Bitwise Operators

Bitwise operators manipulate the individual bits of integer values. They are the building blocks of flag sets, masks, packed data, and low-level protocol code.

Where to go deeper

This page is the C++ operator reference. For the algorithmic side — counting bits, isolating the lowest set bit, parity tricks — see Bit Manipulation Techniques.

The operators

Operator	Name	Effect
&	AND	bit set in result iff set in both operands
|	OR	bit set iff set in either operand
^	XOR	bit set iff set in exactly one operand
~	NOT	flips every bit (unary)
<<	left shift	multiply by 2ⁿ (bits shifted in are zero)
>>	right shift	divide by 2ⁿ (sign-dependent — see below)

unsigned a = 0b1100;
unsigned b = 0b1010;
a & b;    // 0b1000  (8)
a | b;    // 0b1110  (14)
a ^ b;    // 0b0110  (6)
~a;       // all bits flipped — depends on width of the type
a << 1;   // 0b11000 (24)
a >> 1;   // 0b0110  (6)

Flag sets — the canonical use

enum Perm : unsigned { Read = 1u << 0, Write = 1u << 1, Exec = 1u << 2 };

unsigned p = Read | Write;     // set bits         -> 0b011
p |=  Exec;                    // add a flag       -> 0b111
p &= ~Write;                   // clear a flag     -> 0b101
bool can_read = p & Read;      // test a flag      (non-zero == set)
p ^=  Exec;                    // toggle a flag

Shifts: the sharp edges

Two real traps

• Shift by ≥ the type width is undefined behaviour. x << 32 on a 32-bit int is UB, not 0.
• Right-shifting a negative signed value is implementation-defined (arithmetic on every mainstream compiler, but not guaranteed by the standard pre-C++20).

Do bit work on unsigned types. uint32_t/uint64_t make width explicit and shifts well-defined.

uint32_t mask = 1u << 31;     // OK: uint32_t literal, bit 31 set
// int bad   = 1  << 31;      // UB on most platforms: overflows signed int

Bitwise vs logical

A recurring beginner bug is writing &/| where &&/|| was meant. Bitwise operators do not short-circuit and compare bit patterns, not truth values:

if (a & b)  { ... }   // true when a and b share any set bit
if (a && b) { ... }   // true when both a and b are non-zero

See Logical Operators for the short-circuiting pair.

std::bitset and <bit>

For readable flag handling and portable bit utilities, prefer the library over hand-rolled tricks:

#include <bitset>
#include <bit>                 // C++20
std::bitset<8> bs{0b1010};
bs.set(0); bs.flip(); bs.count();              // legible bit ops
int n = std::popcount(0b1011u);                // 3 — set-bit count
bool p = std::has_single_bit(16u);             // is power of two?

Summary

• & | ^ ~ work per bit; <</>> scale by powers of two.
• Use unsigned, fixed-width types — signed shifts and overflow invite UB.
• Shifting by ≥ the bit width is undefined behaviour.
• &/| are not &&/||: no short-circuit, different meaning.
• Reach for std::bitset and <bit> (popcount, has_single_bit, rotl) before clever tricks.

Related

• Bit Manipulation Techniques — algorithms
• Logical Operators
• Signedness