std::bit_width

 

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has_single_bit (C++20)     bit_cast (C++20) bit_ceil (C++20) bit_floor (C++20) bit_width (C++20) rotl (C++20) rotr (C++20)	popcount (C++20) countl_zero (C++20) countl_one (C++20) countr_zero (C++20) countr_one (C++20) byteswap (C++23) endian (C++20)

 

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

If x is not zero, calculates the number of bits needed to store the value x, that is, \(1 + \lfloor \log_2(x) \rfloor\)1 + floor(log
2(x)). If x is zero, returns zero.

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

-

unsigned integer value

Return value

Zero if x is zero; otherwise, one plus the base-2 logarithm of x, with any fractional part discarded.

Notes

This function is equivalent to return std::numeric_limits<T>::digits - std::countl_zero(x);.

Feature-test macro	Value	Std	Feature
__cpp_lib_int_pow2	202002L	(C++20)	Integral power-of-2 operations

Example

Run this code

#include <bit>
#include <bitset>
#include <iostream>
 
int main()
{
    for (unsigned x{}; x != 010; ++x)
        std::cout << "bit_width( "
                  << std::bitset<4>{x} << " ) = "
                  << std::bit_width(x) << '\n';
}

Output:

bit_width( 0000 ) = 0
bit_width( 0001 ) = 1
bit_width( 0010 ) = 2
bit_width( 0011 ) = 2
bit_width( 0100 ) = 3
bit_width( 0101 ) = 3
bit_width( 0110 ) = 3
bit_width( 0111 ) = 3

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 3656	C++20	the return type of bit_width is the same as the type of its function argument	made it int

See also

countl_zero (C++20)

counts the number of consecutive ​0​ bits, starting from the most significant bit (function template)