一、头文件解析
iostream标准输入输出流
sstream 字符串输入输出流
头文件climits定义了符号常量来表示类型的限制，编译器厂商提供了climits文件，该文件支持了其编译器中的值。例如，在使用16位int的老系统中，climits文件将INT_MAX定义为32676。
#include "base64_kernel_1.h"
#include <iostream>
#include <sstream>
#include <climits>fstream 文件输入输出流
二、换行符CR,LF和CRLF
CR：\r 表示回车，MacIntosh操作系统（即早期的Mac操作系统）采用单个字符CR来进行换行
LF：\n 转义字符表示换行，Unix/Linux/Mac OS X操作系统采用单个字符LF来进行换行
CRLF：\r\n 表示回车并换行，Windows操作系统采用两个字符来进行换行，即CRLF
enum line_ending_type
{
CR, // i.e. "\r"
LF, // i.e. "\n"
CRLF // i.e. "\r\n"
};
// ----------------------------------------------------------------------------------------
base64::line_ending_type base64::
line_ending (
) const
{
return eol_style;
}
// ----------------------------------------------------------------------------------------
void base64::
set_line_ending (
line_ending_type eol_style_
)
{
eol_style = eol_style_;
}
// ----------------------------------------------------------------------------------------根据RFC822规定,BASE64Encoder编码每76个字符,还需要加上一个回车换行，所以dlib这里定义了换行符。
三、构造函数
这里组织了编码、解码的字符数组，下面是关于初始值的说明。
/*!
INITIAL VALUE
- bad_value == 100
- encode_table == a pointer to an array of 64 chars
- where x is a 6 bit value the following is true:
- encode_table[x] == the base64 encoding of x
- decode_table == a pointer to an array of UCHAR_MAX chars
- where x is any char value:
- if (x is a valid character in the base64 coding scheme) then
- decode_table[x] == the 6 bit value that x encodes
- else
- decode_table[x] == bad_value
CONVENTION
- The state of this object never changes so just refer to its
initial value.
!*/源码代码
base64::
base64 (
) :
encode_table(0),
decode_table(0),
bad_value(100),
eol_style(LF)
{
try
{
encode_table = new char[64];
decode_table = new unsigned char[UCHAR_MAX];
}
catch (...)
{
if (encode_table) delete [] encode_table;
if (decode_table) delete [] decode_table;
throw;
}
// now set up the tables with the right stuff
encode_table[0] = 'A';
encode_table[17] = 'R';
encode_table[34] = 'i';
encode_table[51] = 'z';
encode_table[1] = 'B';
encode_table[18] = 'S';
encode_table[35] = 'j';
encode_table[52] = '0';
encode_table[2] = 'C';
encode_table[19] = 'T';
encode_table[36] = 'k';
encode_table[53] = '1';
encode_table[3] = 'D';
encode_table[20] = 'U';
encode_table[37] = 'l';
encode_table[54] = '2';
encode_table[4] = 'E';
encode_table[21] = 'V';
encode_table[38] = 'm';
encode_table[55] = '3';
encode_table[5] = 'F';
encode_table[22] = 'W';
encode_table[39] = 'n';
encode_table[56] = '4';
encode_table[6] = 'G';
encode_table[23] = 'X';
encode_table[40] = 'o';
encode_table[57] = '5';
encode_table[7] = 'H';
encode_table[24] = 'Y';
encode_table[41] = 'p';
encode_table[58] = '6';
encode_table[8] = 'I';
encode_table[25] = 'Z';
encode_table[42] = 'q';
encode_table[59] = '7';
encode_table[9] = 'J';
encode_table[26] = 'a';
encode_table[43] = 'r';
encode_table[60] = '8';
encode_table[10] = 'K';
encode_table[27] = 'b';
encode_table[44] = 's';
encode_table[61] = '9';
encode_table[11] = 'L';
encode_table[28] = 'c';
encode_table[45] = 't';
encode_table[62] = '+';
encode_table[12] = 'M';
