C++ single header base64 decode/encoder for C++ 11 and above.

Base64.h

#ifndef _MACARON_BASE64_H_
#define _MACARON_BASE64_H_

/**
 * The MIT License (MIT)
 * Copyright (c) 2016-2024 tomykaira
 *
 * Permission is hereby granted, free of charge, to any person obtaining
 * a copy of this software and associated documentation files (the
 * "Software"), to deal in the Software without restriction, including
 * without limitation the rights to use, copy, modify, merge, publish,
 * distribute, sublicense, and/or sell copies of the Software, and to
 * permit persons to whom the Software is furnished to do so, subject to
 * the following conditions:
 *
 * The above copyright notice and this permission notice shall be
 * included in all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
 * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
 * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
 * LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
 * OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
 * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 */

#include <cstdint>
#include <string>

namespace macaron {

class Base64 {
public:
  static std::string Encode(const std::string &data) {
    static constexpr char sEncodingTable[] = {
        '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', '+', '/'};

    size_t in_len = data.size();
    size_t out_len = 4 * ((in_len + 2) / 3);
    std::string ret(out_len, '\0');
    size_t i;
    char *p = const_cast<char *>(ret.c_str());

    for (i = 0; in_len > 2 && i < in_len - 2; i += 3) {
      *p++ = sEncodingTable[(data[i] >> 2) & 0x3F];
      *p++ = sEncodingTable[((data[i] & 0x3) << 4) |
                            ((int)(data[i + 1] & 0xF0) >> 4)];
      *p++ = sEncodingTable[((data[i + 1] & 0xF) << 2) |
                            ((int)(data[i + 2] & 0xC0) >> 6)];
      *p++ = sEncodingTable[data[i + 2] & 0x3F];
    }
    if (i < in_len) {
      *p++ = sEncodingTable[(data[i] >> 2) & 0x3F];
      if (i == (in_len - 1)) {
        *p++ = sEncodingTable[((data[i] & 0x3) << 4)];
        *p++ = '=';
      } else {
        *p++ = sEncodingTable[((data[i] & 0x3) << 4) |
                              ((int)(data[i + 1] & 0xF0) >> 4)];
        *p++ = sEncodingTable[((data[i + 1] & 0xF) << 2)];
      }
      *p++ = '=';
    }

    return ret;
  }

  static std::string Decode(const std::string &input, std::string &out) {
    static constexpr unsigned char kDecodingTable[] = {
        64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
        64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
        64, 64, 64, 64, 64, 64, 64, 62, 64, 64, 64, 63, 52, 53, 54, 55, 56, 57,
        58, 59, 60, 61, 64, 64, 64, 64, 64, 64, 64, 0,  1,  2,  3,  4,  5,  6,
        7,  8,  9,  10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24,
        25, 64, 64, 64, 64, 64, 64, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36,
        37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 64, 64, 64,
        64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
        64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
        64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
        64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
        64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
        64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
        64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
        64, 64, 64, 64};

    size_t in_len = input.size();
    if (in_len % 4 != 0)
      return "Input data size is not a multiple of 4";

    size_t out_len = in_len / 4 * 3;
    if (in_len >= 1 && input[in_len - 1] == '=')
      out_len--;
    if (in_len >= 2 && input[in_len - 2] == '=')
      out_len--;

    out.resize(out_len);

    for (size_t i = 0, j = 0; i < in_len;) {
      uint32_t a = input[i] == '='
                       ? 0 & i++
                       : kDecodingTable[static_cast<int>(input[i++])];
      uint32_t b = input[i] == '='
                       ? 0 & i++
                       : kDecodingTable[static_cast<int>(input[i++])];
      uint32_t c = input[i] == '='
                       ? 0 & i++
                       : kDecodingTable[static_cast<int>(input[i++])];
      uint32_t d = input[i] == '='
                       ? 0 & i++
                       : kDecodingTable[static_cast<int>(input[i++])];

      uint32_t triple =
          (a << 3 * 6) + (b << 2 * 6) + (c << 1 * 6) + (d << 0 * 6);

      if (j < out_len)
        out[j++] = (triple >> 2 * 8) & 0xFF;
      if (j < out_len)
        out[j++] = (triple >> 1 * 8) & 0xFF;
      if (j < out_len)
        out[j++] = (triple >> 0 * 8) & 0xFF;
    }

    return "";
  }
};

} // namespace macaron

#endif /* _MACARON_BASE64_H_ */

test.cc

#include <cstdint>
#include <iostream>
#include <vector>

#include "Base64.h"

int test(const std::string &data) {
  std::string out;
  auto b64 = macaron::Base64::Encode(data);
  auto error = macaron::Base64::Decode(b64, out);
  if (!error.empty()) {
    std::cout << "Error: " << error << std::endl;
    return 1;
  }
  if (data == out) {
    std::cout << "OK: " << out << std::endl;
  } else {
    std::cout << "Wrong: " << data << ", " << b64 << ", " << out << std::endl;
  }
  return 0;
}

int main() {
  test("hello");
  test("");
  test("1");
  test("22");
  test("333");
  test("4444");
  return 0;
}

omg this gist is still alive

Yes indeed. I am suprised as well.
Currently I am trying to copying-pasting some codes from one project(which established in 2021) to another, which is using a header only base64 algorithm from here, till now I realize this piece of code still have issues left.

So does anyone know is there any better alternatives for header-only version of base64 algorithm ?

To @tomykaira:
Thanks a lot for your effort, your codes helped us greatly. But according to the discussion above, from my perspective, I would personally recommend to use std::vector for the return value of 'Encode' instead of std::string, and also for the input parameter of 'Decode', since usually the raw data is not human readable text, which is not necessary a 'string' .
What's more, personally I don't think return error message with a string value is a good practice. I would prefer using exception in modern C++, or leverage std::expect in C++20, or, the old school style way, use an predefined integer error code, since you don't have too many error types here, but you can still put some more later .

What's more, about frequently discussed 'const_cast' thing, what about using std::string::iterator instead ? So this line

   char *p = const_cast<char *>(ret.c_str());

Will become

    std::string::iterator p = ret.begin();

Pretty neat isn't it ? And better add if (p == ret.end()) within the loop for safety .