std::countr_one

 

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countl_zero (C++20)

countl_one (C++20)

countr_zero (C++20)

countr_one (C++20)

popcount (C++20)

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Defined in header <bit>
template< class T > constexpr int countr_one( T x ) noexcept;		(since C++20)

Returns the number of consecutive 1 bits in the value of x, starting from the least significant bit ("right").

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

The number of consecutive 1 bits in the value of x, starting from the least significant bit.

Notes

Feature-test macro	Value	Std	Feature
__cpp_lib_bitops	201907L	(C++20)	Bit operations

Example

Run this code

#include <bit>
#include <bitset>
#include <cstdint>
#include <iostream>
 
int main()
{
    for (const std::uint8_t i : {0, 0b11111111, 0b11111110, 0b11100011})
        std::cout << "countr_one( " << std::bitset<8>(i) << " ) = "
                  << std::countr_one(i) << '\n';
}

Output:

countr_one( 00000000 ) = 0
countr_one( 11111111 ) = 8
countr_one( 11111110 ) = 0
countr_one( 11100011 ) = 2

See also

countl_zero (C++20)	counts the number of consecutive ​0​ bits, starting from the most significant bit (function template)
countl_one (C++20)	counts the number of consecutive 1 bits, starting from the most significant bit (function template)
countr_zero (C++20)	counts the number of consecutive ​0​ bits, starting from the least significant bit (function template)
popcount (C++20)	counts the number of 1 bits in an unsigned integer (function template)
has_single_bit (C++20)	checks if a number is an integral power of 2 (function template)
count	returns the number of bits set to true (public member function of std::bitset<N>)
allanynone	checks if all, any or none of the bits are set to true (public member function of std::bitset<N>)