将 unsigned char * 转换为十六进制字符串

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 我需要通过网络发送到接收进程。 我选择十六进制字符串，因为无符号字符可以在 0 到 255 的范围内，并且 127 之后没有可打印的字符。 下面的代码只是告诉我困扰的部分。其在评论中。

#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.

C++20方式：

unsigned char* data = new unsigned char[]{ "Hello world\n\t\r\0" };
std::size_t data_size = sizeof("Hello world\n\t\r\0") - 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));
                });

或者如果你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