C++标准模板（STL）- 类型支持（数值极限，round_style，is_iec559，is

数值极限

定义于头文件 <limits>

定义于头文件 `<limits>`
template< class T > class numeric_limits;

numeric_limits 类模板提供查询各种算术类型属性的标准化方式（例如 int 类型的最大可能值是 std::numeric_limits<int>::max() ）。

鉴别类型所用的舍入模式

std::numeric_limits<T>::round_style

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

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

注意

这些值是常量，且不反映 std::fesetround 所做的舍入模式更改。被更改的值可从 FLT_ROUNDS 或 std::fegetround 获得。

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-745 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。

调用示例

#include <iostream>
#include <string>
#include <limits>

struct SName
{
};

//偏特化
struct SPartSpec
{
};

namespace std
{
template<>
struct numeric_limits<SPartSpec>
{
    static _GLIBCXX_USE_CONSTEXPR bool is_specialized   = true;
    static _GLIBCXX_USE_CONSTEXPR bool is_signed        = true;
    static _GLIBCXX_USE_CONSTEXPR bool is_integer       = true;
    static _GLIBCXX_USE_CONSTEXPR bool is_exact         = true;
    static _GLIBCXX_USE_CONSTEXPR bool has_infinity     = true;
    static _GLIBCXX_USE_CONSTEXPR bool has_quiet_NaN    = true;
    static _GLIBCXX_USE_CONSTEXPR bool has_signaling_NaN = true;
    static _GLIBCXX_USE_CONSTEXPR float_denorm_style has_denorm     = denorm_present;
    static _GLIBCXX_USE_CONSTEXPR bool has_denorm_loss  = true;
    static _GLIBCXX_USE_CONSTEXPR float_round_style round_style     = round_toward_neg_infinity;
};
}

int main()
{
    std::cout << std::boolalpha;
    std::cout << "std::numeric_limits<bool>::round_style:                 "
              << std::numeric_limits<bool>::round_style << std::endl;
    std::cout << "std::numeric_limits<char>::round_style:                 "
              << std::numeric_limits<char>::round_style << std::endl;
    std::cout << "std::numeric_limits<signed char>::round_style:          "
              << std::numeric_limits<signed char>::round_style << std::endl;
    std::cout << "std::numeric_limits<unsigned char>::round_style:        "
              << std::numeric_limits<unsigned char>::round_style << std::endl;
    std::cout << "std::numeric_limits<wchar_t>::round_style:              "
              << std::numeric_limits<wchar_t>::round_style << std::endl;
    std::cout << "std::numeric_limits<char16_t>::round_style:             "
              << std::numeric_limits<char16_t>::round_style << std::endl;
    std::cout << "std::numeric_limits<char32_t>::round_style:             "
              << std::numeric_limits<char32_t>::round_style << std::endl;
    std::cout << "std::numeric_limits<short>::round_style:                "
              << std::numeric_limits<short>::round_style << std::endl;
    std::cout << "std::numeric_limits<unsigned short>::round_style:       "
              << std::numeric_limits<unsigned short>::round_style << std::endl;
    std::cout << "std::numeric_limits<int>::round_style:                  "
              << std::numeric_limits<int>::round_style << std::endl;
    std::cout << "std::numeric_limits<unsigned int>::round_style:         "
              << std::numeric_limits<unsigned int>::round_style << std::endl;
    std::cout << "std::numeric_limits<long>::round_style:                 "
              << std::numeric_limits<long>::round_style << std::endl;
    std::cout << "std::numeric_limits<unsigned long>::round_style:        "
              << std::numeric_limits<unsigned long>::round_style << std::endl;
    std::cout << "std::numeric_limits<long long>::round_style:            "
              << std::numeric_limits<long long>::round_style << std::endl;
    std::cout << "std::numeric_limits<unsigned long long>::round_style:   "
              << std::numeric_limits<unsigned long long>::round_style << std::endl;
    std::cout << "std::numeric_limits<float>::round_style:                "
              << std::numeric_limits<float>::round_style << std::endl;
    std::cout << "std::numeric_limits<double>::round_style:               "
              << std::numeric_limits<double>::round_style << std::endl;
    std::cout << "std::numeric_limits<long double>::round_style:          "
              << std::numeric_limits<long double>::round_style << std::endl;
    std::cout << "std::numeric_limits<std::string>::round_style:          "
              << std::numeric_limits<std::string>::round_style << std::endl;
    std::cout << "std::numeric_limits<SName>::round_style:                "
              << std::numeric_limits<SName>::round_style << std::endl;
    std::cout << "std::numeric_limits<SPartSpec>::round_style:            "
              << std::numeric_limits<SPartSpec>::round_style << std::endl;
    return 0;
}

