【】std::integral_constant语法

 1 #include <iostream>
 2 
 3 template<class T, T v>
 4 struct integral_constant {
 5     static constexpr T value = v;
 6     using value_type = T;
 7     using type = integral_constant; // 使用注入类名
 8     constexpr operator value_type() const noexcept {
 9         return value;
10     }
11     constexpr value_type operator()() const noexcept {
12         return value;    // C++14 起
13     }
14 };
15 
16 int main() {
17     std::cout << "integral_constant<int, 5> == "
18               << std::integral_constant<int, 5>::value << std::endl;
19     std::cout << "integral_constant<bool, false> == " << std::boolalpha
20               << std::integral_constant<bool, false>::value << std::endl;
21 
22     return (0);
23 }