写复制或值语义。）

        隐式共享类对内部数据操作敏感。任何成员函数修改了共享数据，在修改数据之前都会分离共享数据块。

        QPen使用了隐式共享。任何成员函数修改内部数据，都会导致共享数据块的分离。

        代码片段：

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：

        下面列出的类在对象即将改变时，会自动从共享数据块分离。程序员甚至都没注意到对象的共享。因此，应该将这些单独的实例作为单独的对象对待。它们看上去就像单独的对象，但是却有共享对象的优点。因此，你可以将这些类的实例作为函数参数传递而不需要担心拷贝的开销。

例如：

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();

在这个例子中，p1和p2共享数据，直到QPainter::begin对p2调用，因为绘制操作会修改共享对象。

警告：当你使用非常量的标准模板库风格迭代器迭代隐式共享容器(QMap、QVector等等)时，不要拷贝这些容器。

QBitmap	The QBitmap class provides monochrome (1-bit depth) pixmaps.
QIcon	The QIcon class provides scalable icons in different modes and states.
QImage	The QImage class provides a hardware-independent image representation that allows direct access to the pixel data, and can be used as a paint device.
QPicture	The QPicture class is a paint device that records and replays QPainter commands.
QPixmap	The QPixmap class is an off-screen image representation that can be used as a paint device.
QCursor	The QCursor class provides a mouse cursor with an arbitrary shape.
QKeySequence	The QKeySequence class encapsulates a key sequence as used by shortcuts.
QPalette	The QPalette class contains color groups for each widget state.
QOpenGLDebugMessage	The QOpenGLDebugMessage class wraps an OpenGL debug message.
QBrush	The QBrush class defines the fill pattern of shapes drawn by QPainter.
QGradient	The QGradient class is used in combination with QBrush to specify gradient fills.
QPainterPath	The QPainterPath class provides a container for painting operations, enabling graphical shapes to be constructed and reused.
QPen	The QPen class defines how a QPainter should draw lines and outlines of shapes.
QPolygon	The QPolygon class provides a vector of points using integer precision.
QPolygonF	The QPolygonF class provides a vector of points using floating point precision.
QRegion	The QRegion class specifies a clip region for a painter.
QFont	The QFont class specifies a font used for drawing text.
QFontInfo	The QFontInfo class provides general information about fonts.
QFontMetrics	The QFontMetrics class provides font metrics information.
QFontMetricsF	The QFontMetricsF class provides font metrics information.
QGlyphRun	The QGlyphRun class provides direct access to the internal glyphs in a font.
QRawFont	The QRawFont class provides access to a single physical instance of a font.
QStaticText	The QStaticText class enables optimized drawing of text when the text and its layout is updated rarely.
QTextCursor	The QTextCursor class offers an API to access and modify QTextDocuments.
QTextDocumentFragment	The QTextDocumentFragment class represents a piece of formatted text from a QTextDocument.
QTextBlockFormat	The QTextBlockFormat class provides formatting information for blocks of text in a QTextDocument.
QTextCharFormat	The QTextCharFormat class provides formatting information for characters in a QTextDocument.
QTextFormat	The QTextFormat class provides formatting information for a QTextDocument.
QTextFrameFormat	The QTextFrameFormat class provides formatting information for frames in a QTextDocument.
QTextImageFormat	The QTextImageFormat class provides formatting information for images in a QTextDocument.
QTextListFormat	The QTextListFormat class provides formatting information for lists in a QTextDocument.
QTextTableCellFormat	The QTextTableCellFormat class provides formatting information for table cells in a QTextDocument.
QTextTableFormat	The QTextTableFormat class provides formatting information for tables in a QTextDocument.
QNetworkCacheMetaData	The QNetworkCacheMetaData class provides cache information.
QHttpPart	The QHttpPart class holds a body part to be used inside a HTTP multipart MIME message.
QNetworkCookie	The QNetworkCookie class holds one network cookie.
QNetworkRequest	The QNetworkRequest class holds a request to be sent with QNetworkAccessManager.
QNetworkConfiguration	The QNetworkConfiguration class provides an abstraction of one or more access point configurations.
QDnsDomainNameRecord	The QDnsDomainNameRecord class stores information about a domain name record.
QDnsHostAddressRecord	The QDnsHostAddressRecord class stores information about a host address record.
QDnsMailExchangeRecord	The QDnsMailExchangeRecord class stores information about a DNS MX record.
QDnsServiceRecord	The QDnsServiceRecord class stores information about a DNS SRV record.
QDnsTextRecord	The QDnsTextRecord class stores information about a DNS TXT record.
QNetworkAddressEntry	The QNetworkAddressEntry class stores one IP address supported by a network interface, along with its associated netmask and broadcast address.
QNetworkInterface	The QNetworkInterface class provides a listing of the host's IP addresses and network interfaces.
QNetworkProxy	The QNetworkProxy class provides a network layer proxy.
QNetworkProxyQuery	The QNetworkProxyQuery class is used to query the proxy settings for a socket.
QSslCertificate	The QSslCertificate class provides a convenient API for an X509 certificate.
QSslCertificateExtension	The QSslCertificateExtension class provides an API for accessing the extensions of an X509 certificate.
QSslCipher	The QSslCipher class represents an SSL cryptographic cipher.
QSslConfiguration	The QSslConfiguration class holds the configuration and state of an SSL connection
QSslError	The QSslError class provides an SSL error.
QSslKey	The QSslKey class provides an interface for private and public keys.
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
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
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
QCache	Template class that provides a cache
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