将 unsigned char * 转换为 hexstring

Question

下面的代码采用一个十六进制字符串（每个字节都表示为其对应的十六进制值）将其转换为 unsigned char * 缓冲区，然后再转换回十六进制字符串。此代码正在测试从 unsigned char* 缓冲区到十六进制字符串的转换我需要通过网络将其发送到接收进程。我选择了十六进制字符串，因为 unsigned char 可以在 0 到 255 的范围内，并且 127 之后没有可打印字符。下面的代码只是告诉了让我烦恼的部分。它在评论中。

#include <iostream>
#include <sstream>
#include <iomanip>

using namespace std;
// converts a hexstring to corresponding integer. i.e "c0" - > 192
int convertHexStringToInt(const string & hexString)
{
  stringstream geek;
  int x=0;

  geek << std::hex << hexString;
  geek >> x;

  return x;
}

// converts a complete hexstring to unsigned char * buffer
void convertHexStringToUnsignedCharBuffer(string hexString, unsigned char* 
hexBuffer)
{
  int i=0;
  while(hexString.length())
  {
    string hexStringPart = hexString.substr(0,2);
    hexString = hexString.substr(2);
    int hexStringOneByte = convertHexStringToInt (hexStringPart);
    hexBuffer[i] = static_cast<unsigned char>((hexStringOneByte & 0xFF)) ;
    i++;
  }
}

int main()
{
  //below hex string is a hex representation of a unsigned char * buffer.
  //this is generated by an excryption algorithm in unsigned char* format
  //I am converting it to hex string to make it printable for verification pupose.
  //and takes the hexstring as inpuit here to test the conversion logic.
  string inputHexString = "552027e33844dd7b71676b963c0b8e20";
  string outputHexString;
  stringstream geek;

  unsigned char * hexBuffer = new unsigned char[inputHexString.length()/2];
  convertHexStringToUnsignedCharBuffer(inputHexString, hexBuffer);

  for (int i=0;i<inputHexString.length()/2;i++)
  {
    geek <<std::hex << std::setw(2) << std::setfill('0')<<(0xFF&hexBuffer[i]); // this works
    //geek <<std::hex << std::setw(2) << std::setfill('0')<<(hexBuffer[i]); -- > this does not work
    // I am not able to figure out why I need to do the bit wise and operation with unsigned char "0xFF&hexBuffer[i]"
    // without this the conversion does not work for individual bytes having ascii values more than 127.
  }

  geek >> outputHexString;

  cout << "input hex string:  " << inputHexString<<endl;
  cout << "output hex string: " << outputHexString<<endl;
  if(0 == inputHexString.compare(outputHexString))
    cout<<"hex encoding successful"<<endl;
  else
    cout<<"hex encoding failed"<<endl;

  if(NULL != hexBuffer)
      delete[] hexBuffer;

  return 0;
}

// output
// can some one explain ? I am sure its something silly that I am missing.

Answer 1

unsigned char 的输出类似于 char 的输出，这显然不是 OP 所期望的。

我在 coliru 上测试了以下内容：

#include <iomanip>
#include <iostream>

int main()
{
  std::cout << "Output of (unsigned char)0xc0: "
    << std::hex << std::setw(2) << std::setfill('0') << (unsigned char)0xc0 << '\n';
  return 0;
}

并得到：

Output of (unsigned char)0xc0: 0�

这是从可用运算符中选择的 std::ostream::operator<<() 引起的。我查看了 cppreference

找到

template< class Traits >
basic_ostream<char,Traits>& operator<<( basic_ostream<char,Traits>& os,
                                        unsigned char ch );

前者（在 M.M 的帮助下）。

OP 建议修复：按位并使用 0xff，这似乎有效。在 coliru.com 中检查：

#include <iomanip>
#include <iostream>

int main()
{
  std::cout << "Output of (unsigned char)0xc0: "
    << std::hex << std::setw(2) << std::setfill('0') << (0xff & (unsigned char)0xc0) << '\n';
  return 0;
}

输出：

Output of (unsigned char)0xc0: c0

真的，这似乎行得通。为什么？

0xff 是一个 int 常量（严格来说：一个 integer literal）并且类型为 int。因此，按位 And 也将 (unsigned char)0xc0 提升为 int，产生类型 int 的结果，因此，应用 int 的 std::ostream::operator<< .

这是解决这个问题的一个选项。我可以提供另一个 - 只需将 unsigned char 转换为 unsigned.

unsigned char 到 int 的提升引入了可能的符号位扩展（在这种情况下是不希望的），当 unsigned char 转换为时不会发生这种情况unsigned。 unsigned 的输出流运算符也提供了预期的输出：

#include <iomanip>
#include <iostream>

int main()
{
  std::cout << "Output of (unsigned char)0xc0: "
    << std::hex << std::setw(2) << std::setfill('0') << (unsigned)(unsigned char)0xc0 << '\n';
  const unsigned char c = 0xc0;
  std::cout << "Output of unsigned char c = 0xc0: "
    << std::hex << std::setw(2) << std::setfill('0') << (unsigned)c << '\n';
  return 0;
}

输出：

Output of (unsigned char)0xc0: c0
Output of unsigned char c = 0xc0: c0

Live Demo on coliru

Answer 2

C++20 方式：

unsigned char* data = new unsigned char[]{ "Hello world\n\t\r[=10=]" };
std::size_t data_size = sizeof("Hello world\n\t\r[=10=]") - 1;

auto sp = std::span(data, data_size );
std::transform( sp.begin(), sp.end(),
                std::ostream_iterator<std::string>(std::cout),
                [](unsigned char c) -> std::string {
                    return std::format("{:02X}", int(c));
                });

或者如果您想将结果存储到字符串中：

std::string result{};
result.reserve(size * 2 + 1);
std::transform( sp.begin(), sp.end(),
                std::back_inserter(result),
                [](unsigned char c) -> std::string {
                    return std::format("{:02X}", int(c));
                });

Output:
48656C6C6F20776F726C640A090D00

将 unsigned char * 转换为 hexstring

Converting unsigned char * to hexstring

c++

stringstream

bitwise-operators