Bitwise Operators and Bit Flags

C++ Bitwise Operators

Numbers live inside the computer as bits: 0 and 1. Bitwise operators work directly on those bits - great for fast flags and hardware.

Binary look at a number

Use std::bitset from <bitset> to show the bit pattern:

#include <bitset>
cout << bitset<8>(5) << endl;   // 00000101

Here 5 is 00000101 in binary.

The six bitwise operators

• & AND: 1 only when both bits are 1.
• | OR: 1 when at least one bit is 1.
• ^ XOR: 1 when the bits differ.
• ~ NOT: flips every bit.
• << shift left: move the bits left.
• >> shift right: move the bits right.

A small program

#include <iostream>
#include <bitset>
using namespace std;

int main() {
  cout << bitset<4>(6 & 3) << endl;  // 0010
  cout << bitset<4>(6 | 3) << endl;  // 0111
  cout << bitset<4>(6 ^ 3) << endl;  // 0101
  cout << (1 << 3) << endl;          // 8
  return 0;
}

Shifts: quick multiplication and division

Moving bits left by one doubles the number; moving right by one halves it. 1 << 3 moves the 1-bit three spots, giving 8.

Bit flags: many booleans in one int

A common trick is to pack several true/false settings into a single integer, one bit each. Each bit is a flag.

int flags = 0;              // everything off
flags = flags | 1;          // turn on bit 0
flags = flags | (1 << 2);   // turn on bit 2
bool on = (flags & (1 << 2)) != 0;  // is bit 2 on?

Bit flags save memory (one value instead of many booleans) and are used a lot in systems programming.

TL;DR

• Bitwise ops work per bit: &, |, ^, <<, <<, ~.
• std::bitset<8> prints the bits of a number.
• << shifts left, >> shifts right.
• Store many true/false flags in one int with the bit-flag pattern.