std::integral_constant

STD：积分[医]常量

Defined in header
template< class T, T v > struct integral_constant;		(since C++11)

std::integral_constant包装指定类型的静态常量。它是C++类型特征的基类。

Helper templates A helper alias template std::bool_constant is defined for the common case where T is bool. template using bool_constant = integral_constant;	template using bool_constant = integral_constant;			(since C++17)
template <bool B> using bool_constant = integral_constant<bool, B>;

两种类型用于常见情况T是bool提供：

在标头<type中定义[医]性状>

*。

类型定义

千真万确[医]STD类型：：整型[医]常数<bool，true>

假的[医]STD类型：：整型[医]常数<bool，false>

成员类型

Type	Definition
value_type	T
type	std::integral_constant<T,v>

成员常数

Name	Value
constexpr T value static	static constant of type T with value v (public static member constant)

成员函数

operator value_type	returns the wrapped value (public member function)
operator() (C++14)	returns the wrapped value (public member function)

STD：积分[医]常数：：运算符值[医]类型

constexpr operator value_type() const;

转换函数。返回包装的值。

例外

noexcept规格：

noexcept

STD：积分[医]常数：：运算符%28%29

constexpr value_type operator()() const;		(since C++14)

返回包装的值。此函数启用std::integral_constant作为编译时函数对象的来源。

例外

noexcept规格：

noexcept

可能的实施

模板<类T，Tv>结构积分[医]常数{静态常数T值=v；[医]类型[医]常量类型；//使用注入的类名称conexpr运算符值[医]类型%28%29 const no{返回值除外；}constexpr值)[医]类型运算符%28%29%28%29 const no除{返回值；}//自c++14}；

*。

例

二次

#include <iostream>
#include <type_traits>
 
int main() 
{
    typedef std::integral_constant<int, 2> two_t;
    typedef std::integral_constant<int, 4> four_t;
 
//  static_assert(std::is_same<two_t, four_t>::value,
//                "two_t and four_t are not equal!" 
//  error: static assertion failed: "two_t and four_t are not equal!"
 
    static_assert(two_t::value*2 == four_t::value,
       "2*2 != 4"
    
 
    enum class my_e {
       e1,
       e2
    };
    typedef std::integral_constant<my_e, my_e::e1> my_e_e1;
    typedef std::integral_constant<my_e, my_e::e2> my_e_e2;
 
//  static_assert(my_e_e1::value == my_e::e2,
//               "my_e_e1::value != my_e::e2"
//  error: static assertion failed: "my_e_e1::value != my_e::e2"
 
    static_assert(std::is_same<my_e_e2, my_e_e2>::value,
                  "my_e_e2 != my_e_e2"
}

二次

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