# std::numeric_limits<T>::round_style

< cpp‎ | types‎ | numeric limits

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 (C++20) | std::round_toward_zero
char16_t (C++11) | std::round_toward_zero
char32_t (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 (C++11) | std::round_toward_zero
unsigned long long (C++11) | 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_ROUNDS 或 std::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
与 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
与 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

float_round_style | 指示浮点舍入模式 (枚举)