namespace std {
  template<class E> class initializer_list {
  public:
    using value_type = E;
    using reference = const E&;
    using const_reference = const E&;
    using size_type = size_t;
 
    using iterator = const E*;
    using const_iterator = const E*;
 
    constexpr initializer_list() noexcept;
 
    constexpr size_t size() const noexcept;     // 元素数量
    constexpr const E* begin() const noexcept;  // 首元素
    constexpr const E* end() const noexcept;    // 末元素后一位置
  };
 
  // initializer_list 范围访问
  template<class E> constexpr const E* begin(initializer_list<E> il) noexcept;
  template<class E> constexpr const E* end(initializer_list<E> il) noexcept;
}