Unit.h

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/*
 * Copyright 2015-present Facebook, Inc.
 *
 * 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.
 */

#pragma once

#include <type_traits>

namespace folly {

struct Unit {
  constexpr bool operator==(const Unit& /*other*/) const {
    return true;
  }
  constexpr bool operator!=(const Unit& /*other*/) const {
    return false;
  }
};

constexpr Unit unit{};

template <typename T>
struct lift_unit {
  using type = T;
};
template <>
struct lift_unit<void> {
  using type = Unit;
};
template <typename T>
using lift_unit_t = typename lift_unit<T>::type;

template <typename T>
struct drop_unit {
  using type = T;
};
template <>
struct drop_unit<Unit> {
  using type = void;
};
template <typename T>
using drop_unit_t = typename drop_unit<T>::type;

} // namespace folly