#include <compare>
 
namespace std {
  class type_index;
  template<class T> struct hash;
  template<> struct hash<type_index>;
}

class type_index {
  public:
    type_index(const type_info& rhs) noexcept;
    bool operator==(const type_index& rhs) const noexcept;
    bool operator< (const type_index& rhs) const noexcept;
    bool operator> (const type_index& rhs) const noexcept;
    bool operator<=(const type_index& rhs) const noexcept;
    bool operator>=(const type_index& rhs) const noexcept;
    strong_ordering operator<=>(const type_index& rhs) const noexcept;
    size_t hash_code() const noexcept;
    const char* name() const noexcept;
 
  private:
    const type_info* target; // 仅用于阐释
    // 注意，此处使用了指针而非引用，
    // 表示预置的复制/移动构造函数及赋值运算符均有所提供并按预期工作。
  };
}