std::numeric_limits<T>:: round_style

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

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 .

#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';
}

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