Base64 url encode/decode. C++ adapted from some old code by someone else...

base64.cpp

#include <string>
#include <vector>

/*
Base64 translates 24 bits into 4 ASCII characters at a time. First,
3 8-bit bytes are treated as 4 6-bit groups. Those 4 groups are
translated into ASCII characters. That is, each 6-bit number is treated
as an index into the ASCII character array.

If the final set of bits is less 8 or 16 instead of 24, traditional base64
would add a padding character. However, if the length of the data is
known, then padding can be eliminated.

One difference between the "standard" Base64 is two characters are different.
See RFC 4648 for details.
This is how we end up with the Base64 URL encoding.
*/

const char base64_url_alphabet[] = {
    'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M',
    'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z',
    'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm',
    'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z',
    '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '-', '_'
};

std::string base64_encode(const std::string & in) {
  std::string out;
  int val =0, valb=-6;
  size_t len = in.length();
  unsigned int i = 0;
  for (i = 0; i < len; i++) {
    unsigned char c = in[i];
    val = (val<<8) + c;
    valb += 8;
    while (valb >= 0) {
      out.push_back(base64_url_alphabet[(val>>valb)&0x3F]);
      valb -= 6;
    }
  }
  if (valb > -6) {
    out.push_back(base64_url_alphabet[((val<<8)>>(valb+8))&0x3F]);
  }
  return out;
}

std::string base64_decode(const std::string & in) {
  std::string out;
  std::vector<int> T(256, -1);
  unsigned int i;
  for (i =0; i < 64; i++) T[base64_url_alphabet[i]] = i;

  int val = 0, valb = -8;
  for (i = 0; i < in.length(); i++) {
    unsigned char c = in[i];
    if (T[c] == -1) break;
    val = (val<<6) + T[c];
    valb += 6;
    if (valb >= 0) {
      out.push_back(char((val>>valb)&0xFF));
      valb -= 8;
    }
  }
  return out;
}

Thank you very much for this code, It worked great for my minimal implementation of JSON Web Token, I made a few adjustments after running SonarCloud static analyzer. Since I need to encode string_views and also vector I used a template, and also incorporated std::range:for_each and lambdas to modernize the loops (suggested by SonarCloud):

        template<typename T>
	std::string base64_encode(T in) 
	{
	  std::string out;
	  out.reserve(127);
	  int val = 0;
	  int valb = -6;
	  std::ranges::for_each(in, [&](unsigned char c) {
		val = (val<<8) + c;
		valb += 8;
		while (valb >= 0) {
		  out.push_back(base64_url_alphabet[(val>>valb)&0x3F]);
		  valb -= 6;
		}
	  });
	  if (valb > -6) 
		out.push_back(base64_url_alphabet[((val<<8)>>(valb+8))&0x3F]);
	  return out;
	}

	std::string base64_decode(std::string_view in) 
	{
	  std::string out;
	  out.reserve(127);
	  std::vector<int> T(256, -1);
	  for (unsigned int i = 0; i < 64; i++) 
		  T[base64_url_alphabet[i]] = i;
	  int val = 0; 
	  int valb = -8;
          std::ranges::for_each(in, [&](unsigned char c) {
		if (T[c] == -1) return;
		val = (val<<6) + T[c];
		valb += 6;
		if (valb >= 0) {
		  out.push_back(char((val>>valb)&0xFF));
		  valb -= 8;
		}
	  });
	  return out;
	}