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

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?

Each element of the array must have its own address, so each element must occupy at least one byte. Maybe you want to use bit-fields instead of enum arrays? — user17732522, Feb 25 '22 at 12:51
C++ does not support arrays of bit-fields. You can declare a struct with five bit-fields 2 bits long each, but you won't be able to access them *by indexing*. — , Feb 25 '22 at 13:22
You could define a class template to hold an array of "bitfields", similar to `std::vector`. That won't be true C++ bitfields, though. And there's no direct way to find out how many bits you need per State, although you could derive that from `unused` and `off`. — MSalters, Feb 25 '22 at 13:39

Raildex · Answer 1 · 2022-02-25T13:19:56.080

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);
}

@YvesDaoust *sweats nervously* – Raildex Feb 25 '22 at 13:20 — Raildex, Feb 25 '22 at 13:20

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

1 Answers1