opaque_id.h

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/******************************************************************************
 * Copyright 2022 ETC 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.
 *******************************************************************************
 * This file is a part of EtcPal. For more information, go to:
 * https://github.com/ETCLabs/EtcPal
 ******************************************************************************/
 
 
#ifndef ETCPAL_CPP_OPAQUE_ID_H_
#define ETCPAL_CPP_OPAQUE_ID_H_
 
#include <functional>
#include "etcpal/cpp/common.h"
 
namespace etcpal
{
 
template <class IdType, class ValueType, ValueType InvalidValue>
class OpaqueId
{
public:
  OpaqueId() = default;
  constexpr explicit OpaqueId(ValueType value);
 
  static constexpr OpaqueId Invalid();
 
  constexpr ValueType value() const;
  constexpr bool      IsValid() const;
  constexpr explicit  operator bool() const;
  constexpr bool      operator!() const;
 
  ETCPAL_CONSTEXPR_14 void SetValue(const ValueType& new_value);
  ETCPAL_CONSTEXPR_14 void Clear();
 
private:
  ValueType value_{InvalidValue};
};
 
template <class IdType, class ValueType, ValueType InvalidValue>
constexpr OpaqueId<IdType, ValueType, InvalidValue> OpaqueId<IdType, ValueType, InvalidValue>::Invalid()
{
  return OpaqueId();
}
 
template <class IdType, class ValueType, ValueType InvalidValue>
constexpr OpaqueId<IdType, ValueType, InvalidValue>::OpaqueId(ValueType value) : value_(value)
{
}
 
template <class IdType, class ValueType, ValueType InvalidValue>
constexpr ValueType OpaqueId<IdType, ValueType, InvalidValue>::value() const
{
  return value_;
}
 
template <class IdType, class ValueType, ValueType InvalidValue>
constexpr bool OpaqueId<IdType, ValueType, InvalidValue>::IsValid() const
{
  return !(value_ == InvalidValue);
}
 
template <class IdType, class ValueType, ValueType InvalidValue>
constexpr OpaqueId<IdType, ValueType, InvalidValue>::operator bool() const
{
  return IsValid();
}
 
template <class IdType, class ValueType, ValueType InvalidValue>
constexpr bool OpaqueId<IdType, ValueType, InvalidValue>::operator!() const
{
  return !IsValid();
}
 
template <class IdType, class ValueType, ValueType InvalidValue>
ETCPAL_CONSTEXPR_14_OR_INLINE void OpaqueId<IdType, ValueType, InvalidValue>::SetValue(const ValueType& new_value)
{
  value_ = new_value;
}
 
template <class IdType, class ValueType, ValueType InvalidValue>
ETCPAL_CONSTEXPR_14_OR_INLINE void OpaqueId<IdType, ValueType, InvalidValue>::Clear()
{
  value_ = InvalidValue;
}
 
 
 
template <class IdType, class ValueType, ValueType InvalidValue>
constexpr bool operator==(const OpaqueId<IdType, ValueType, InvalidValue>& a,
                          const OpaqueId<IdType, ValueType, InvalidValue>& b)
{
  return a.value() == b.value();
}
 
template <class IdType, class ValueType, ValueType InvalidValue>
constexpr bool operator!=(const OpaqueId<IdType, ValueType, InvalidValue>& a,
                          const OpaqueId<IdType, ValueType, InvalidValue>& b)
{
  return !(a == b);
}
 
 
};  // namespace etcpal
 
 
namespace std
{
// Inject a std::less specialization for any template specialization for OpaqueId. This takes the
// place of operator< for the purposes of OpaqueId usage in ordered containers. We don't provide
// relational operators other than == and != to avoid the temptation to treat OpaqueIds like
// numbers.
template <class IdType, class ValueType, ValueType InvalidValue>
struct less<etcpal::OpaqueId<IdType, ValueType, InvalidValue>>
{
  ETCPAL_CONSTEXPR_14 bool operator()(const etcpal::OpaqueId<IdType, ValueType, InvalidValue>& a,
                                      const etcpal::OpaqueId<IdType, ValueType, InvalidValue>& b) const
  {
    return a.value() < b.value();
  }
};
 
// Inject a new std::hash specialization for any template specialization of OpaqueId, so that
// OpaqueIds can be used in hash-based containers (e.g. unordered_map and unordered_set) without
// a user needing to create a hash specialization.
//
// The std::hash specialization simply passes through to a hash of the underlying value. If the
// underlying value is not hashable, this specialization should simply be eliminated without a
// compile error through SFINAE.
template <class IdType, class ValueType, ValueType InvalidValue>
struct hash<etcpal::OpaqueId<IdType, ValueType, InvalidValue>>
{
  std::size_t operator()(const etcpal::OpaqueId<IdType, ValueType, InvalidValue>& id) const noexcept
  {
    return std::hash<ValueType>()(id.value());
  }
};
};  // namespace std
 
 
#endif  // ETCPAL_CPP_OPAQUE_ID_H_