C++ infinite hex string to bytes (of any junk size)

hex2bytes.h

/*
 *  hex2bytes - hex string of unlimited length to a continous block of bytes
 *  Copyright (C) 2018  Bernhard Arnold <bernhard.arnold@cern.ch>
 *
 *  This program is free software: you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation, either version 3 of the License, or
 *  (at your option) any later version.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with this program.  If not, see <https://www.gnu.org/licenses/>.
 *
 *
 * Function template converts hex string of unlimited length to a continous
 * block of bytes wrapped by a STL vector.
 *
 * Examples:
 *
 *   hex2bytes<uint8_t>("a"); // [0xa]
 *   hex2bytes<uint8_t>("ab"); // [0xab]
 *   hex2bytes<uint8_t>("abc"); // [0xbc, 0xa]
 *   hex2bytes<uint8_t>("abcd"); // [0xcd, 0xab]
 *   hex2bytes<uint8_t>("abcde"); // [0xde, 0xbc, 0xa]
 *   hex2bytes<uint16_t>("a11bb22cc33dd"); // [0x33dd, 0x22cc, 0x11bb, 0xa]
 *
 */

#ifndef _gist_hex2bytes_h
#define _gist_hex2bytes_h

#include <string>
#include <vector>

template<typename T>
std::vector<T> hex2bytes(const std::string& s)
{
    constexpr size_t width = sizeof(T) * 2;
    std::vector<T> v;
    v.reserve((s.size() + width - 1) / width);
    for (auto it = s.crbegin(); it < s.crend(); it += width)
    {
        auto begin = std::min(s.crend(), it + width).base();
        auto end = it.base();
        std::string slice(begin, end);
        T value = std::stoul(slice, 0, 16);
        v.push_back(value);
    }
    return v;
}

#endif

unittest.cc

#include "hex2bytes.h"

#include <algorithm>
#include <cassert>
#include <cstdlib>

template<typename T>
void assert_hex2bytes(const std::string& s, const std::vector<T>& ref)
{
    auto res = hex2bytes<T>(s);
    assert(std::equal(ref.begin(), ref.end(), res.begin()));
}

int main()
{
    assert_hex2bytes<uint8_t>("", {});
    assert_hex2bytes<uint8_t>("a", {0xa});
    assert_hex2bytes<uint8_t>("ab", {0xab});
    assert_hex2bytes<uint8_t>("abc", {0xbc, 0xa});
    assert_hex2bytes<uint8_t>("abcd", {0xcd, 0xab});
    assert_hex2bytes<uint8_t>("abcde", {0xde, 0xbc, 0xa});
    assert_hex2bytes<uint8_t>("abcdef", {0xef, 0xcd, 0xab});

    assert_hex2bytes<uint16_t>("", {});
    assert_hex2bytes<uint16_t>("a", {0xa});
    assert_hex2bytes<uint16_t>("ab", {0xab});
    assert_hex2bytes<uint16_t>("abc", {0xabc});
    assert_hex2bytes<uint16_t>("abcd", {0xabcd});
    assert_hex2bytes<uint16_t>("abcde", {0xbcde, 0xa});
    assert_hex2bytes<uint16_t>("abcdef", {0xcdef, 0xab});

    assert_hex2bytes<uint32_t>("bbbbcccc", {0xbbbbcccc});
    assert_hex2bytes<uint32_t>("abbbbcccc", {0xbbbbcccc, 0xa});
    assert_hex2bytes<uint32_t>("aabbbbcccc", {0xbbbbcccc, 0xaa});
    assert_hex2bytes<uint32_t>("aaabbbbcccc", {0xbbbbcccc, 0xaaa});
    assert_hex2bytes<uint32_t>("aaaabbbbcccc", {0xbbbbcccc, 0xaaaa});

    assert_hex2bytes<uint64_t>("bbbbccccddddeeee", {0xbbbbccccddddeeee});
    assert_hex2bytes<uint64_t>("abbbbccccddddeeee", {0xbbbbccccddddeeee, 0xa});
    assert_hex2bytes<uint64_t>("aabbbbccccddddeeee", {0xbbbbccccddddeeee, 0xaa});
    assert_hex2bytes<uint64_t>("aaabbbbccccddddeeee", {0xbbbbccccddddeeee, 0xaaa});
    assert_hex2bytes<uint64_t>("aaaabbbbccccddddeeee", {0xbbbbccccddddeeee, 0xaaaa});
  
    return EXIT_SUCCESS;
}