How to get array of bits in a structure?

Question

I was pondering (and therefore am looking for a way to learn this, and not a better solution) if it is possible to get an array of bits in a structure.

Answer 1

C++ would use std::vector or std::bitset.

In C, to emulate std::vector semantics, you use a struct like this:

struct Bits {
    Word word[];
    size_t word_count;
};

where Word is an implementation-defined type equal in width to the data bus of the CPU; wordsize, as used later on, is equal to the width of the data bus.

E.g. Word is uint32_fast_t for 32-bit machines, uint64_fast_t for 64-bit machines; wordsize is 32 for 32-bit machines, and 64 for 64-bit machines.

You use functions/macros to set/clear bits.

To extract a bit, use GET_BIT(bits, bit) (((bits)->)word[(bit)/wordsize] & (1 << ((bit) % wordsize))).

To set a bit, use SET_BIT(bits, bit) (((bits)->)word[(bit)/wordsize] |= (1 << ((bit) % wordsize))).

To clear a bit, use CLEAR_BIT(bits, bit) (((bits)->)word[(bit)/wordsize] &= ~(1 << ((bit) % wordsize))).

To flip a bit, use FLIP_BIT(bits, bit) (((bits)->)word[(bit)/wordsize] ^= (1 << ((bit) % wordsize))).

To add resizeability as per std::vector, make a resize function which calls realloc on Bits.word and changes Bits.word_count accordingly. The exact details of this is left as a problem.

The same applies for proper range-checking of bit indices.