std::bit_width

 

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bit_width (C++20)

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Defined in header <bit>
template<class T > constexprint 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(log2(x)). If x is zero, returns zero.

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

-

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 returnstd::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)