std::has_single_bit

 

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has_single_bit (C++20)

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Defined in header <bit>
template<class T > constexprbool 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

x

-

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 strict_unsigned_integral =std::unsigned_integral<T>&& neither<T, bool, char, char8_t, char16_t, char32_t, wchar_t>;   // First versionconstexprbool has_single_bit(strict_unsigned_integral auto x)noexcept{return x &&!(x &(x -1));}   // Second versionconstexprbool has_single_bit(strict_unsigned_integral auto x)noexcept{returnstd::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>)