【】std::integral_constant语法

#include <iostream>

template<class T, T v>
struct integral_constant {
    static constexpr T value = v;
    using value_type = T;
    using type = integral_constant; // 使用注入类名
    constexpr operator value_type() const noexcept {
        return value;
    }
    constexpr value_type operator()() const noexcept {
        return value;    // C++14 起
    }
};

int main() {
    std::cout << "integral_constant<int, 5> == "
              << std::integral_constant<int, 5>::value << std::endl;
    std::cout << "integral_constant<bool, false> == " << std::boolalpha
              << std::integral_constant<bool, false>::value << std::endl;

    return (0);
}