std::bit_width(3)

NAME
std::bit_width - std::bit_width

Synopsis
   Defined in header <bit>
   template< class T > (since C++20)
   constexpr int bit_width( T x ) noexcept;
   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{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
   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 made it int
                       type of its function argument

See also
   countl_zero counts the number of consecutive 0 bits, starting from the most
   (C++20) significant bit
               (function template)