std::numeric_limits::round_style(3)

NAME
std::numeric_limits::round_style - std::numeric_limits::round_style

Synopsis
   static const std::float_round_style round_style; (until C++11)
   static constexpr std::float_round_style round_style; (since C++11)
   The value of std::numeric_limits<T>::round_style identifies the rounding style used
   by the floating-point type T whenever a value that is not one of the exactly
   repesentable values of T is stored in an object of that type.

Standard specializations
   T value of std::numeric_limits<T>::round_style
   /* non-specialized */ std::round_toward_zero
   bool std::round_toward_zero
   char std::round_toward_zero
   signed char std::round_toward_zero
   unsigned char std::round_toward_zero
   wchar_t std::round_toward_zero
   char8_t (since C++20) std::round_toward_zero
   char16_t (since C++11) std::round_toward_zero
   char32_t (since C++11) std::round_toward_zero
   short std::round_toward_zero
   unsigned short std::round_toward_zero
   int std::round_toward_zero
   unsigned int std::round_toward_zero
   long std::round_toward_zero
   unsigned long std::round_toward_zero
   long long (since C++11) std::round_toward_zero
   unsigned long long (since C++11) std::round_toward_zero
   float usually std::round_to_nearest
   double usually std::round_to_nearest
   long double usually std::round_to_nearest

Notes
   These values are constants, and do not reflect the changes to the rounding made by
   std::fesetround. The changed values may be obtained from FLT_ROUNDS or
   std::fegetround.

Example
   The decimal value 0.1 cannot be represented by a binary floating-point type. When
   stored in an IEEE-754 double, it falls between 0x1.9999999999999*2-4
   and 0x1.999999999999a*2-4
   . Rounding to nearest representable value results in 0x1.999999999999a*2-4
   .
   Similarly, the decimal value 0.3, which is between 0x1.3333333333333*2-2
   and 0x1.3333333333334*2-2
   is rounded to nearest and is stored as 0x1.3333333333333*2-2
   . // Run this code
 #include <iostream>
 #include <limits>
 auto print(std::float_round_style frs)
 {
     switch (frs)
     {
         case std::round_indeterminate:
             return "Rounding style cannot be determined";
         case std::round_toward_zero:
             return "Rounding toward zero";
         case std::round_to_nearest:
             return "Rounding toward nearest representable value";
         case std::round_toward_infinity:
             return "Rounding toward positive infinity";
         case std::round_toward_neg_infinity:
             return "Rounding toward negative infinity";
     }
     return "unknown round style";
 }
 int main()
 {
     std::cout << std::hexfloat
               << "The decimal 0.1 is stored in a double as "
               << 0.1 << '\n'
               << "The decimal 0.3 is stored in a double as "
               << 0.3 << '\n'
               << print(std::numeric_limits<double>::round_style) << '\n';
 }

Output:
 The decimal 0.1 is stored in a double as 0x1.999999999999ap-4
 The decimal 0.3 is stored in a double as 0x1.3333333333333p-2
 Rounding toward nearest representable value

See also
   float_round_style indicates floating-point rounding modes
                     (enum)