Bitwise OR Operation Basics

Understanding Bitwise OR (|)

The OR operator compares each pair of corresponding bits. The result bit is 1 when at least one of the input bits is 1. It is 0 only when both bits are 0.

Truth Table

A | B | A | B
--|---|------
0 | 0 |   0
0 | 1 |   1
1 | 0 |   1
1 | 1 |   1

Worked Example

Let's compute 42 | 15:

  42  = 00101010
| 15  = 00001111
──────────────────
  47  = 00101111

Every bit position that has a 1 in either operand produces a 1 in the result.

Setting Bits with OR

The primary use of OR is to set specific bits to 1 without affecting other bits. ORing any bit with 1 forces it to 1, while ORing with 0 leaves the original value unchanged. This is the complement of AND's masking behavior.

Combining Flags

In flag-based systems, each flag occupies a unique bit position. You combine multiple flags using OR:

const READ    = 0b001;  // 1
const WRITE   = 0b010;  // 2
const EXECUTE = 0b100;  // 4

const permissions = READ | WRITE;  // 0b011 = 3

This pattern is used extensively in operating systems, graphics APIs (OpenGL, Vulkan), and configuration libraries.

Idempotence

OR is idempotent for setting bits: if a bit is already 1, ORing it with 1 again still produces 1. This means you can safely "add" a flag without checking whether it is already set.