xref: /aosp_15_r20/external/federated-compute/fcp/base/move_to_lambda.h (revision 14675a029014e728ec732f129a32e299b2da0601)

/*
 * Copyright 2018 Google LLC
 *
 * 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 FCP_BASE_MOVE_TO_LAMBDA_H_
#define FCP_BASE_MOVE_TO_LAMBDA_H_

#include <utility>

namespace fcp {

/**
 * Copyable wrapper for a move-only value. See MoveToLambda.
 * The value is accessible with the * and -> operators.
 *
 * You must be careful to avoid accidnetal copies of this type. Copies are
 * destructive (by design), so accidental copies might lead to using a
 */
template <typename T>
class MoveToLambdaWrapper {
 public:
  explicit MoveToLambdaWrapper(T t) : value_(std::move(t)) {}

  // The copy and move constructors are intentionally non-const.

  MoveToLambdaWrapper(MoveToLambdaWrapper const& other)
      : value_(std::move(other.value_)) {}

  MoveToLambdaWrapper& operator=(MoveToLambdaWrapper const& other) {
    value_ = std::move(other.value_);
    return *this;
  }

  // We respect const-ness of the wrapper when dereferencing, so that 'mutable'
  // is required on the lambda depending on usage of the value; changes
  // to a captured value persist across calls to the lambda, which is rarely
  // desired.

  T const& operator*() const & {
    return value_;
  }

  T const* operator->() const & {
    return &value_;
  }

  T& operator*() & {
    return value_;
  }

  T* operator->() & {
    return &value_;
  }

 private:
  mutable T value_;
};

/**
 * Allows capturing a value into a lambda 'by move', before C++14. This is
 * implemented by a copyable wrapper, which actually moves its value.
 *
 *     auto moving = MoveToLambda(value);
 *     DoSometing([moving]{ V const& v = *moving; ... });
 */
template <typename T>
MoveToLambdaWrapper<std::remove_reference_t<T>> MoveToLambda(T&& value) {
  static_assert(
      std::is_rvalue_reference<T&&>::value,
      "Expected an rvalue: If the value is copied anyway (to this function), "
      "you might as well put it in the lambda-capture list directly.");
  return MoveToLambdaWrapper<std::remove_reference_t<T>>(
      std::forward<T>(value));
}

}  // namespace fcp

#endif  // FCP_BASE_MOVE_TO_LAMBDA_H_