POCO C++库学习和分析 -- 流（二）

柔情只为你懂 2022-03-09 11:28 641阅读 0赞

# POCO C++库学习和分析 -- 流 （二） #

## 2. 创建自己的流 ##

### 2.1 自定义流的结构 ###

在Poco中提供了一个框架用于创建自己的流，并且创建的流都符合C++标准。想一下标准库中流的层次和结构。每一个流都必须有对应的流缓冲，并且在流初始化时提供此流缓冲的指针。Poco中提供了3种流缓冲供选择，分别是BasicBufferedStreamBuf、BasicUnbufferedStreamBuf、BasicBufferedBidirectionalStreamBuf。首先来看一个例子：

#include "Poco/UnbufferedStreamBuf.h"
    #include <ostream>
    #include <cctype>
    class UpperStreamBuf: public UnbufferedStreamBuf
    {
    public:
    	UpperStreamBuf(std::ostream& ostr): _ostr(ostr)
    	{}
    protected:
    	int writeToDevice(char c)
    	{
    		_ostr.put(toupper(c));
    		return charToInt(c);
    	}
    private:
    	std::ostream& _ostr;
    };
    
    class UpperIOS: public virtual std::ios
    {
    public:
    	UpperIOS(std::ostream& ostr): _buf(ostr)
    	{
    		poco_ios_init(&_buf);
    	}
    protected:
    	UpperStreamBuf _buf;
    };
    
    class UpperOutputStream: public UpperIOS, public std::ostream
    {
    public:
    	UpperOutputStream(std::ostream& ostr):
    	  UpperIOS(ostr),
    		  std::ostream(&_buf)
    	  {}
    };
    
    int main(int argc, char** argv)
    {
    	UpperOutputStream upper(std::cout);
    	upper << "Hello, world!" << std::endl;
    	return 0;
    }

从上面的例子中，我们看到用户自定流缓冲类型UpperStreamBuf，并自定义流类型UpperIOS和UpperOutputStream。其中UpperIOS从std::ios虚拟继承。注意这里是虚拟继承，这是为了保证UpperOutputStream多重继承时的菱形结构(std::ostream也是从std::ios虚拟继承的)。再回忆一下，std::ios的作用，其中最重要的就是定义了缓冲区指针。那么很自然的，定义UpperIOS，并让UpperOutputStream从UpperIOS继承的原因，就是为了让UpperOutputStream类拥有自己的缓冲区。再来看从流std::ostream继承的原因，很简单，为了继承"<<"操作符。那么清楚了，如果是输入流，从std::istream继承；是输入输出流从std::iostream继承。

再从上面的例子看：为什么说，Poco中流框架提供的是一个中介流的框架。当创建UpperOutputStream对象时，我们看它做了什么.  
          在调用UpperOutputStream upper(std::cout)时，传入参数是一个std::cout，这是一个已定义的流对象。这个流对象被赋予给UpperIOS类，用来初始化UpperIOS类中的UpperStreamBuf对象；而UpperStreamBuf又并不负责直接输出，它的输出依赖于内部的std::ostream对象，在这里就是std::cout。另一方面由于UpperOutputStream对象同时也继承自std::ostream，而std::ostream必须要求用一个流缓冲对象来初始化，所以在UpperIOS类中的UpperStreamBuf对象初始化后，又被赋给std::ostream对象用作初始化。  
  
          再来看 "upper << "Hello, world!" << std::endl "语句的执行过程：  
          1. "<<"操作符是std::ostream的成员函数。也就是说 "Hello, world!"被首先调用了std::ostream函数"<<"。

2.  我们知道，流的输入输出是委托给其内部的缓冲区对象的。也就是说，在这里"<<"操作符，会委托到std::ostream类内部关联的流缓冲UpperIOS::UpperStreamBuf对象\_buf上。  
          2. UpperStreamBuf类型的\_buf对象并不输出，它把输出任务继续委托给内部的流对象\_ostr。在程序里，\_ost对象为std::cout。  
          3. std::cout对象在收到数据后，委托给std::cout内部关联的流缓冲对象，最终输出。至于std::cout对象关联的流对象是什么，那就不用去管了。这是系统内部实现的。  
  
