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std::has_single_bit(3)	       C++ Standard Libary	  std::has_single_bit(3)

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 un-
     signed 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>)

http://cppreference.com 	   2024.06.10		  std::has_single_bit(3)
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