std::ilogb,std::ilogbf,std::ilogbl(3)

NAME
std::ilogb,std::ilogbf,std::ilogbl - std::ilogb,std::ilogbf,std::ilogbl

Synopsis
   Defined in header <cmath>
   int ilogb ( float arg ); (1) (since C++11)
   int ilogbf( float arg ); (constexpr since C++23)
   int ilogb ( double arg ); (2) (since C++11)
                        (constexpr since C++23)
   int ilogb ( long double arg ); (3) (since C++11)
   int ilogbl( long double arg ); (constexpr since C++23)
   int ilogb ( IntegralType arg ); (4) (since C++11)
                        (constexpr since C++23)
   #define FP_ILOGB0 /*implementation-defined*/ (5) (since C++11)
   #define FP_ILOGBNAN /*implementation-defined*/ (6) (since C++11)
   1-3) Extracts the value of the unbiased exponent from the floating-point argument
   arg, and returns it as a signed integer value.
   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).
   5) Expands to integer constant expression whose value is either INT_MIN or -INT_MAX.
   6) Expands to integer constant expression whose value is either INT_MIN or +INT_MAX.
   Formally, the unbiased exponent is the integral part of log
   r|arg| as a signed integral value, for non-zero arg, where r is
   std::numeric_limits<T>::radix and T is the floating-point type of arg.

Parameters
   arg - floating point value

Return value
   If no errors occur, the unbiased exponent of arg is returned as a signed int value.
   If arg is zero, FP_ILOGB0 is returned.
   If arg is infinite, INT_MAX is returned.
   If arg is a NaN, FP_ILOGBNAN is returned.
   If the correct result is greater than INT_MAX or smaller than INT_MIN, the return
   value is unspecified.

Error handling
   Errors are reported as specified in math_errhandling.
   A domain error or range error may occur if arg is zero, infinite, or NaN.
   If the correct result is greater than INT_MAX or smaller than INT_MIN, a domain
   error or a range error may occur
   If the implementation supports IEEE floating-point arithmetic (IEC 60559),
     * If the correct result is greater than INT_MAX or smaller than INT_MIN,
       FE_INVALID is raised.
     * If arg is ±0, ±∞, or NaN, FE_INVALID is raised.
     * In all other cases, the result is exact (FE_INEXACT is never raised) and the
       current rounding mode is ignored

Notes
   If arg is not zero, infinite, or NaN, the value returned is exactly equivalent to
   static_cast<int>(std::logb(arg))
   POSIX requires that a domain error occurs if arg is zero, infinite, NaN, or if the
   correct result is outside of the range of int.
   POSIX also requires that, on XSI-conformant systems, the value returned when the
   correct result is greater than INT_MAX is INT_MAX and the value returned when the
   correct result is less than INT_MIN is INT_MIN.
   The correct result can be represented as int on all known implementations. For
   overflow to occur, INT_MAX must be less than LDBL_MAX_EXP*log2(FLT_RADIX) or INT_MIN
   must be greater than LDBL_MIN_EXP-LDBL_MANT_DIG)*log2(FLT_RADIX).
   The value of the exponent returned by std::ilogb is always 1 less than the exponent
   retuned by std::frexp because of the different normalization requirements: for the
   exponent e returned by std::ilogb, |arg*r-e
   | is between 1 and r (typically between 1 and 2), but for the exponent e returned by
   std::frexp, |arg*2-e
   | is between 0.5 and 1.

Example
   Compares different floating-point decomposition functions // Run this code
 #include <iostream>
 #include <cmath>
 #include <limits>
 #include <cfenv>
 // #pragma STDC FENV_ACCESS ON
 int main()
 {
     double f = 123.45;
     std::cout << "Given the number " << f << " or " << std::hexfloat
               << f << std::defaultfloat << " in hex,\n";
     double f3;
     double f2 = std::modf(f, &f3);
     std::cout << "modf() makes " << f3 << " + " << f2 << '\n';
     int i;
     f2 = std::frexp(f, &i);
     std::cout << "frexp() makes " << f2 << " * 2^" << i << '\n';
     i = std::ilogb(f);
     std::cout << "logb()/ilogb() make " << f/std::scalbn(1.0, i) << " * "
               << std::numeric_limits<double>::radix
               << "^" << std::ilogb(f) << '\n';
     // error handling
     std::feclearexcept(FE_ALL_EXCEPT);
     std::cout << "ilogb(0) = " << std::ilogb(0) << '\n';
     if (std::fetestexcept(FE_INVALID))
         std::cout << " FE_INVALID raised\n";
 }

Possible output:
 Given the number 123.45 or 0x1.edccccccccccdp+6 in hex,
 modf() makes 123 + 0.45
 frexp() makes 0.964453 * 2^7
 logb()/ilogb() make 1.92891 * 2^6
 ilogb(0) = -2147483648
     FE_INVALID raised

See also
   frexp
   frexpf decomposes a number into significand and a power of 2
   frexpl (function)
   (C++11)
   (C++11)
   logb
   logbf
   logbl extracts exponent of the number
   (C++11) (function)
   (C++11)
   (C++11)
   scalbn
   scalbnf
   scalbnl
   scalbln
   scalblnf
   scalblnl multiplies a number by FLT_RADIX raised to a power
   (C++11) (function)
   (C++11)
   (C++11)
   (C++11)
   (C++11)
   (C++11)