输出

鉴别 IEC 559/IEEE 754 浮点类型

std::numeric_limits<T>::is_iec559

static const bool is_iec559;		(C++11 前)
static constexpr bool is_iec559;		(C++11 起)

std::numeric_limits<T>::is_iec559 的值对于所有符合 IEC 559 （ IEEE 754 ）标准要求的浮点类型 T 为 true 。若 std::numeric_limits<T>::is_iec559 为 true ，则 std::numeric_limits<T>::has_infinity 、 std::numeric_limits<T>::has_quiet_NaN 与 std::numeric_limits<T>::has_signaling_NaN 亦为 true 。

标准特化

`T`	std::numeric_limits<T>::is_iec559 的值
/* non-specialized */	false
bool	false
char	false
signed char	false
unsigned char	false
wchar_t	false
char8_t	false
char16_t	false
char32_t	false
short	false
unsigned short	false
int	false
unsigned int	false
long	false
unsigned long	false
long long	false
unsigned long long	false
float	通常为 true
double	通常为 true
long double	通常为 true

调用示例

#include <iostream>
#include <string>
#include <limits>

struct SName
{
};

//偏特化
struct SPartSpec
{
};

namespace std
{
template<>
struct numeric_limits<SPartSpec>
{
    static _GLIBCXX_USE_CONSTEXPR bool is_specialized   = true;
    static _GLIBCXX_USE_CONSTEXPR bool is_signed        = true;
    static _GLIBCXX_USE_CONSTEXPR bool is_integer       = true;
    static _GLIBCXX_USE_CONSTEXPR bool is_exact         = true;
    static _GLIBCXX_USE_CONSTEXPR bool has_infinity     = true;
    static _GLIBCXX_USE_CONSTEXPR bool has_quiet_NaN    = true;
    static _GLIBCXX_USE_CONSTEXPR bool has_signaling_NaN = true;
    static _GLIBCXX_USE_CONSTEXPR float_denorm_style has_denorm     = denorm_present;
    static _GLIBCXX_USE_CONSTEXPR bool has_denorm_loss  = true;
    static _GLIBCXX_USE_CONSTEXPR float_round_style round_style     = round_toward_neg_infinity;
    static _GLIBCXX_USE_CONSTEXPR bool is_iec559        = true;
};
}

