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std::has_single_bit

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

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

value of unsigned integer type

Return value

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

Notes

Feature-test macro	Value	Std	Comment
`__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; }

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

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