在编译时扩展整数序列模板参数

Question

我有点不确定如何在编译时实现可变参数扩展。考虑下面的用例，我想找出序列中最大元素的索引：

template<A>
struct doSomething {
  static constexpr int getIndexOfMaxValue() {
    // recursive variadic expansion
  }

  static constexpr int value = getIndexOfMaxValue();
}

int main() {
   std::cout << doSomething<std::integer_sequence<int, 1, 2, 3, 4, 3>::value <<std::endl;
}

谁能帮我解释一下？

Answer 1

大概是这样的：

template <typename T, T... Vals, size_t... Is>
constexpr int getIndexOfMaxValueHelper(
    std::integer_sequence<T, Vals...>,
    std::index_sequence<Is...>) {
  constexpr T max_val = std::max({Vals...});
  constexpr size_t N = sizeof...(Is);
  return std::min({(Vals == max_val ? Is : N)...});
}

template <typename A>
struct doSomething {
  static constexpr int getIndexOfMaxValue() {
    return getIndexOfMaxValueHelper(A{}, std::make_index_sequence<A::size()>{});
  }

  static constexpr int value = getIndexOfMaxValue();
};

int main() {
   std::cout << doSomething<std::integer_sequence<int, 1, 2, 3, 4, 3>>::value <<std::endl;
}

Demo

在编译时扩展整数序列模板参数

Expanding integer-sequence template-param in compile time

variadic-templates

c++11

c++14