Arrays of enum's packed into bit fields in MSVC++ - c++

Question

Unsing MS Studio 2022 I am trying to pack two items into a union of size 16 bits but I am having problems with the correct syntax.
The first item is an unsigned short int so no problems there. The other is an array of 5 items, all two bits long. So imagine:
enum States {unused, on, off};
// Should be able to store this in a 2 bit field
then I want
States myArray[5];
// Should be able to fit in 10 bits and
// be unioned with my unsigned short
Unfortunatly I am completely failing to work out the correct syntax which leads to my array fitting into 16 bits. Any ideas?

Answer 1

You can't do that. An array is an array, not some packed bits.
What you can do is using manual bit manipulation:
#include <iostream>
#include <cstdint>
#include <bitset>
#include <climits>
enum status {
on = 0x03,
off = 0x01,
unused = 0x00
};
constexpr std::uint8_t status_bit_width = 2;
std::uint16_t encode(status s,std::uint8_t id, std::uint16_t status_vec) {
if(id >= (CHAR_BIT * sizeof(std::uint16_t)) / status_bit_width) {
std::cout << "illegal id" << std::endl;
return status_vec;
}
std::uint8_t bit_value = s;
status_vec |= bit_value << (id*status_bit_width);
return status_vec;
};
int main(void) {
std::uint16_t bits = 0;
bits = encode(on,1,bits);
std::cout << std::bitset<16>(bits) << std::endl;
bits = encode(off,2,bits);
std::cout << std::bitset<16>(bits) << std::endl;
bits = encode(unused,3,bits);
std::cout << std::bitset<16>(bits) << std::endl;
bits = encode(off,4,bits);
std::cout << std::bitset<16>(bits) << std::endl;
bits = encode(off,7,bits);
std::cout << std::bitset<16>(bits) << std::endl;
bits = encode(on,8,bits);
}