/* Copyright 2021 The TensorFlow Authors. All Rights Reserved.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
==============================================================================*/
#ifndef TENSORFLOW_CC_EXPERIMENTAL_LIBTF_IMPL_NONE_H_
#define TENSORFLOW_CC_EXPERIMENTAL_LIBTF_IMPL_NONE_H_

#include <iosfwd>
#include <utility>

namespace tf {
namespace libtf {
namespace impl {
/// @brief The Singleton `None` class.
///
/// This class is not user-constructible. To create a `None` instance, use
/// None::GetInstance().

class None final {
 public:
  /// Retrieves the `None` instance.
  ///
  /// @return Returns the `None` singleton.
  static None& GetInstance();

  /// Equality operator.
  bool operator==(const None& other) const { return true; }

  /// Overload AbslHashValue.
  template <typename H>
  friend H AbslHashValue(H h, const None& n) {
    return H::combine(std::move(h), 34559);
  }

 private:
  // Private contructor.
  None() {}
};

// Defined in iostream.cc.
std::ostream& operator<<(std::ostream& o, const None& none);

}  // namespace impl
}  // namespace libtf
}  // namespace tf

#endif  // TENSORFLOW_CC_EXPERIMENTAL_LIBTF_IMPL_NONE_H_
