隐式共享是QT设计的一个重要特点。
QT隐式共享的原文是这样的(来源于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.”
总结几句话就是：
1）QT里对很多C++类采用了隐式共享的方法；隐式共享保证了高效共享数据；
2）隐式共享实质就是浅拷贝；QT认为深拷贝过于耗费内存和CPU；
3）如果用隐式共享，用QSharedData and QSharedDataPointer 这两种类。
由上可以得知，所谓隐式共享，不过是浅拷贝；同时，保证了浅拷贝有引用计数，避免删除。
QSharedData和QSharedDataPointer是可以用于隐式共享的QT方法。
目前，QT里默认隐式共享的方法是：
| QGLColormap class is used for installing custom colormaps into a QGLWidget |
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| Array of bits |
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| Monochrome (1-bit depth) pixmaps |
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| Defines the fill pattern of shapes drawn by QPainter |
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| Array of bytes |
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| Template class that provides a cache |
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| Template class that provides a contiguous cache |
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| Mouse cursor with an arbitrary shape |
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| Access to directory structures and their contents |
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| System-independent file information |
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| Specifies a font used for drawing text |
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| General information about fonts |
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| Font metrics information |
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| Font metrics information |
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| Used in combination with QBrush to specify gradient fills |
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| Template class that provides a hash-table-based dictionary |
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| Scalable icons in different modes and states |
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| Hardware-independent image representation that allows direct access to the pixel data, and can be used as a paint device |
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| Encapsulates a key sequence as used by shortcuts |
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| Template class that provides linked lists |
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| Template class that provides lists |
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| Converts between numbers and their string representations in various languages |
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| Template class that provides a skip-list-based dictionary |
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| Convenience QHash subclass that provides multi-valued hashes |
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| Convenience QMap subclass that provides multi-valued maps |
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| Container for painting operations, enabling graphical shapes to be constructed and reused |
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| Contains color groups for each widget state |
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| Defines how a QPainter should draw lines and outlines of shapes |
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| Paint device that records and replays QPainter commands |
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| Off-screen image representation that can be used as a paint device |
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| Vector of points using integer precision |
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| Vector of points using floating point precision |
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| Generic container that provides a queue |
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| Pattern matching using regular expressions |
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| Specifies a clip region for a painter |
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| Template class that provides a hash-table-based set |
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| Manipulates the fields in SQL database tables and views |
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| Means of executing and manipulating SQL statements |
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| Encapsulates a database record |
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| Template class that provides a stack |
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| Unicode character string |
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| List of strings |
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| Way of finding Unicode text boundaries in a string |
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| Offers an API to access and modify QTextDocuments |
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| Represents a piece of formatted text from a QTextDocument |
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| Formatting information for a QTextDocument |
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| Convenient interface for working with URLs |
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| Acts like a union for the most common Qt data types |
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| Template class that provides a dynamic array |
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| Information about the X display configuration |