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, unsignedchar, unsignedshort, unsignedint, unsignedlong, unsignedlonglong, 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

Prior to P1956R1, the proposed name for this function template was ispow2.

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

Possible implementation

template<typename T, typename ... U>
concept neither = (!std::same_as<T, U> && ...);

template<typename T>
concept unsigned_integer = std::unsigned_integral<T> &&
    neither<T, bool, char, char8_t, char16_t, char32_t, wchar_t>;

// First version
constexpr bool has_single_bit(unsigned_integer auto x) noexcept
{
    return x && !(x & (x - 1));
}

// Second version
constexpr bool has_single_bit(unsigned_integer auto x) noexcept
{
    return std::popcount(x) == 1;
}

Example

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

int main()
{
    for (auto u{0u}; u != 0B1010; ++u)
    {
        std::cout << "u = " << u << " = " << std::bitset<4>(u);
        if (std::has_single_bit(u))
            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