RValueReferenceWrapper.h

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/*
 * Copyright 2017-present Facebook, 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.
 */

#pragma once

#include <cassert>
#include <memory>
#include <utility>

namespace folly {

template <class T>
class rvalue_reference_wrapper {
 public:
  using type = T;

  rvalue_reference_wrapper() noexcept : ptr_(nullptr) {}

  explicit rvalue_reference_wrapper(T&& ref) noexcept
      : ptr_(std::addressof(ref)) {}

  explicit rvalue_reference_wrapper(T&) noexcept = delete;

  rvalue_reference_wrapper(rvalue_reference_wrapper<T>&& other) noexcept
      : ptr_(other.ptr_) {
    other.ptr_ = nullptr;
  }

  rvalue_reference_wrapper& operator=(
      rvalue_reference_wrapper&& other) noexcept {
    ptr_ = other.ptr_;
    other.ptr_ = nullptr;
    return *this;
  }

  /* implicit */ operator T &&() && noexcept {
    return static_cast<rvalue_reference_wrapper&&>(*this).get();
  }

  T&& get() && noexcept {
    assert(valid());
    T& ref = *ptr_;
    ptr_ = nullptr;
    return static_cast<T&&>(ref);
  }

  template <class... Args>
      decltype(auto) operator()(Args&&... args) &&
      noexcept(noexcept(std::declval<T>()(std::forward<Args>(args)...))) {
    return static_cast<rvalue_reference_wrapper&&>(*this).get()(
        std::forward<Args>(args)...);
  }

  bool valid() const noexcept {
    return ptr_ != nullptr;
  }

 private:
  // Disallow copy construction and copy assignment, to make it harder to
  // accidentally use an rvalue reference multiple times.
  rvalue_reference_wrapper(const rvalue_reference_wrapper&) = delete;
  rvalue_reference_wrapper& operator=(const rvalue_reference_wrapper&) = delete;

  T* ptr_;
};

template <typename T>
rvalue_reference_wrapper<T> rref(T&& value) noexcept {
  return rvalue_reference_wrapper<T>(std::move(value));
}
template <typename T>
rvalue_reference_wrapper<T> rref(T&) noexcept = delete;
} // namespace folly