NAME¶

std::has_single_bit - std::has_single_bit

Synopsis¶

Defined in header <bit>
template< class T > (since C++20)
constexpr bool has_single_bit( T x ) noexcept;

Checks if x is an integral power of two.

This overload participates in overload resolution only if T is an unsigned integer
type (that is, unsigned char, unsigned short, unsigned int, unsigned long, unsigned
long long, or an extended unsigned integer type).

Parameters¶

x - value of unsigned integer type

Return value¶

true if x is an integral power of two; otherwise false.

Notes¶

Feature-test macro Value Std Feature
__cpp_lib_int_pow2 202002L (C++20) Integral power-of-2 operations

Possible implementation¶

First version¶

template<std::unsigned_integral T>
requires !std::same_as<T, bool> && !std::same_as<T, char> &&
!std::same_as<T, char8_t> && !std::same_as<T, char16_t> &&
!std::same_as<T, char32_t> && !std::same_as<T, wchar_t>
constexpr bool has_single_bit(T x) noexcept
{
return x && !(x & (x - 1));
}

Second version¶

template<std::unsigned_integral T>
requires !std::same_as<T, bool> && !std::same_as<T, char> &&
!std::same_as<T, char8_t> && !std::same_as<T, char16_t> &&
!std::same_as<T, char32_t> && !std::same_as<T, wchar_t>
constexpr bool has_single_bit(T x) noexcept
{
return std::popcount(x) == 1;
}

Example¶

// Run this code

#include <bit>
#include <bitset>
#include <cmath>
#include <iostream>

int main()
{
for (auto u = 0u; u != 10; ++u)
{
std::cout << "u = " << u << " = " << std::bitset<4>(u);
if (std::has_single_bit(u)) // `ispow2` before P1956R1
std::cout << " = 2^" << std::log2(u) << " (is power of two)";
std::cout << '\n';
}
}

Output:¶

u = 0 = 0000
u = 1 = 0001 = 2^0 (is power of two)
u = 2 = 0010 = 2^1 (is power of two)
u = 3 = 0011
u = 4 = 0100 = 2^2 (is power of two)
u = 5 = 0101
u = 6 = 0110
u = 7 = 0111
u = 8 = 1000 = 2^3 (is power of two)
u = 9 = 1001

See also¶

popcount counts the number of 1 bits in an unsigned integer
(C++20) (function template)
count returns the number of bits set to true
(public member function of std::bitset<N>)
test accesses specific bit
(public member function of std::bitset<N>)