C++ 参考手册

static const std::float_round_style round_style;
(C++11 前)
static constexpr std::float_round_style round_style;
(C++11 起)

std::numeric_limits<T>::round_style 的值鉴别浮点类型 T 在凡将不能以 T 的值表示的值存储于该类型的对象时所用的舍入模式。

标准特化

 
T std::numeric_limits<T>::round_style 的值
/* 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 std::round_toward_zero
char16_t std::round_toward_zero
char32_t 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 std::round_toward_zero
unsigned long long std::round_toward_zero
float 通常为 std::round_to_nearest
double 通常为 std::round_to_nearest
long double 通常为 std::round_to_nearest

注解

这些值是常量,且不反映 std::fesetround 所做的舍入模式更改。可从 FLT_ROUNDSstd::fegetround 获得更改后的值。

示例

十进制值 0.1 不能表示成二进制浮点类型。在存储于 IEEE-754 double 时,它落入 0x1.9999999999999*2-4
0x1.999999999999a*2-4
之间。舍入到最近可表示结果导致 0x1.999999999999a*2-4

同样地,十进制值 0.3 在 0x1.3333333333333*2-2
0x1.3333333333334*2-2
之间,舍入到最近值后存储为 0x1.3333333333333*2-2

#include <iostream>
#include <limits>
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'
              << "The rounding style is " << std::numeric_limits<double>::round_style << '\n';
}

输出:

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
The rounding style is 1

参阅

指示浮点舍入模式
(枚举)