WhileLoop.cpp

//
// Created by light on 20-1-5.
//

#include <iostream>

using namespace std;

// 使用template实现while循环

template <bool condition, typename Body> struct WhileLoop;

template <typename Body> struct WhileLoop<true, Body> {
  typedef
      typename WhileLoop<Body::cond_value, typename Body::next_type>::type type;
};

template <typename Body> struct WhileLoop<false, Body> {
  typedef typename Body::res_type type;
};

template <typename Body> struct While {
  typedef typename WhileLoop<Body::cond_value, Body>::type type;
};
template <typename Body> using While_t = WhileLoop<Body::cond_value, Body>;

namespace my {
template <class T, T v> struct integral_constant {
  static const T value = v;
  typedef T value_type;
  typedef integral_constant<T, v> type;
};
} // namespace my
template <int result, int n> struct SumLoop {

  // 循环的条件
  static const bool cond_value = n != 0;

  // 循环后的结果
  static const int res_value = result;

  // 循环时的状态
  typedef my::integral_constant<int, res_value> res_type;

  // 循环执行一次时的状态
  typedef SumLoop<result + n, n - 1> next_type;
};

template <int n> struct Sum { typedef SumLoop<0, n> type; };

template <int n> using Sum_t = SumLoop<0, n>;

int main() {
  cout << While<Sum<6>::type>::type::value << endl;
  cout << While_t<Sum_t<6>>::type::value << endl;
  return 0;
}