QT設計重要特點：QT隱式共享（Implicit Sharing）

隱式共享是QT設計的一個重要特點。

QT隱式共享的原文是這樣的(來源於https://doc.qt.io/archives/qt-4.8/implicit-sharing.html)：

“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.”

“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.”

“When implementing your own implicitly shared classes, use the QSharedData and QSharedDataPointer classes.”

總結幾句話就是：

1）QT裏對很多C++類採用了隱式共享的方法；隱式共享保證了高效共享數據；

2）隱式共享實質就是淺拷貝；QT認爲深拷貝過於耗費內存和CPU；

3）如果用隱式共享，用QSharedData and QSharedDataPointer 這兩種類。

由上可以得知，所謂隱式共享，不過是淺拷貝；同時，保證了淺拷貝有引用計數，避免刪除。

QSharedData和QSharedDataPointer是可以用於隱式共享的QT方法。

目前，QT裏默認隱式共享的方法是：

OpenGL	QGLColormap class is used for installing custom colormaps into a QGLWidget
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
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