Standard library header <any> (C++17)

namespace std {
  // class bad_any_cast
  class bad_any_cast;
 
  // class any
  class any;
 
  // non-member functions
  void swap(any& x, any& y) noexcept;
 
  template<class T, class... Args>
    any make_any(Args&&... args);
  template<class T, class U, class... Args>
    any make_any(initializer_list<U> il, Args&&... args);
 
  template<class T>
    T any_cast(const any& operand);
  template<class T>
    T any_cast(any& operand);
  template<class T>
    T any_cast(any&& operand);
 
  template<class T>
    const T* any_cast(const any* operand) noexcept;
  template<class T>
    T* any_cast(any* operand) noexcept;
}

namespace std {
  class bad_any_cast : public bad_cast {
  public:
    // see [exception] for the specification of the special member functions
    const char* what() const noexcept override;
  };
}

namespace std {
  class any {
  public:
    // construction and destruction
    constexpr any() noexcept;
 
    any(const any& other);
    any(any&& other) noexcept;
 
    template<class T>
      any(T&& value);
 
    template<class T, class... Args>
      explicit any(in_place_type_t<T>, Args&&...);
    template<class T, class U, class... Args>
      explicit any(in_place_type_t<T>, initializer_list<U>, Args&&...);
 
    ~any();
 
    // assignments
    any& operator=(const any& rhs);
    any& operator=(any&& rhs) noexcept;
 
    template<class T>
      any& operator=(T&& rhs);
 
    // modifiers
    template<class T, class... Args>
      decay_t<T>& emplace(Args&&...);
    template<class T, class U, class... Args>
      decay_t<T>& emplace(initializer_list<U>, Args&&...);
    void reset() noexcept;
    void swap(any& rhs) noexcept;
 
    // observers
    bool has_value() const noexcept;
    const type_info& type() const noexcept;
  };
}