How to convert hex representation from URL (%) to std::string (chinese text)?

Question

Intro

I have some input that I need to convert to the correct Chinese characters but I think I'm stuck at the final number to string conversion. I have checked using this hex to text converter online tool that e6b9af corresponds to the text湯.

MWE

Here is a minimal example that I made to illustrate the problem. The input is "%e6%b9%af" (obtained from an URL somewhere else).

#include <iostream>
#include <string>

std::string attempt(std::string path)
{
  std::size_t i = path.find("%");
  while (i != std::string::npos)
  {
    std::string sub = path.substr(i, 9);
    sub.erase(i + 6, 1);
    sub.erase(i + 3, 1);
    sub.erase(i, 1);
    std::size_t s = std::stoul(sub, nullptr, 16);
    path.replace(i, 9, std::to_string(s));
    i = path.find("%");
  }
  return path;
}

int main()
{
  std::string input = "%E6%B9%AF";
  std::string goal = "湯";

  // convert input to goal
  input = attempt(input);

  std::cout << goal << " and " << input << (input == goal ? " are the same" : " are not the same") << std::endl;

  return 0;
}

Output

湯 and 15120815 are not the same

Expected output

湯 and 湯 are the same

Additional question

Are all characters in foreign languages represented in 3 bytes or is that just for Chinese? Since my attempt assumes blocks of 3 bytes, is that a good assumption?

Answer 1

Based on your suggestions and changing an example from this other post . This is what I came up with.

#include <iostream>
#include <string>
#include <sstream>

std::string decode_url(const std::string& path)
{
  std::stringstream decoded;
  for (std::size_t i = 0; i < path.size(); i++)
  {
    if (path[i] != '%')
    {
      if (path[i] == '+')
        decoded << ' ';
      else
        decoded << path[i];
    }
    else
    {
      unsigned int j;
      sscanf(path.substr(i + 1, 2).c_str(), "%x", &j);
      decoded << static_cast<char>(j);
      i += 2;
    }
  }
  return decoded.str();
}

int main()
{
  std::string input = "%E6%B9%AF";
  std::string goal = "湯";

  // convert input to goal
  input = decode_url(input);

  std::cout << goal << " and " << input << (input == goal ? " are the same" : " are not the same") << std::endl;

  return 0;
}

Output

湯 and 湯 are the same