          转了半天，输出最终还是依赖于系统的默认输出。如果仅是如此，就没有意思了。Poco流在类BasicBufferedStreamBuf、BasicUnbufferedStreamBuf、BasicBufferedBidirectionalStreamBuf上开了一个小口，供用户自定义中间操作。这两个小口就是:

virtual int readFromDevice(char_type* buffer, std::streamsize length)；
    virtual int writeToDevice(const char_type* buffer, std::streamsize length)；

通过子类继承，用户可以自定义转换行为。任何从Poco流自定义框架出来的类都必须重新定义这两个虚函数。readFromDevice和writeToDevice会在std::ostream中的虚函数overflow和underflow中被调用。  
  
          在了解了Poco自定义流类的流转次序后，再去看Poco中的自定义流Base64Decoder，Base64Encoder等的数据流转，会发现过程都是类似的。下面就不再对这一方面进行叙述。只是讲一下Poco中已定义的3个流缓冲类BasicBufferedStreamBuf、BasicUnbufferedStreamBuf、BasicBufferedBidirectionalStreamBuf。

### 2.2 BasicUnbufferedStreamBuf ###

Poco::BasicUnbufferedStreamBuf是一个使用char作为模板参数类型的偏特化模板类。其完整定义为：

template <typename ch, typename tr> 
    class BasicUnbufferedStreamBuf: public std::basic_streambuf<ch, tr>
    	/// This is an implementation of an unbuffered streambuf
    	/// that greatly simplifies the implementation of
    	/// custom streambufs of various kinds.
    	/// Derived classes only have to override the methods
    	/// readFromDevice() or writeToDevice().
    {
    	// .....
    	int_type _pb;
    	bool     _ispb;
    }
    
    typedef BasicUnbufferedStreamBuf<char, std::char_traits<char> > UnbufferedStreamBuf;

Poco::BasicUnbufferedStreamBuf可以说是最简单的创建用户自定流的方法，在其内部没有任何缓冲对象。从Poco::BasicUnbufferedStreamBuf继承的类需要重载readFromDevice和writeToDevice接口。  
  
          对应重载的要求如下：

int readFromDevice()

读取并返回单字节无符号字符。如果没有更多的合适数据时，返回char\_traits::eof()(-1)。需要注意的是，不要直接返回一个char类型值，因为char类型值是一个有符号数。调用int charToInt(char c)返回一个char转换到int的值。

int writeToDevice(char c)

写入一个单字节字符，如果成功返回写入的字符(已整形方式)，否则返回char\_traits::eof() (-1).  
  
          在Poco中从UnbufferedStreamBuf继承的类见下图:

![20130426165751763][]

### 2.3 BasicBufferedStreamBuf ###

Poco::BasicBufferedStreamBuf也同样是使用char作为模板参数类型的偏特化模板类。其完整定义为：

template <typename ch, typename tr, typename ba = BufferAllocator<ch> > 
    class BasicBufferedStreamBuf: public std::basic_streambuf<ch, tr>
    	/// This is an implementation of a buffered streambuf
    	/// that greatly simplifies the implementation of
    	/// custom streambufs of various kinds.
    	/// Derived classes only have to override the methods
    	/// readFromDevice() or writeToDevice().
    	///
    	/// This streambuf only supports unidirectional streams.
    	/// In other words, the BasicBufferedStreamBuf can be
    	/// used for the implementation of an istream or an
    	/// ostream, but not for an iostream.
    {
    	// ...
    	std::streamsize _bufsize;
    	char_type*      _pBuffer;
    	openmode        _mode;
    }
    
    typedef BasicBufferedStreamBuf<char, std::char_traits<char> > BufferedStreamBuf;

在BasicBufferedStreamBuf内部存在一个\_pBuffer，用作缓冲，供输出使用。Poco::BufferedStreamBuf的实例支持读写，但不支持同时读写。从Poco::BasicBufferedStreamBuf继承的类需要重载readFromDevice和writeToDevice接口。  
  
          对应重载的要求如下：

int readFromDevice(char* buffer, std::streamsize length)

读取指定长度的字符，并把它们放置在buffer中。返回读到的字节数，或者-1(当错误发生时)

int writeToDevice(const char* buffer, std::streamsize length)

写入buffer中指定数目的字节。返回读到的字节数，或者-1(当错误发生时)  
  
