头文件：#include<type_traits>

可能的实现:

template<class T,T v>
struct integral_constant{
    static constexpr T value = v;
    using value_type = T;
    using type = integral_constant;
    constexpr operator value_type() const noexcept { return value; }
    constexpr value_type operator()() const noexcept { return value; } // if __cplusplus > 201103L.
}

std::integral_constant 包装(wrap)一个指定类型T的一个静态常量，它是C++类型特征(The C++ type traits)的基类。

• Member constants

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

• Member constants

Name	Value
constanexpr T value [static]	static constant of type T with value v (public static member constant)

• Member function

operator	value_type
operator value_type	returns the wrapped value (public member function)
operator()	returns the wrapped value (public member function)