datastream解析

　　在EOS的eosiolib模块中有一个datasteam.hpp文件，它几乎实现了所有类型对字节流的转换，是一个非常强大的工具类，在这里对它的做一个简单的提取，也加强一下自己对它的理解。在下面的工程中有三个头文件与EOS源码对应如下：

　　#include"datastream.h"     -->  eoscontractseosiolibdatastream.hpp　　//去掉了异常，改为代码处理
　　#include"serialize.h"　　　　-->　  eoscontractseosiolibserialize.hpp   //未修改
    #include"varint.h"　　　　　　-->　　eoscontractseosiolibvarint.hpp　　　//未修改

示例代码编译环境： ubuntu 16.04  boost 1.67  Qt Creator

下面我们从我的简单实例进行分析，相信理解了这部分原题，eos中的datastream也自己可以进行修改了。

先来看看我们需要实现的功能，main函数如下编写：

 1 #include <iostream>
 2 #include<vector>
 3 #include<functional>
 4 #include<algorithm>
 5 #include <iterator>
 6 #include<string>
 7 #include<vector>
 8 #include<set>
 9 #include<map>
10 #include"datastream.h"
11 #include"serialize.h"
12 
13 class CBase{
14 public:
15     std::string m_strBase = "bright";
16     std::vector<char> m_vcBase;
17 
18     EOSLIB_SERIALIZE( CBase, (m_strBase)(m_vcBase) )
19 };
20 
21 class CDerive:public CBase {
22 public:
23      std::string m_strDerive = "Derive";
24      std::set<std::string> m_derSet;
25 
26     EOSLIB_SERIALIZE_DERIVED( CDerive, CBase, (m_strDerive) (m_derSet) )
27 };
28 
29 void printderive(const CDerive& derive)
30 {
31     std::cout << derive.m_strBase.data() << " " << derive.m_strDerive.data() << std::endl;
32     copy(derive.m_vcBase.begin(), derive.m_vcBase.end(), std::ostream_iterator<char>(std::cout, " "));
33     std::cout << "
";
34     copy(derive.m_derSet.begin(), derive.m_derSet.end(), std::ostream_iterator<std::string>(std::cout, " "));
35 }
36 
37 int main()
38 {
39     CDerive derive;
40     derive.m_vcBase = { 'a', 'b' , 'c', 'd', 'e', 'f', 'g', 'h', 'i'};
41     derive.m_derSet = {"one", "two", "three", "four", "five"};
42 
43     printderive(derive);
44 
45     bytes packed_derive = pack(derive);
46     size_t size = packed_derive.size();
47     int32_t data = 0;
48 
49      std::cout << "
";
50     for(bytes::iterator iter = packed_derive.begin(); iter !=packed_derive.end(); ++iter)
51     {
52         data = *iter;
53         if(data <'a')
54         {
55             std::cout << data << " ";
56         }
57         else
58         {
59             std::cout << *iter << " ";
60         }
61     }
62 
63     std::cout << "
";
64 
65     CDerive  copy_derive;
66     copy_derive = unpack<CDerive>(packed_derive);
67     printderive(copy_derive);
68 
69     return 0;
70 }

　　运行后的打印信息如下

bright Derive
a b c d e f g h i 
five four one three two 
6 b r i g h t 9 a b c d e f g h i 6 68 e r i v e 5 4 f i v e 4 f o u r 3 o n e 5 t h r e e 3 t w o 
bright Derive
a b c d e f g h i 
five four one three two

　　从第四行的打印信息我们可以清楚地看到类对象的数据变成了一种格式：长度+内容。所有的数据从基数开始依次被放入到流中，在前面加入了长度，而对于容器类型std::set<std::string>，首先会记录set的实际数据长度，再记录string的长度。因此，在我里我们就可以联想到，任何一种数据我们都可以按照自己的想法去转化成datastream，如map,tuple,deque及自定义类型。那在上面的功能中最关键的为两个宏:EOSLIB_SERIALIZE与EOSLIB_SERIALIZE_DERIVED。他们如何实现的呢？请看serialize.h文件的实现：

