How can I access a specific group of bits from a variable?

Question

I have a variable with \"x\" number of bits. How can I extract a specific group of bits and then work on them in C?

Answer 1

If you're dealing with a primitive then just use bitwise operations:

int bits = 0x0030;
bool third_bit = bits & 0x0004;  // bits & 00000100
bool fifth_bit = bits & 0x0010;  // bits & 00010000

If x can be larger than a trivial primitive but is known at compile-time then you can use std::bitset<> for the task:

#include
#include

// ...
std::bitset<512> b(std::string("001"));
b.set(2, true);
std::cout << b[1] << ' ' << b[2] << '\n';

std::bitset<32>  bul(0x0010ul);

If x is not known at compile-time then you can use std::vector and then use bit-manipulation at runtime. It's more work, the intent reads less obvious than with std::bitset and it's slower, but that's arguably your best option for x varying at runtime.

#include

// ...
std::vector v(256);
v[2] = 1;
bool eighteenth_bit = v[2] & 0x02;  // second bit of third byte