encode_table[29] = 'd';
encode_table[46] = 'u';
encode_table[63] = '/';
encode_table[13] = 'N';
encode_table[30] = 'e';
encode_table[47] = 'v';
encode_table[14] = 'O';
encode_table[31] = 'f';
encode_table[48] = 'w';
encode_table[15] = 'P';
encode_table[32] = 'g';
encode_table[49] = 'x';
encode_table[16] = 'Q';
encode_table[33] = 'h';
encode_table[50] = 'y';
// we can now fill out the decode_table by using the encode_table
for (int i = 0; i < UCHAR_MAX; ++i)
{
decode_table[i] = bad_value;
}
for (unsigned char i = 0; i < 64; ++i)
{
decode_table[(unsigned char)encode_table[i]] = i;
}
}
四、析构函数
这里删除了构造函数里面的数组。
base64::
~base64 (
)
{
delete [] encode_table;
delete [] decode_table;
}五、编码函数
1、这里首先了解streambuf::sgetn，是用于从streambuf对象的缓冲区中获取字符序列，获取 get 指针后面的nCount个字符并将它们存储在从pch开始的区域中。当streambuf对象中剩余的字符少于nCount时， sgetn获取剩余的任何字符。该函数重新定位 get 指针以跟随获取的字符。
2、每76个字符,还需要加上一个回车换行，所以看到counter = 19，这里的76个字符是指写入到输出流的。
void base64::
encode (
std::istream& in_,
std::ostream& out_
) const
{
using namespace std;
streambuf& in = *in_.rdbuf();
streambuf& out = *out_.rdbuf();
unsigned char inbuf[3];
unsigned char outbuf[4];
streamsize status = in.sgetn(reinterpret_cast<char*>(&inbuf),3);
unsigned char c1, c2, c3, c4, c5, c6;
int counter = 19;
// while we haven't hit the end of the input stream
while (status != 0)
{
if (counter == 0)
{
counter = 19;
// write a newline
char ch;
switch (eol_style)
{
case CR:
ch = '\r';
if (out.sputn(&ch,1)!=1)
throw std::ios_base::failure("error occurred in the base64 object");
break;
case LF:
ch = '\n';
if (out.sputn(&ch,1)!=1)
throw std::ios_base::failure("error occurred in the base64 object");
break;
case CRLF:
ch = '\r';
if (out.sputn(&ch,1)!=1)
throw std::ios_base::failure("error occurred in the base64 object");
ch = '\n';
if (out.sputn(&ch,1)!=1)
throw std::ios_base::failure("error occurred in the base64 object");
break;
default:
DLIB_CASSERT(false,"this should never happen");
}
}
--counter;
if (status == 3)
{
// encode the bytes in inbuf to base64 and write them to the output stream
c1 = inbuf[0]&0xfc;
c2 = inbuf[0]&0x03;
c3 = inbuf[1]&0xf0;
c4 = inbuf[1]&0x0f;
c5 = inbuf[2]&0xc0;
c6 = inbuf[2]&0x3f;
outbuf[0] = c1>>2;
outbuf[1] = (c2<<4)|(c3>>4);
outbuf[2] = (c4<<2)|(c5>>6);
outbuf[3] = c6;
outbuf[0] = encode_table[outbuf[0]];
outbuf[1] = encode_table[outbuf[1]];
outbuf[2] = encode_table[outbuf[2]];
outbuf[3] = encode_table[outbuf[3]];
// write the encoded bytes to the output stream
if (out.sputn(reinterpret_cast<char*>(&outbuf),4)!=4)
{
throw std::ios_base::failure("error occurred in the base64 object");
}
// get 3 more input bytes
status = in.sgetn(reinterpret_cast<char*>(&inbuf),3);
continue;
}
else if (status == 2)
{
// we are at the end of the input stream and need to add some padding
// encode the bytes in inbuf to base64 and write them to the output stream
c1 = inbuf[0]&0xfc;
c2 = inbuf[0]&0x03;
c3 = inbuf[1]&0xf0;
c4 = inbuf[1]&0x0f;
c5 = 0;
outbuf[0] = c1>>2;
outbuf[1] = (c2<<4)|(c3>>4);
outbuf[2] = (c4<<2)|(c5>>6);
outbuf[3] = '=';
outbuf[0] = encode_table[outbuf[0]];
