std::has_single_bit

 

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has_single_bit (C++20)

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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.

Parameters

x	-	a value to check
Type requirements
T	-	must be an unsigned integer type (that is, unsigned char, unsigned short, unsigned int, unsigned long, unsigned long long, or an extended unsigned integer type) in order to participate in overload resolution.

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<std::unsigned_integral T> requires neither<T, bool, char, char8_t, char16_t, char32_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 != 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

See also

popcount (C++20)	counts the number of 1 bits in an unsigned integer (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>)