 1 #ifndef SERIALIZE_H
 2 #define SERIALIZE_H
 3 #include<iostream>
 4 #include<string>
 5 #include"datastream.h"
 6 #include <boost/preprocessor/seq/enum.hpp>
 7 #include <boost/preprocessor/seq/size.hpp>
 8 #include <boost/preprocessor/seq/seq.hpp>
 9 #include<boost/preprocessor/seq/for_each.hpp>
10 
11 
12 #define EOSLIB_REFLECT_MEMBER_OP( r, OP, elem ) 
13   OP t.elem
14 
15 #define EOSLIB_SERIALIZE( TYPE,  MEMBERS ) 
16  template<typename DataStream> 
17  friend DataStream& operator << ( DataStream& ds, const TYPE& t ){ 
18     return ds BOOST_PP_SEQ_FOR_EACH( EOSLIB_REFLECT_MEMBER_OP, <<, MEMBERS );
19  }
20  template<typename DataStream> 
21  friend DataStream& operator >> ( DataStream& ds, TYPE& t ){ 
22     return ds BOOST_PP_SEQ_FOR_EACH( EOSLIB_REFLECT_MEMBER_OP, >>, MEMBERS );
23  }
24 
25 #define EOSLIB_SERIALIZE_DERIVED( TYPE, BASE, MEMBERS ) 
26  template<typename DataStream> 
27  friend DataStream& operator << ( DataStream& ds, const TYPE& t ){ 
28     ds << static_cast<const BASE&>(t); 
29     return ds BOOST_PP_SEQ_FOR_EACH( EOSLIB_REFLECT_MEMBER_OP, <<, MEMBERS );
30  }
31  template<typename DataStream> 
32  friend DataStream& operator >> ( DataStream& ds, TYPE& t ){ 
33     ds >> static_cast<BASE&>(t); 
34     return ds BOOST_PP_SEQ_FOR_EACH( EOSLIB_REFLECT_MEMBER_OP, >>, MEMBERS );
35  }
36 
37 #endif // SERIALIZE_H

把main.cpp文件用上面的宏替换，可以看出其实就是在每个类中实现了自己的输入输出流。其中 BOOST_PP_SEQ_FOR_EACH( EOSLIB_REFLECT_MEMBER_OP, >>, MEMBERS ); 的意思是把对象的所需要的多个成员变量按宏依次展开。比如在CBase中的此句

　　ds BOOST_PP_SEQ_FOR_EACH( EOSLIB_REFLECT_MEMBER_OP, <<, MEMBERS );

会替换成：

　　ds << CBase.m_strBase << CBase.m_vcBase ;

因为返回值为datastream,故会循环调用直至无成员变量。另外，还有以下两个特点：

1.派生类的会调用基类的，直到最底层的基类(多重继承我没有测试过)；

2.在ds << CBase.m_strBase << CBase.m_vcBase ; 中，类中的成员变量使用<<和 >>时是需要自己定义类型的转换的，即类中的成员变量和datastream是如何相互转换的，这也是我们接下来讨论的问题。

