std::countl_one

 

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countr_one (C++20)

popcount (C++20)

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

Returns the number of consecutive 1 ("one") bits in the value of x, starting from the most significant bit ("left").

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 most significant bit.

Notes

Feature-test macro:

__cpp_lib_bitops

Example

Run this code

#include <bit>
#include <bitset>
#include <cstdint>
#include <iostream>
 
int main()
{
    for (const std::uint8_t i : { 0, 0b11111111, 0b01111111, 0b11100011 }) {
        std::cout << "countl_one( " << std::bitset<8>(i) << " ) = "
                  << std::countl_one(i) << '\n';
    }
}

Output:

countl_one( 00000000 ) = 0
countl_one( 11111111 ) = 8
countl_one( 01111111 ) = 0
countl_one( 11100011 ) = 3

See also

countl_zero (C++20)	counts the number of consecutive 0 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)
countr_one (C++20)	counts the number of consecutive 1 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 two (function template)
count	returns the number of bits set to true (public member function of std::bitset<N>)
allanynone (C++11)	checks if all, any or none of the bits are set to true (public member function of std::bitset<N>)