in bitset, can i use "to_ulong" for a specific range of bits?

Question

I'm working on something that requires me to get access to specific bits and ranges of bits. I decided to use bitset because it is easy to get access to specific bits; how can I extract a range (subset) of bits?

Answer 1

Method A:

return (the_bitset >> start_bit).to_ulong();

Method B (faster than method A by 100 times on my machine):

unsigned long mask = 1;
unsigned long result = 0;
for (size_t i = start_bit; i < end_bit; ++ i) {
    if (the_bitset.test(i))
       result |= mask;
    mask <<= 1;
}
return result;

Answer 2

One complete example:

#include <bitset>
#include <iostream>
#include <spdlog/spdlog.h>

template<std::size_t FROM, std::size_t TO, std::size_t SIZE>
std::bitset<TO - FROM> subBitSet(std::bitset<SIZE> &fromBitSet) {
    std::bitset<TO - FROM> toBitSet;
    for (int i = 0; i < TO - FROM; ++i) toBitSet.set(i, fromBitSet[i + FROM]);
    return toBitSet;
}

int main() {
    std::bitset<8> bitSet("10110000");
    std::cout << "BitSet: " << bitSet << std::endl;
    std::cout << "(0,4]: " << subBitSet<0, 4>(bitSet) << std::endl;
    std::cout << "(2,6]: " << subBitSet<2, 6>(bitSet) << std::endl;
    std::cout << "(4,8]: " << subBitSet<4, 8>(bitSet) << std::endl;
}

The output

BitSet: 10110000
(0,4]: 0000
(2,6]: 1100
(4,8]: 1011