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

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?

CodePudding user response：

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);
}

CodePudding user response：

Thanks to all who tried to help me and for clarifying my mis-understanding of what an array is.

Thanks to Raildex and Yves for pointing out what appear to be the only viable solutions. I think I'll probably go with the encoding idea rather than a structure.