std::bit_width

 

C++

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Technical Specifications

 

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Bit operations

bit_cast (C++20)

has_single_bit (C++20)

bit_ceil (C++20)

bit_floor (C++20)

bit_width (C++20)

rotl (C++20)

rotr (C++20)

countl_zero (C++20)

countl_one (C++20)

countr_zero (C++20)

countr_one (C++20)

popcount (C++20)

endian (C++20)

 

Defined in header <bit>
template< class T > constexpr T 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 only participates in overload resolution 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).

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);.

Example

Run this code

#include <bit>
#include <bitset>
#include <iostream>
 
auto main() -> int
{
    for (unsigned x{0}; x != 8; ++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

See also

countl_zero (C++20)

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