int main()
{
    std::cout << std::boolalpha;
    std::cout << "std::numeric_limits<bool>::is_iec559:                 "
              << std::numeric_limits<bool>::is_iec559 << std::endl;
    std::cout << "std::numeric_limits<char>::is_iec559:                 "
              << std::numeric_limits<char>::is_iec559 << std::endl;
    std::cout << "std::numeric_limits<signed char>::is_iec559:          "
              << std::numeric_limits<signed char>::is_iec559 << std::endl;
    std::cout << "std::numeric_limits<unsigned char>::is_iec559:        "
              << std::numeric_limits<unsigned char>::is_iec559 << std::endl;
    std::cout << "std::numeric_limits<wchar_t>::is_iec559:              "
              << std::numeric_limits<wchar_t>::is_iec559 << std::endl;
    std::cout << "std::numeric_limits<char16_t>::is_iec559:             "
              << std::numeric_limits<char16_t>::is_iec559 << std::endl;
    std::cout << "std::numeric_limits<char32_t>::is_iec559:             "
              << std::numeric_limits<char32_t>::is_iec559 << std::endl;
    std::cout << "std::numeric_limits<short>::is_iec559:                "
              << std::numeric_limits<short>::is_iec559 << std::endl;
    std::cout << "std::numeric_limits<unsigned short>::is_iec559:       "
              << std::numeric_limits<unsigned short>::is_iec559 << std::endl;
    std::cout << "std::numeric_limits<int>::is_iec559:                  "
              << std::numeric_limits<int>::is_iec559 << std::endl;
    std::cout << "std::numeric_limits<unsigned int>::is_iec559:         "
              << std::numeric_limits<unsigned int>::is_iec559 << std::endl;
    std::cout << "std::numeric_limits<long>::is_iec559:                 "
              << std::numeric_limits<long>::is_iec559 << std::endl;
    std::cout << "std::numeric_limits<unsigned long>::is_iec559:        "
              << std::numeric_limits<unsigned long>::is_iec559 << std::endl;
    std::cout << "std::numeric_limits<long long>::is_iec559:            "
              << std::numeric_limits<long long>::is_iec559 << std::endl;
    std::cout << "std::numeric_limits<unsigned long long>::is_iec559:   "
              << std::numeric_limits<unsigned long long>::is_iec559 << std::endl;
    std::cout << "std::numeric_limits<float>::is_iec559:                "
              << std::numeric_limits<float>::is_iec559 << std::endl;
    std::cout << "std::numeric_limits<double>::is_iec559:               "
              << std::numeric_limits<double>::is_iec559 << std::endl;
    std::cout << "std::numeric_limits<long double>::is_iec559:          "
              << std::numeric_limits<long double>::is_iec559 << std::endl;
    std::cout << "std::numeric_limits<std::string>::is_iec559:          "
              << std::numeric_limits<std::string>::is_iec559 << std::endl;
    std::cout << "std::numeric_limits<SName>::is_iec559:                "
              << std::numeric_limits<SName>::is_iec559 << std::endl;
    std::cout << "std::numeric_limits<SPartSpec>::is_iec559:            "
              << std::numeric_limits<SPartSpec>::is_iec559 << std::endl;
    return 0;
}

输出

鉴别表示有限值集合的类型

std::numeric_limits<T>::is_bounded

static const bool is_bounded;		(C++11 前)
static constexpr bool is_bounded;		(C++11 起)

std::numeric_limits<T>::is_bounded 对所有表示有限的值集合的算术类型 T 为 true 。所有基础类型均有界，不过此常量会在 std::numeric_limits 对库提供的任意精度算术类型的特化中为 false 。

标准特化

`T`	std::numeric_limits<T>::is_bounded 的值
/* non-specialized */	false
bool	true
char	true
signed char	true
unsigned char	true
wchar_t	true
char8_t	true
char16_t	true
char32_t	true
short	true
unsigned short	true
int	true
unsigned int	true
long	true
unsigned long	true
long long	true
unsigned long long	true
float	true
double	true
long double	true

调用示例

#include <iostream>
#include <string>
#include <limits>

struct SName
{
};

