std::log2,std::log2f,std::log2l(3)

NAME
std::log2,std::log2f,std::log2l - std::log2,std::log2f,std::log2l

Synopsis
   Defined in header <cmath>
   float log2 ( float arg ); (1) (since C++11)
   float log2f( float arg );
   double log2 ( double arg ); (2) (since C++11)
   long double log2 ( long double arg ); (3) (since C++11)
   long double log2l( long double arg );
   double log2 ( IntegralType arg ); (4) (since C++11)
   1-3) Computes the binary (base-2) logarithm of arg.
   4) A set of overloads or a function template accepting an argument of any integral
   type. Equivalent to (2) (the argument is cast to double).

Parameters
   arg - value of floating-point or Integral type

Return value
   If no errors occur, the base-2 logarithm of arg (log
   2(arg) or lb(arg)) is returned.
   If a domain error occurs, an implementation-defined value is returned (NaN where
   supported)
   If a pole error occurs, -HUGE_VAL, -HUGE_VALF, or -HUGE_VALL is returned.

Error handling
   Errors are reported as specified in math_errhandling.
   Domain error occurs if arg is less than zero.
   Pole error may occur if arg is zero.
   If the implementation supports IEEE floating-point arithmetic (IEC 60559),
     * If the argument is ±0, -∞ is returned and FE_DIVBYZERO is raised.
     * If the argument is 1, +0 is returned
     * If the argument is negative, NaN is returned and FE_INVALID is raised.
     * If the argument is +∞, +∞ is returned
     * If the argument is NaN, NaN is returned

Notes
   For integer arg, the binary logarithm can be interpreted as the zero-based index of
   the most significant 1 bit in the input.

Example
// Run this code
 #include <iostream>
 #include <cmath>
 #include <cerrno>
 #include <cstring>
 #include <cfenv>
 // #pragma STDC FENV_ACCESS ON
 int main()
 {
     std::cout << "log2(65536) = " << std::log2(65536) << '\n'
               << "log2(0.125) = " << std::log2(0.125) << '\n'
               << "log2(0x020f) = " << std::log2(0x020f)
               << " (highest set bit is in position 9)\n"
               << "base-5 logarithm of 125 = " << std::log2(125)/std::log2(5) << '\n';
     // special values
     std::cout << "log2(1) = " << std::log2(1) << '\n'
               << "log2(+Inf) = " << std::log2(INFINITY) << '\n';
     // error handling
     errno = 0;
     std::feclearexcept(FE_ALL_EXCEPT);
     std::cout << "log2(0) = " << std::log2(0) << '\n';
     if (errno == ERANGE)
         std::cout << " errno == ERANGE: " << std::strerror(errno) << '\n';
     if (std::fetestexcept(FE_DIVBYZERO))
         std::cout << " FE_DIVBYZERO raised\n";
 }

Possible output:
 log2(65536) = 16
 log2(0.125) = -3
 log2(0x020f) = 9.04166 (highest set bit is in position 9)
 base-5 logarithm of 125 = 3
 log2(1) = 0
 log2(+Inf) = inf
 log2(0) = -inf
     errno == ERANGE: Numerical result out of range
     FE_DIVBYZERO raised

See also
   log
   logf computes natural (base e) logarithm (\({\small \ln{x} }\)ln(x))
   logl (function)
   (C++11)
   (C++11)
   log10
   log10f computes common (base 10) logarithm (\({\small \log_{10}{x} }\)log[10](x))
   log10l (function)
   (C++11)
   (C++11)
   log1p
   log1pf natural logarithm (to base e) of 1 plus the given number (\({\small
   log1pl \ln{(1+x)} }\)ln(1+x))
   (C++11) (function)
   (C++11)
   (C++11)
   exp2
   exp2f
   exp2l returns 2 raised to the given power (\({\small 2^x}\)2^x)
   (C++11) (function)
   (C++11)
   (C++11)