datastream的实现如下：

  1 #ifndef DATASTREAM_H
  2 #define DATASTREAM_H
  3 
  4 #include<iostream>
  5 #include<stdint.h>
  6 #include<memory>
  7 #include<cstring>
  8 #include<vector>
  9 #include<set>
 10 #include<map>
 11 #include"serialize.h"
 12 #include"varint.h"
 13 
 14  
 15 template<typename T>
 16 class datastream {
 17 public:
 18     datastream(T start, size_t s)
 19         :_start(start),_pos(start),_end(start + s) {}
 20 
 21     inline void skip( size_t s) { _pos += s; }
 22     inline bool read( char* d, size_t s){
 23         if( size_t(_end - _pos) < (size_t)s )
 24         {
 25             return false;
 26         }
 27 
 28         memcpy(d, _pos, s);
 29         _pos += s;
 30 
 31         return true;
 32     }
 33 
 34     inline bool write(const char* d, size_t s){
 35         if(_end -_pos < (int32_t)s )
 36         {
 37             return false;
 38         }
 39         memcpy((void*)_pos, d, s);
 40         _pos += s;
 41 
 42         return true;
 43     }
 44 
 45     inline bool put(char c) {
 46         if(_pos >= _end) {
 47             return false;
 48         }
 49         *_pos = c;
 50         ++_pos;
 51         return true;
 52       }
 53 
 54     inline bool get( unsigned char& c ) { return get( *(char*)&c ); }
 55 
 56     inline bool get( char& c )
 57       {
 58         if(_pos >= _end)
 59         {
 60             return false;
 61         }
 62 
 63         c = *_pos;
 64         ++_pos;
 65         return true;
 66       }
 67 
 68     T pos()const { return _pos; }
 69     inline bool valid()const { return _pos <= _end && _pos >= _start;  }
 70 
 71     inline bool seekp(size_t p) { _pos = _start + p; return _pos <= _end; }
 72 
 73     inline size_t tellp()const      { return size_t(_pos - _start); }
 74 
 75     inline size_t remaining()const  { return _end - _pos; }
 76 
 77 private:
 78     T _start;
 79     T _pos;
 80     T _end;
 81 };
 82 
 83 template<>
 84 class datastream<size_t> {
 85    public:
 86      datastream( size_t init_size = 0):_size(init_size){}
 87      inline bool     skip( size_t s )                 { _size += s; return true;  }
 88      inline bool     write( const char* ,size_t s )  { _size += s; return true;  }
 89      inline bool     put(char )                      { ++_size; return  true;    }
 90      inline bool     valid()const                     { return true;              }
 91      inline bool     seekp(size_t p)                  { _size = p;  return true;  }
 92      inline size_t   tellp()const                     { return _size;             }
 93      inline size_t   remaining()const                 { return 0;                 }
 94   private:
 95      size_t _size;
 96 };
 97 
 98 typedef std::vector<char> bytes;
 99 
100 template<typename DataStream>
101 DataStream& operator << ( DataStream& ds, const std::string& v ) {
102    ds << unsigned_int( v.size() );
103    if (v.size())
104       ds.write(v.data(), v.size());
105    return ds;
106 }
107 
108 template<typename DataStream>
109 DataStream& operator >> ( DataStream& ds, std::string& v ) {
