Add aidl::nullable type for nullable in AIDL

/*

 * Copyright (C) 2020 The Android Open Source Project

 *

 * 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 <optional>

#include <utility>

namespace android {

namespace aidl {

// nullable/make_nullable provide source-level compatibility between std::opional and std::unique_ptr

// usage:

//     nullable<Foo> a;

//     nullable<Foo> b = make_nullable<Foo>(...);

//     auto c = make_nullable<Foo>(...);

//     c.reset();

//     c = make_nullable<Foo>(...);

//     c = std::move(a);

template <typename T>

using nullable = std::optional<T>;

template <typename T, typename... Args>

inline nullable<T> make_nullable(Args&&... args) {

    return std::make_optional<T>(std::forward<Args>(args)...);

}

} // namespace aidl

} // namespace android

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