std::has_single_bit

 

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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.

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:

__cpp_lib_int_pow2

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 != 0 && (x & (x - 1)) == 0; }
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 <iostream>
 
int main()
{
    std::cout << std::boolalpha;
    for (auto i = 0u; i < 10u; ++i) {
        std::cout << "has_single_bit( " << std::bitset<4>(i) << " ) = "
                  << std::has_single_bit(i) // `ispow2` before P1956R1
                  << '\n';
    }
}

Output:

has_single_bit( 0000 ) = false
has_single_bit( 0001 ) = true
has_single_bit( 0010 ) = true
has_single_bit( 0011 ) = false
has_single_bit( 0100 ) = true
has_single_bit( 0101 ) = false
has_single_bit( 0110 ) = false
has_single_bit( 0111 ) = false
has_single_bit( 1000 ) = true
has_single_bit( 1001 ) = false

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>)