  
          在Poco中从BasicBufferedStreamBuf继承的类见下图:

![20130426165819763][]

### 2.4 BasicBufferedBidirectionalStreamBuf ###

Poco::BasicBufferedBidirectionalStreamBuf一样是使用char作为模板参数类型的偏特化模板类。其完整定义为：

template <typename ch, typename tr, typename ba = BufferAllocator<ch> > 
    class BasicBufferedBidirectionalStreamBuf: public std::basic_streambuf<ch, tr>
    	/// This is an implementation of a buffered bidirectional 
    	/// streambuf that greatly simplifies the implementation of
    	/// custom streambufs of various kinds.
    	/// Derived classes only have to override the methods
    	/// readFromDevice() or writeToDevice().
    	///
    	/// In contrast to BasicBufferedStreambuf, this class supports
    	/// simultaneous read and write access, so in addition to
    	/// istream and ostream this streambuf can also be used
    	/// for implementing an iostream.
    {
    	// ....
    	std::streamsize _bufsize;
    	char_type*      _pReadBuffer;
    	char_type*      _pWriteBuffer;
    	openmode        _mode;
    }
    
    typedef BasicBufferedBidirectionalStreamBuf<char, std::char_traits<char> > BufferedBidirectionalStreamBuf;

在BasicBufferedBidirectionalStreamBuf内部存在两个缓冲，\_pReadBuffer和\_pWriteBuffer，作为缓冲，分别供输入或输出使用。  
          Poco::BasicBufferedBidirectionalStreamBuf的实例支持同时读写。从Poco::BasicBufferedBidirectionalStreamBuf继承的类需要重载readFromDevice和writeToDevice接口。  
  
          对应重载的要求如下：

int readFromDevice(char* buffer, std::streamsize length)

读取指定长度的字符，并把它们放置在buffer中。返回读到的字节数，或者-1(当错误发生时)

int writeToDevice(const char* buffer, std::streamsize length)

写入buffer中指定数目的字节。返回读到的字节数，或者-1(当错误发生时)  
  
          在Poco中从BasicBufferedBidirectionalStreamBuf继承的类见下图:

![20130426165843449][]

## 3. Poco::Base64Encoder和Poco::Base64Decoder ##

### 3.1 Base64编码的概念 ###

关于Base64编码的概念的概念出自 [wiki百科][wiki]。想要得到更加详细的资料，可以去看 [RFC 4648][]。

Base64是一种基于64个可打印字符来表示二进制数据的表示方法。由于2的6次方等于64，所以每6个位元为一个单元，对应某个可打印字符。三个字节有24个位元，对应于4个Base64单元，即3个字节需要用4个可打印字符来表示。它可用来作为电子邮件的传输编码。在Base64中的可打印字符包括字母A-Z、a-z、数字0-9 ，这样共有62个字符，此外两个可打印符号在不同的系统中而不同。  
          Base64常用于在通常处理文本数据的场合，表示、传输、存储一些二进制数据。包括MIME的email，email via MIME, 在XML中存储复杂数据.

