Bit Field for c++, inspired by https://codereview.stackexchange.com/questions/54342/template-for-endianness-free-code-data-always-packed-as-big-endian and https://blog.codef00.com/2014/12/06/portable-bitfields-using-c11/

bitfield.cpp

#include <stdint.h>
#include <cstddef>
#include <type_traits>
#include <cstring>
#include <cstdio>

template <size_t Size>
struct MinimumTypeHelper {
    typedef
        typename std::conditional<Size == 0 , void,
        typename std::conditional<Size <= 8 , uint8_t,
        typename std::conditional<Size <= 16, uint16_t,
        typename std::conditional<Size <= 32, uint32_t,
        typename std::conditional<Size <= 64, uint64_t,
        void>::type>::type>::type>::type>::type type;
};

template<int firstBit, int bitSize>
struct BitFieldMember
{
    using self_t = BitFieldMember<firstBit, bitSize>;
    using T = typename MinimumTypeHelper<bitSize>::type;

    static constexpr size_t firstByte = firstBit / 8;
    static constexpr size_t lastByte = (firstBit + bitSize - 1) / 8;
    static constexpr size_t tail = ((-(firstBit + bitSize)) % 8 + 8) % 8;
    static constexpr size_t mask = (T(1) << bitSize) - 1;
 
    uint8_t *_this()
    {
        return reinterpret_cast<uint8_t *>(this);
    }

    operator T() const {
        T ret = 0;
        for (size_t ii = firstByte; ii <= lastByte; ++ii ) {
            ret = (ret << 8) | _this()[ii];
        }
        return (ret >> tail) & mask;
    }

    self_t& operator = (T m) {
        m = (m & mask);
        T write_mask = mask;
        int ii = lastByte;
        if (tail) { // take care of wrapping
            uint8_t m1 = (m  << tail) & 0xff;
            uint8_t wm1 = (write_mask  << tail) & 0xff;
            
            uint8_t* ref = _this() + ii;
            (*ref) &= (~wm1) & 0xff;
            (*ref) |= (m1 & 0xff);

            m >>= (8-tail);
            write_mask >>= (8-tail);
            ii--;
        }
        for (; ii >= 0 && 0 != write_mask; ii--) {
            uint8_t* ref = _this() + ii;
            (*ref) &= (~write_mask) & 0xff;
            (*ref) |= (m & 0xff);
            m >>= 8;
            write_mask >>= 8;
        }
        return *this;
    }
};