110    std::vector<char> tmp;
111    ds >> tmp;
112    if( tmp.size() )
113       v = std::string(tmp.data(),tmp.data()+tmp.size());
114    else
115       v = std::string();
116    return ds;
117 }
118 
119 template<typename DataStream, typename T>
120 DataStream& operator << ( DataStream& ds, const std::vector<T>& v ) {
121    ds << unsigned_int( v.size() );
122    for( const auto& i : v )
123       ds << i;
124    return ds;
125 }
126 
127 template<typename DataStream>
128 DataStream& operator << ( DataStream& ds, const std::vector<char>& v ) {
129    ds << unsigned_int( v.size() );
130    ds.write( v.data(), v.size() );
131    return ds;
132 }
133 
134 template<typename DataStream, typename T>
135 DataStream& operator >> ( DataStream& ds, std::vector<T>& v ) {
136    unsigned_int s;
137    ds >> s;
138    v.resize(s.value);
139    for( auto& i : v )
140       ds >> i;
141    return ds;
142 }
143 
144 template<typename DataStream>
145 DataStream& operator >> ( DataStream& ds, std::vector<char>& v ) {
146    unsigned_int s;
147    ds >> s;
148    v.resize( s.value );
149    ds.read( v.data(), v.size() );
150    return ds;
151 }
152 
153 template<typename DataStream, typename T>
154 DataStream& operator << ( DataStream& ds, const std::set<T>& s ) {
155    ds << unsigned_int( s.size() );
156    for( const auto& i : s ) {
157       ds << i;
158    }
159    return ds;
160 }
161 
162 template<typename DataStream, typename T>
163 DataStream& operator >> ( DataStream& ds, std::set<T>& s ) {
164    s.clear();
165    unsigned_int sz; ds >> sz;
166 
167    for( uint32_t i = 0; i < sz.value; ++i ) {
168       T v;
169       ds >> v;
170       s.emplace( std::move(v) );
171    }
172    return ds;
173 }
174 
175 template<typename DataStream, typename K, typename V>
176 DataStream& operator << ( DataStream& ds, const std::map<K,V>& m ) {
177    ds << unsigned_int( m.size() );
178    for( const auto& i : m ) {
179       ds << i.first << i.second;
180    }
181    return ds;
182 }
183 
184 template<typename DataStream, typename K, typename V>
185 DataStream& operator >> ( DataStream& ds, std::map<K,V>& m ) {
186    m.clear();
187    unsigned_int s; ds >> s;
188 
189    for (uint32_t i = 0; i < s.value; ++i) {
190       K k; V v;
191       ds >> k >> v;
192       m.emplace( std::move(k), std::move(v) );
193    }
194    return ds;
195 }
196 
197 
198 template<typename T>
199 size_t pack_size( const T& value ) {
200   datastream<size_t> ps;
201   ps << value;
202   return ps.tellp();
203 }
204 
205 template<typename T>
206 bytes pack( const T& value ) {
207   bytes result;
208   result.resize(pack_size(value));
209 
210   datastream<char*> ds( result.data(), result.size() );
211   ds << value;
212   return result;
213 }
214 
215 template<typename T>
216 T unpack( const char* buffer, size_t len ) {
217    T result;
218    datastream<const char*> ds(buffer,len);
219    ds >> result;
220    return result;
221 }
222 
223 template<typename T>
224  T unpack( const std::vector<char>& bytes ) {
225    return unpack<T>( bytes.data(), bytes.size() );
226 }
227 
228 #endif // DATASTREAM_H