**          Base64索引表：**

<table style="font-size:15px; margin:1em 0px; background-color:rgb(249,249,249); border:1px solid rgb(170,170,170); border-collapse:collapse; color:rgb(0,0,0); font-family:sans-serif; line-height:22.078125px; text-align:center"> 
 <tbody> 
  <tr> 
   <th style="border:1px solid rgb(170,170,170); padding:0.2em; background-color:rgb(242,242,242)"> Value</th> 
   <th style="border:1px solid rgb(170,170,170); padding:0.2em; background-color:rgb(242,242,242)"> Char</th> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">&nbsp;</td> 
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   <th style="border:1px solid rgb(170,170,170); padding:0.2em; background-color:rgb(242,242,242)"> Char</th> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">&nbsp;</td> 
   <th style="border:1px solid rgb(170,170,170); padding:0.2em; background-color:rgb(242,242,242)"> Value</th> 
   <th style="border:1px solid rgb(170,170,170); padding:0.2em; background-color:rgb(242,242,242)"> Char</th> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">&nbsp;</td> 
   <th style="border:1px solid rgb(170,170,170); padding:0.2em; background-color:rgb(242,242,242)"> Value</th> 
   <th style="border:1px solid rgb(170,170,170); padding:0.2em; background-color:rgb(242,242,242)"> Char</th> 
  </tr> 
  <tr> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">0</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">A</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">16</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">Q</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">32</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">g</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">48</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">w</td> 
  </tr> 
  <tr> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">1</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">B</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">17</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">R</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">33</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">h</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">49</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">x</td> 
  </tr> 
  <tr> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">2</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">C</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">18</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">S</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">34</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">i</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">50</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">y</td> 
  </tr> 
  <tr> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">3</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">D</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">19</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">T</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">35</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">j</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">51</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">z</td> 
  </tr> 
  <tr> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">4</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">E</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">20</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">U</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">36</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">k</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">52</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">0</td> 
  </tr> 
  <tr> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">5</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">F</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">21</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">V</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">37</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">l</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">53</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">1</td> 
  </tr> 
  <tr> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">6</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">G</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">22</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">W</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">38</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">m</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">54</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">2</td> 
  </tr> 
  <tr> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">7</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">H</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">23</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">X</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">39</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">n</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">55</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">3</td> 
  </tr> 
  <tr> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">8</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">I</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">24</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">Y</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">40</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">o</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">56</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">4</td> 
  </tr> 
  <tr> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">9</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">J</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">25</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">Z</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">41</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">p</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">57</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">5</td> 
  </tr> 
  <tr> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">10</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">K</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">26</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">a</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">42</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">q</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">58</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">6</td> 
  </tr> 
  <tr> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">11</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">L</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">27</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">b</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">43</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">r</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">59</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">7</td> 
  </tr> 
  <tr> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">12</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">M</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">28</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">c</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">44</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">s</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">60</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">8</td> 
  </tr> 
  <tr> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">13</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">N</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">29</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">d</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">45</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">t</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">61</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">9</td> 
  </tr> 
  <tr> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">14</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">O</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">30</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">e</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">46</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">u</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">62</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">+</td> 
  </tr> 
  <tr> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">15</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">P</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">31</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">f</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">47</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">v</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">63</td> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em">/</td> 
  </tr> 
 </tbody> 
</table>

**          例子：**  
          举例来说，一段引用自托马斯·霍布斯的利维坦的文句：

Man is distinguished, not only by his reason, but by this singular passion from other animals, which is a lust of the mind, that by a perseverance of delight in the continued and indefatigable generation of knowledge, exceeds the short vehemence of any carnal pleasure.

经过base64编码之后变成：

TWFuIGlzIGRpc3Rpbmd1aXNoZWQsIG5vdCBvbmx5IGJ5IGhpcyByZWFzb24sIGJ1dCBieSB0aGlz
    IHNpbmd1bGFyIHBhc3Npb24gZnJvbSBvdGhlciBhbmltYWxzLCB3aGljaCBpcyBhIGx1c3Qgb2Yg
    dGhlIG1pbmQsIHRoYXQgYnkgYSBwZXJzZXZlcmFuY2Ugb2YgZGVsaWdodCBpbiB0aGUgY29udGlu
    dWVkIGFuZCBpbmRlZmF0aWdhYmxlIGdlbmVyYXRpb24gb2Yga25vd2xlZGdlLCBleGNlZWRzIHRo
    ZSBzaG9ydCB2ZWhlbWVuY2Ugb2YgYW55IGNhcm5hbCBwbGVhc3VyZS4=

** 编码“Man”**

在此例中，Base64算法将三个字符编码为4个字符

基本上说，如果那天你看到一段以“=”或者"=="结束的字符，你就可以判断这段字符用了Base64编码。就应用而言，我只在邮件内容发送和MD5 或者SHA密码保存时见过Base64编码。

实现上Base64算法很简单，把流变成二进制，然后按6位转换为字符保存。

### 3.2 Poco::Base64Encode ###

Poco::Base64Encode的类图如下:

![20130427092310735][]

### 3.3 Poco::Base64Decoder ###

Poco::Base64Decoder的类图如下:

![20130427092304490][]

### 3.4 例子 ###

下面是一个例子

#include "Poco/Base64Encoder.h"
    #include <iostream>
    using Poco::Base64Encoder;
    int main(int argc, char** argv)
    {
           Base64Encoder encoder(std::cout);
           encoder << "Hello, world!";
           return 0;
    }

