util/include/chre/util/nested_data_ptr.h

/*
 * Copyright (C) 2019 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.
 */

#ifndef UTIL_CHRE_NESTED_DATA_PTR_H_
#define UTIL_CHRE_NESTED_DATA_PTR_H_

namespace chre {

/**
 * A template that provides the ability to store data inside of a void pointer
 * to avoid allocating space on the heap in the case where the data is smaller
 * than the size of a void pointer.
 */
template <typename DataType>
union NestedDataPtr {
  static_assert(sizeof(DataType) <= sizeof(void *),
                "Requested data type must fit in a void* to use NestedDataPtr");
  // If the sizeof() check passes, then this is unlikely to be an issue, and in
  // many usage scenarios this wouldn't be an issue (e.g. reinterpreting a value
  // stored in a register), but it's included here just to be safe.
  static_assert(alignof(DataType) <= alignof(void *),
                "Additional alignment in NestedDataPtr can't be guaranteed");

  NestedDataPtr() = default;

  explicit NestedDataPtr(DataType nestedData) : data(nestedData) {}
  explicit NestedDataPtr(void *ptr) : dataPtr(ptr) {}

  // Implicit conversions
  operator DataType() const {
    return data;
  }

  operator void *() const {
    return dataPtr;
  }

  void *dataPtr;
  DataType data;
};

}  // namespace chre

#endif  // UTIL_CHRE_NESTED_DATA_PTR_H_