ByteAccess

// GNU++98
#include <iostream>
#include <stdint.h>
#include <bitset>

template <class T, unsigned _pos, unsigned _size>
class ByAccess {
public:
    typedef T data_type;

    void set(data_type v)
    {
        static const data_type mask = (1 << _size) - 1;
        data_type res = v & mask;
        res <<= _pos;
        data &= ~ (mask << _pos);
        data |= res;
    }

    data_type get()
    {
        static const unsigned move = sizeof(data_type)*8 - _size;
        data_type result = data << (move - _pos);
        result >>= move;
        result &= (1 << _size) - 1;
        return result;
    }

    operator data_type() { return get(); }
    // non-lvalue;
    ByAccess& operator= (const data_type& v) { set(v); return *this; }
private:
    data_type data;
};

struct A {
    union {
        int16_t a;
        ByAccess<int16_t, 0, 3> a1;
        ByAccess<int16_t, 3, 2> a2;
        ByAccess<int16_t, 5, 3> a3;
        ByAccess<int16_t, 8, 4> a4;
        ByAccess<int16_t, 12, 4> a5;
    };
};

int main()
{
    A pack;
    pack.a  = 0xAC9B;
    std::cout << "a= " << std::bitset<16>(pack.a) << "\n";
    pack.a1 = 7;
    std::cout << "1) a= " << std::bitset<16>(pack.a) << "\n";
    pack.a2 = 3;
    std::cout << "2) a= " << std::bitset<16>(pack.a) << "\n";
    pack.a2 = pack.a3 = 5;
    std::cout << "3) a= " << std::bitset<16>(pack.a) << "\n";

    std::cout << "a1= " << std::bitset<16>(pack.a1) << "\n";
    std::cout << "a2= " << std::bitset<16>(pack.a2) << "\n";
    std::cout << "a3= " << std::bitset<16>(pack.a3) << "\n";
    std::cout << "a4= " << std::bitset<16>(pack.a4) << "\n";
    std::cout << "a5= " << std::bitset<16>(pack.a5) << "\n";
}