## 4. Poco::HexBinaryEncoder和Poco::HexBinaryDecoder ##

### 4.1 HexBinary编码编码概念 ###

HexBinary编码又叫Base 16编码，Base16是一种基于16个可打印字符来表示二进制数据的表示方法。由于2的4次方等于16，所以每4个位元为一个单元，对应某个可打印字符。三个字节有24个位元，对应于6个Base64单元，即3个字节需要用6个可打印字符来表示。

**          Base16索引表：**

The Base 16 Alphabet
    
             Value Encoding  Value Encoding  Value Encoding  Value Encoding
                 0 0             4 4             8 8            12 C
                 1 1             5 5             9 9            13 D
                 2 2             6 6            10 A            14 E
                 3 3             7 7            11 B            15 F

还是以编码“Man”举例：

<table>     
 <tbody> 
  <tr> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em"><strong>文本</strong></td> 
   <td colspan="8" style="border:1px solid rgb(170,170,170); padding:0.2em"> <strong>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; M</strong></td> 
   <td colspan="8" style="border:1px solid rgb(170,170,170); padding:0.2em"> <strong>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;a</strong></td> 
   <td colspan="8" style="border:1px solid rgb(170,170,170); padding:0.2em"> <strong>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;n</strong></td> 
  </tr> 
  <tr> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em"><strong>ASCII编码</strong></td> 
   <td colspan="8" style="border:1px solid rgb(170,170,170); padding:0.2em"> &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; 77</td> 
   <td colspan="8" style="border:1px solid rgb(170,170,170); padding:0.2em"> &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; 97</td> 
   <td colspan="8" style="border:1px solid rgb(170,170,170); padding:0.2em"> &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;110</td> 
  </tr> 
  <tr> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em"><strong>二进制位</strong></td> 
   <td align="right" style="border:1px solid rgb(170,170,170); padding:0.2em">0</td> 
   <td align="right" style="border:1px solid rgb(170,170,170); padding:0.2em">1</td> 
   <td align="right" style="border:1px solid rgb(170,170,170); padding:0.2em">0</td> 
   <td align="right" style="border:1px solid rgb(170,170,170); padding:0.2em">0</td> 
   <td align="right" style="border:1px solid rgb(170,170,170); padding:0.2em">1</td> 
   <td align="right" style="border:1px solid rgb(170,170,170); padding:0.2em">1</td> 
   <td align="right" style="border:1px solid rgb(170,170,170); padding:0.2em">0</td> 
   <td align="right" style="border:1px solid rgb(170,170,170); padding:0.2em">1</td> 
   <td align="right" style="border:1px solid rgb(170,170,170); padding:0.2em">0</td> 
   <td align="right" style="border:1px solid rgb(170,170,170); padding:0.2em">1</td> 
   <td align="right" style="border:1px solid rgb(170,170,170); padding:0.2em">1</td> 
   <td align="right" style="border:1px solid rgb(170,170,170); padding:0.2em">0</td> 
   <td align="right" style="border:1px solid rgb(170,170,170); padding:0.2em">0</td> 
   <td align="right" style="border:1px solid rgb(170,170,170); padding:0.2em">0</td> 
   <td align="right" style="border:1px solid rgb(170,170,170); padding:0.2em">0</td> 
   <td align="right" style="border:1px solid rgb(170,170,170); padding:0.2em">1</td> 
   <td align="right" style="border:1px solid rgb(170,170,170); padding:0.2em">0</td> 
   <td align="right" style="border:1px solid rgb(170,170,170); padding:0.2em">1</td> 
   <td align="right" style="border:1px solid rgb(170,170,170); padding:0.2em">1</td> 
   <td align="right" style="border:1px solid rgb(170,170,170); padding:0.2em">0</td> 
   <td align="right" style="border:1px solid rgb(170,170,170); padding:0.2em">1</td> 
   <td align="right" style="border:1px solid rgb(170,170,170); padding:0.2em">1</td> 
   <td align="right" style="border:1px solid rgb(170,170,170); padding:0.2em">1</td> 
   <td align="right" style="border:1px solid rgb(170,170,170); padding:0.2em">0</td> 
  </tr> 
  <tr> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em"><strong>索引</strong></td> 
   <td colspan="4" style="border:1px solid rgb(170,170,170); padding:0.2em">&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; 4</td> 
   <td colspan="4" style="border:1px solid rgb(170,170,170); padding:0.2em">&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;13</td> 
   <td colspan="4" style="border:1px solid rgb(170,170,170); padding:0.2em">&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;6</td> 
   <td colspan="4" style="border:1px solid rgb(170,170,170); padding:0.2em">&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;1&nbsp;</td> 
   <td colspan="4" style="border:1px solid rgb(170,170,170); padding:0.2em">&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;6</td> 
   <td colspan="4" style="border:1px solid rgb(170,170,170); padding:0.2em">&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; 14</td> 
  </tr> 
  <tr> 
   <td style="border:1px solid rgb(170,170,170); padding:0.2em"><strong>base 16编码</strong></td> 
   <td colspan="4" style="border:1px solid rgb(170,170,170); padding:0.2em"><strong>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; 4</strong></td> 
   <td colspan="4" style="border:1px solid rgb(170,170,170); padding:0.2em"><strong>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; D</strong></td> 
   <td colspan="4" style="border:1px solid rgb(170,170,170); padding:0.2em"><strong>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;6</strong></td> 
   <td colspan="4" style="border:1px solid rgb(170,170,170); padding:0.2em"><strong>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;1</strong></td> 
   <td colspan="4" style="border:1px solid rgb(170,170,170); padding:0.2em"><strong>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;6</strong></td> 
   <td colspan="4" style="border:1px solid rgb(170,170,170); padding:0.2em"><strong>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; E</strong></td> 
  </tr> 
 </tbody> 
</table>

