Qxt中的d-pointer技术及改进

Qxt是一个Qt的扩展库。其中也用到了d-pointer技术，但是与Qt的区别很大，比起Qt有点不足之处。这里我先简单看下它的实现，然后吸取Qt中的优点进行改进。(最新代码在 https://github.com/wang-bin/d-pointer )

之前写过一篇关于d-pointer的文章 http://blog.youkuaiyun.com/ibingow/article/details/7522157  ，这里很多细节就不重复了。

在qxtglobal.h中相关代码

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** This file is derived from code bearing the following notice:
** The sole author of this file, Adam Higerd, has explicitly disclaimed all
** copyright interest and protection for the content within. This file has
** been placed in the public domain according to United States copyright
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#define QXT_DECLARE_PRIVATE(PUB) friend class PUB##Private; QxtPrivateInterface<PUB, PUB##Private> qxt_d;
#define QXT_DECLARE_PUBLIC(PUB) friend cl