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/*
* Copyright 2018 Google
*
* 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 FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_UTIL_COMPARATOR_HOLDER_H_
#define FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_UTIL_COMPARATOR_HOLDER_H_
#include <type_traits>
namespace firebase {
namespace firestore {
namespace util {
/**
* A holder of some comparator (e.g. std::less or util::Comparator) that
* implements the empty base optimization for the common case where the
* comparator occupies no storage.
*/
template <typename C, bool = std::is_empty<C>::value>
class ComparatorHolder {
protected:
explicit ComparatorHolder(const C& comparator) noexcept
: comparator_(comparator) {
}
public:
const C& comparator() const noexcept {
return comparator_;
}
private:
C comparator_;
};
// Implementation to use when C is empty.
template <typename C>
class ComparatorHolder<C, true> : private C {
protected:
explicit ComparatorHolder(const C& /* comparator */) noexcept {
}
public:
const C& comparator() const noexcept {
return *this;
}
};
} // namespace util
} // namespace firestore
} // namespace firebase
#endif // FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_UTIL_COMPARATOR_HOLDER_H_
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