实现上Base16算法很简单，把流变成二进制，然后按4位转换为字符保存。

### 4.2 Poco::HexBinaryEncoder和Poco::HexBinaryDecoder ###

###      Poco::HexBinaryEncoder 和Poco::HexBinaryDecoder的类图同Poco::Base64Encoder和Poco::Base64Decoder是类似。 ###

###        HexBinaryDecoderBuf定义如下： ###

class Foundation_API HexBinaryDecoderBuf: public UnbufferedStreamBuf
    	/// This streambuf decodes all hexBinary-encoded data read
    	/// from the istream connected to it.
    	/// In hexBinary encoding, each binary octet is encoded as a character tuple,  
    	/// consisting of two hexadecimal digits ([0-9a-fA-F]) representing the octet code.
    	/// See also: XML Schema Part 2: Datatypes (http://www.w3.org/TR/xmlschema-2/),
    	/// section 3.2.15.
    	///
    	/// Note: For performance reasons, the characters 
    	/// are read directly from the given istream's 
    	/// underlying streambuf, so the state
    	/// of the istream will not reflect that of
    	/// its streambuf.
    {
    public:
    	HexBinaryDecoderBuf(std::istream& istr);
    	~HexBinaryDecoderBuf();
    	
    private:
    	int readFromDevice();
    	int readOne();
    
    	std::streambuf& _buf;
    };

###  ###

###        构造文件为： ###

HexBinaryDecoderBuf::HexBinaryDecoderBuf(std::istream& istr): 
    	_buf(*istr.rdbuf())
    {
    }

###  ###

###        我们看到 HexBinaryDecoderBuf直接使用了istream对象内部的缓冲作为实现。同2.1节中的例子相比，可以减少函数调用的流程. ###

[20130426165751763]: /images/20220309/0cc2c484990049008d6af04d521c1955.png
[20130426165819763]: /images/20220309/156864d6561740c59b9357a38cacbdbd.png
[20130426165843449]: /images/20220309/17ccf5f8a83546028d904f969d80ef53.png
[wiki]: http://zh.wikipedia.org/wiki/Base64
[RFC 4648]: http://datatracker.ietf.org/doc/rfc4648/?include_text=1
[20130427092310735]: /images/20220309/fb49efc4e7ca44fc9f09d8bad4f220eb.png
[20130427092304490]: /images/20220309/d31ffab59a164c269ff2ba0a0996f6d1.png
[http_blog.csdn.net_arau_sh_article_details_8851662]: http://blog.csdn.net/arau_sh/article/details/8851662