std::numeric_limits<T>::round_style
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
.
#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
| indicates floating-point rounding modes (enum) |