outbuf[1] = encode_table[outbuf[1]];
outbuf[2] = encode_table[outbuf[2]];
// write the encoded bytes to the output stream
if (out.sputn(reinterpret_cast<char*>(&outbuf),4)!=4)
{
throw std::ios_base::failure("error occurred in the base64 object");
}
break;
}
else // in this case status must be 1
{
// we are at the end of the input stream and need to add some padding
// encode the bytes in inbuf to base64 and write them to the output stream
c1 = inbuf[0]&0xfc;
c2 = inbuf[0]&0x03;
c3 = 0;
outbuf[0] = c1>>2;
outbuf[1] = (c2<<4)|(c3>>4);
outbuf[2] = '=';
outbuf[3] = '=';
outbuf[0] = encode_table[outbuf[0]];
outbuf[1] = encode_table[outbuf[1]];
// write the encoded bytes to the output stream
if (out.sputn(reinterpret_cast<char*>(&outbuf),4)!=4)
{
throw std::ios_base::failure("error occurred in the base64 object");
}
break;
}
} // while (status != 0)
// make sure the stream buffer flushes to its I/O channel
out.pubsync();
}六、解码函数
void base64::
decode (
std::istream& in_,
std::ostream& out_
) const
{
using namespace std;
streambuf& in = *in_.rdbuf();
streambuf& out = *out_.rdbuf();
unsigned char inbuf[4];
unsigned char outbuf[3];
int inbuf_pos = 0;
streamsize status = in.sgetn(reinterpret_cast<char*>(inbuf),1);
// only count this character if it isn't some kind of filler
if (status == 1 && decode_table[inbuf[0]] != bad_value )
++inbuf_pos;
unsigned char c1, c2, c3, c4, c5, c6;
streamsize outsize;
// while we haven't hit the end of the input stream
while (status != 0)
{
// if we have 4 valid characters
if (inbuf_pos == 4)
{
inbuf_pos = 0;
// this might be the end of the encoded data so we need to figure out if
// there was any padding applied.
outsize = 3;
if (inbuf[3] == '=')
{
if (inbuf[2] == '=')
outsize = 1;
else
outsize = 2;
}
// decode the incoming characters
inbuf[0] = decode_table[inbuf[0]];
inbuf[1] = decode_table[inbuf[1]];
inbuf[2] = decode_table[inbuf[2]];
inbuf[3] = decode_table[inbuf[3]];
// now pack these guys into bytes rather than 6 bit chunks
c1 = inbuf[0]<<2;
c2 = inbuf[1]>>4;
c3 = inbuf[1]<<4;
c4 = inbuf[2]>>2;
c5 = inbuf[2]<<6;
c6 = inbuf[3];
outbuf[0] = c1|c2;
outbuf[1] = c3|c4;
outbuf[2] = c5|c6;
// write the encoded bytes to the output stream
if (out.sputn(reinterpret_cast<char*>(&outbuf),outsize)!=outsize)
{
throw std::ios_base::failure("error occurred in the base64 object");
}
}
// get more input characters
status = in.sgetn(reinterpret_cast<char*>(inbuf + inbuf_pos),1);
// only count this character if it isn't some kind of filler
if ((decode_table[inbuf[inbuf_pos]] != bad_value || inbuf[inbuf_pos] == '=') &&
status != 0)
++inbuf_pos;
} // while (status != 0)
if (inbuf_pos != 0)
{
ostringstream sout;
sout << inbuf_pos << " extra characters were found at the end of the encoded data."
<< " This may indicate that the data stream has been truncated.";
// this happens if we hit EOF in the middle of decoding a 24bit block.
throw decode_error(sout.str());
}
// make sure the stream buffer flushes to its I/O channel
out.pubsync();
}
https://github.com/davisking/dlib/tree/master/dlib/base64