//偏特化
struct SPartSpec
{
};

namespace std
{
template<>
struct numeric_limits<SPartSpec>
{
    static _GLIBCXX_USE_CONSTEXPR bool is_specialized   = true;
    static _GLIBCXX_USE_CONSTEXPR bool is_signed        = true;
    static _GLIBCXX_USE_CONSTEXPR bool is_integer       = true;
    static _GLIBCXX_USE_CONSTEXPR bool is_exact         = true;
    static _GLIBCXX_USE_CONSTEXPR bool has_infinity     = true;
    static _GLIBCXX_USE_CONSTEXPR bool has_quiet_NaN    = true;
    static _GLIBCXX_USE_CONSTEXPR bool has_signaling_NaN = true;
    static _GLIBCXX_USE_CONSTEXPR float_denorm_style has_denorm     = denorm_present;
    static _GLIBCXX_USE_CONSTEXPR bool has_denorm_loss  = true;
    static _GLIBCXX_USE_CONSTEXPR float_round_style round_style     = round_toward_neg_infinity;
    static _GLIBCXX_USE_CONSTEXPR bool is_iec559        = true;
    static _GLIBCXX_USE_CONSTEXPR bool is_bounded       = true;
};
}

int main()
{
    std::cout << std::boolalpha;
    std::cout << "std::numeric_limits<bool>::is_bounded:                 "
              << std::numeric_limits<bool>::is_bounded << std::endl;
    std::cout << "std::numeric_limits<char>::is_bounded:                 "
              << std::numeric_limits<char>::is_bounded << std::endl;
    std::cout << "std::numeric_limits<signed char>::is_bounded:          "
              << std::numeric_limits<signed char>::is_bounded << std::endl;
    std::cout << "std::numeric_limits<unsigned char>::is_bounded:        "
              << std::numeric_limits<unsigned char>::is_bounded << std::endl;
    std::cout << "std::numeric_limits<wchar_t>::is_bounded:              "
              << std::numeric_limits<wchar_t>::is_bounded << std::endl;
    std::cout << "std::numeric_limits<char16_t>::is_bounded:             "
              << std::numeric_limits<char16_t>::is_bounded << std::endl;
    std::cout << "std::numeric_limits<char32_t>::is_bounded:             "
              << std::numeric_limits<char32_t>::is_bounded << std::endl;
    std::cout << "std::numeric_limits<short>::is_bounded:                "
              << std::numeric_limits<short>::is_bounded << std::endl;
    std::cout << "std::numeric_limits<unsigned short>::is_bounded:       "
              << std::numeric_limits<unsigned short>::is_bounded << std::endl;
    std::cout << "std::numeric_limits<int>::is_bounded:                  "
              << std::numeric_limits<int>::is_bounded << std::endl;
    std::cout << "std::numeric_limits<unsigned int>::is_bounded:         "
              << std::numeric_limits<unsigned int>::is_bounded << std::endl;
    std::cout << "std::numeric_limits<long>::is_bounded:                 "
              << std::numeric_limits<long>::is_bounded << std::endl;
    std::cout << "std::numeric_limits<unsigned long>::is_bounded:        "
              << std::numeric_limits<unsigned long>::is_bounded << std::endl;
    std::cout << "std::numeric_limits<long long>::is_bounded:            "
              << std::numeric_limits<long long>::is_bounded << std::endl;
    std::cout << "std::numeric_limits<unsigned long long>::is_bounded:   "
              << std::numeric_limits<unsigned long long>::is_bounded << std::endl;
    std::cout << "std::numeric_limits<float>::is_bounded:                "
              << std::numeric_limits<float>::is_bounded << std::endl;
    std::cout << "std::numeric_limits<double>::is_bounded:               "
              << std::numeric_limits<double>::is_bounded << std::endl;
    std::cout << "std::numeric_limits<long double>::is_bounded:          "
              << std::numeric_limits<long double>::is_bounded << std::endl;
    std::cout << "std::numeric_limits<std::string>::is_bounded:          "
              << std::numeric_limits<std::string>::is_bounded << std::endl;
    std::cout << "std::numeric_limits<SName>::is_bounded:                "
              << std::numeric_limits<SName>::is_bounded << std::endl;
    std::cout << "std::numeric_limits<SPartSpec>::is_bounded:            "
              << std::numeric_limits<SPartSpec>::is_bounded << std::endl;
    return 0;
}