上面的datastream文件我只提取了极少的一部分，实际上EOS几乎已经实现了所有类型与datastream的相互转换。datastream用一个泛化版本和特化版本。特化版本中定义了当前使用类型的一个副本并可以使用指针偏移访问任何位置。而我们的CBase,CDerive的成员变量使用了string,vector,set类型，所以我们自己在这里增加了这三个类型与datastream数据流转换函数，如果我们需要使用map类型的成员，那么我们在这个文件里也要增加相应的转换类型，自定义的也是如此。最后，还有一个依赖的头文件内容如下：

  1 struct unsigned_int {
  2     unsigned_int( uint32_t v = 0 ):value(v){}
  3 
  4     template<typename T>
  5     unsigned_int( T v ):value(v){}
  6 
  7     template<typename T>
  8     operator T()const { return static_cast<T>(value); }
  9 
 10     unsigned_int& operator=( uint32_t v ) { value = v; return *this; }
 11 
 12     uint32_t value;
 13 
 14     friend bool operator==( const unsigned_int& i, const uint32_t& v )     { return i.value == v; }
 15     friend bool operator==( const uint32_t& i, const unsigned_int& v )     { return i       == v.value; }
 16     friend bool operator==( const unsigned_int& i, const unsigned_int& v ) { return i.value == v.value; }
 17 
 18     friend bool operator!=( const unsigned_int& i, const uint32_t& v )     { return i.value != v; }
 19     friend bool operator!=( const uint32_t& i, const unsigned_int& v )     { return i       != v.value; }
 20     friend bool operator!=( const unsigned_int& i, const unsigned_int& v ) { return i.value != v.value; }
 21 
 22     friend bool operator<( const unsigned_int& i, const uint32_t& v )      { return i.value < v; }
 23     friend bool operator<( const uint32_t& i, const unsigned_int& v )      { return i       < v.value; }
 24     friend bool operator<( const unsigned_int& i, const unsigned_int& v )  { return i.value < v.value; }
 25 
 26     friend bool operator>=( const unsigned_int& i, const uint32_t& v )     { return i.value >= v; }
 27     friend bool operator>=( const uint32_t& i, const unsigned_int& v )     { return i       >= v.value; }
 28     friend bool operator>=( const unsigned_int& i, const unsigned_int& v ) { return i.value >= v.value; }
 29     template<typename DataStream>
 30     friend DataStream& operator << ( DataStream& ds, const unsigned_int& v ){
 31        uint64_t val = v.value;
 32        do {
 33           uint8_t b = uint8_t(val) & 0x7f;
 34           val >>= 7;
 35           b |= ((val > 0) << 7);
 36           ds.write((char*)&b,1);//.put(b);
 37        } while( val );
 38        return ds;
 39     }
 40 
 41     template<typename DataStream>
 42     friend DataStream& operator >> ( DataStream& ds, unsigned_int& vi ){
 43       uint64_t v = 0; char b = 0; uint8_t by = 0;
 44       do {
 45          ds.get(b);
 46          v |= uint32_t(uint8_t(b) & 0x7f) << by;
 47          by += 7;
 48       } while( uint8_t(b) & 0x80 );
 49       vi.value = static_cast<uint32_t>(v);
 50       return ds;
 51     }
 52 };
 53 
 54 struct signed_int {
 55     signed_int( int32_t v = 0 ):value(v){}
 56     operator int32_t()const { return value; }
 57     template<typename T>
 58     signed_int& operator=( const T& v ) { value = v; return *this; }
 59     signed_int operator++(int) { return value++; }
 60     signed_int& operator++(){ ++value; return *this; }
 61 
 62     int32_t value;
 63 
 64     friend bool operator==( const signed_int& i, const int32_t& v )    { return i.value == v; }
 65     friend bool operator==( const int32_t& i, const signed_int& v )    { return i       == v.value; }
 66     friend bool operator==( const signed_int& i, const signed_int& v ) { return i.value == v.value; }
 67 
 68     friend bool operator!=( const signed_int& i, const int32_t& v )    { return i.value != v; }
 69     friend bool operator!=( const int32_t& i, const signed_int& v )    { return i       != v.value; }
 70     friend bool operator!=( const signed_int& i, const signed_int& v ) { return i.value != v.value; }
 71 
 72     friend bool operator<( const signed_int& i, const int32_t& v )     { return i.value < v; }
 73     friend bool operator<( const int32_t& i, const signed_int& v )     { return i       < v.value; }
 74     friend bool operator<( const signed_int& i, const signed_int& v )  { return i.value < v.value; }
 75 
 76     friend bool operator>=( const signed_int& i, const int32_t& v )    { return i.value >= v; }
 77     friend bool operator>=( const int32_t& i, const signed_int& v )    { return i       >= v.value; }
 78     friend bool operator>=( const signed_int& i, const signed_int& v ) { return i.value >= v.value; }
 79 
 80     template<typename DataStream>
 81     friend DataStream& operator << ( DataStream& ds, const signed_int& v ){
 82       uint32_t val = uint32_t((v.value<<1) ^ (v.value>>31));
 83       do {
 84          uint8_t b = uint8_t(val) & 0x7f;
 85          val >>= 7;
 86          b |= ((val > 0) << 7);
 87          ds.write((char*)&b,1);//.put(b);
 88       } while( val );
 89        return ds;
 90     }
 91     template<typename DataStream>
 92     friend DataStream& operator >> ( DataStream& ds, signed_int& vi ){
 93       uint32_t v = 0; char b = 0; int by = 0;
 94       do {
 95          ds.get(b);
 96          v |= uint32_t(uint8_t(b) & 0x7f) << by;
 97          by += 7;
 98       } while( uint8_t(b) & 0x80 );
 99       vi.value = ((v>>1) ^ (v>>31)) + (v&0x01);
100       vi.value = v&0x01 ? vi.value : -vi.value;
101       vi.value = -vi.value;
102       return ds;
103     }
104 };

其中包括一些类型转换以及数据长度的计算，如此简单。

相关阅读:
vue 开发环境搭建
 签名算法
 MySQL explain执行计划优化
 查看运行时某个java对象占用JVM大小及通过idea查看java的内存占用情况
 jenkins 配置
 CSS布局
 《JavaScript 代码优化指南》
JavaScript 跨域漫游
 iFrame 功能详解
 粘性的底部布局
原文地址：https://www.cnblogs.com/hbright/p/9224874.html