Replaceable.h

Go to the documentation of this file.

/*
 * 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 <initializer_list>
#include <new>
#include <type_traits>
#include <utility>

#include <folly/Portability.h>
#include <folly/Traits.h>
#include <folly/Utility.h>
#include <folly/lang/Launder.h>

namespace folly {
template <class T>
class Replaceable;

namespace replaceable_detail {
/* Mixin templates to give `replaceable<T>` the following properties:
 *
 * 1. Trivial destructor if `T` has a trivial destructor; user-provided
 *    otherwise
 * 2. Move constructor if `T` has a move constructor; deleted otherwise
 * 3. Move assignment operator if `T` has a move constructor; deleted
 *    otherwise
 * 4. Copy constructor if `T` has a copy constructor; deleted otherwise
 * 5. Copy assignment operator if `T` has a copy constructor; deleted
 *    otherwise
 *
 * Has to be done in this way because we can't `enable_if` them away
 */
template <
    class T,
    bool = std::is_destructible<T>::value,
    bool = std::is_trivially_destructible<T>::value>
struct dtor_mixin;

/* Destructible and trivially destructible */
template <class T>
struct dtor_mixin<T, true, true> {};

/* Destructible and not trivially destructible */
template <class T>
struct dtor_mixin<T, true, false> {
  dtor_mixin() = default;
  dtor_mixin(dtor_mixin&&) = default;
  dtor_mixin(dtor_mixin const&) = default;
  dtor_mixin& operator=(dtor_mixin&&) = default;
  dtor_mixin& operator=(dtor_mixin const&) = default;
  ~dtor_mixin() noexcept(std::is_nothrow_destructible<T>::value) {
    T* destruct_ptr = launder(reinterpret_cast<T*>(
        reinterpret_cast<Replaceable<T>*>(this)->storage_));
    destruct_ptr->~T();
  }
};

/* Not destructible */
template <class T, bool A>
struct dtor_mixin<T, false, A> {
  dtor_mixin() = default;
  dtor_mixin(dtor_mixin&&) = default;
  dtor_mixin(dtor_mixin const&) = default;
  dtor_mixin& operator=(dtor_mixin&&) = default;
  dtor_mixin& operator=(dtor_mixin const&) = default;
  ~dtor_mixin() = delete;
};

template <
    class T,
    bool = std::is_default_constructible<T>::value,
    bool = std::is_move_constructible<T>::value>
struct default_and_move_ctor_mixin;

/* Not default-constructible and not move-constructible */
template <class T>
struct default_and_move_ctor_mixin<T, false, false> {
  default_and_move_ctor_mixin() = delete;
  default_and_move_ctor_mixin(default_and_move_ctor_mixin&&) = delete;
  default_and_move_ctor_mixin(default_and_move_ctor_mixin const&) = default;
  default_and_move_ctor_mixin& operator=(default_and_move_ctor_mixin&&) =
      default;
  default_and_move_ctor_mixin& operator=(default_and_move_ctor_mixin const&) =
      default;

 protected:
  inline explicit default_and_move_ctor_mixin(int) {}
};

/* Default-constructible and move-constructible */
template <class T>
struct default_and_move_ctor_mixin<T, true, true> {
  inline default_and_move_ctor_mixin() noexcept(
      std::is_nothrow_constructible<T>::value) {
    ::new (reinterpret_cast<Replaceable<T>*>(this)->storage_) T();
  }
  inline default_and_move_ctor_mixin(
      default_and_move_ctor_mixin&&
          other) noexcept(std::is_nothrow_constructible<T, T&&>::value) {
    ::new (reinterpret_cast<Replaceable<T>*>(this)->storage_)
        T(*std::move(reinterpret_cast<Replaceable<T>&>(other)));
  }
  default_and_move_ctor_mixin(default_and_move_ctor_mixin const&) = default;
  default_and_move_ctor_mixin& operator=(default_and_move_ctor_mixin&&) =
      default;
  inline default_and_move_ctor_mixin& operator=(
      default_and_move_ctor_mixin const&) = default;

 protected:
  inline explicit default_and_move_ctor_mixin(int) {}
};

/* Default-constructible and not move-constructible */
template <class T>
struct default_and_move_ctor_mixin<T, true, false> {
  inline default_and_move_ctor_mixin() noexcept(
      std::is_nothrow_constructible<T>::value) {
    ::new (reinterpret_cast<Replaceable<T>*>(this)->storage_) T();
  }
  default_and_move_ctor_mixin(default_and_move_ctor_mixin&&) = delete;
  default_and_move_ctor_mixin(default_and_move_ctor_mixin const&) = default;
  default_and_move_ctor_mixin& operator=(default_and_move_ctor_mixin&&) =
      default;
  default_and_move_ctor_mixin& operator=(default_and_move_ctor_mixin const&) =
      default;

 protected:
  inline explicit default_and_move_ctor_mixin(int) {}
};

/* Not default-constructible but is move-constructible */
template <class T>
struct default_and_move_ctor_mixin<T, false, true> {
  default_and_move_ctor_mixin() = delete;
  inline default_and_move_ctor_mixin(
      default_and_move_ctor_mixin&&
          other) noexcept(std::is_nothrow_constructible<T, T&&>::value) {
    ::new (reinterpret_cast<Replaceable<T>*>(this)->storage_)
        T(*std::move(reinterpret_cast<Replaceable<T>&>(other)));
  }
  default_and_move_ctor_mixin(default_and_move_ctor_mixin const&) = default;
  default_and_move_ctor_mixin& operator=(default_and_move_ctor_mixin&&) =
      default;
  default_and_move_ctor_mixin& operator=(default_and_move_ctor_mixin const&) =
      default;

 protected:
  inline explicit default_and_move_ctor_mixin(int) {}
};

template <
    class T,
    bool = (std::is_destructible<T>::value) &&
        (std::is_move_constructible<T>::value)>
struct move_assignment_mixin;

/* Not (destructible and move-constructible) */
template <class T>
struct move_assignment_mixin<T, false> {
  move_assignment_mixin() = default;
  move_assignment_mixin(move_assignment_mixin&&) = default;
  move_assignment_mixin(move_assignment_mixin const&) = default;
  move_assignment_mixin& operator=(move_assignment_mixin&&) = delete;
  move_assignment_mixin& operator=(move_assignment_mixin const&) = default;
};

/* Both destructible and move-constructible */
template <class T>
struct move_assignment_mixin<T, true> {
  move_assignment_mixin() = default;
  move_assignment_mixin(move_assignment_mixin&&) = default;
  move_assignment_mixin(move_assignment_mixin const&) = default;
  inline move_assignment_mixin&
  operator=(move_assignment_mixin&& other) noexcept(
      std::is_nothrow_destructible<T>::value&&
          std::is_nothrow_move_constructible<T>::value) {
    T* destruct_ptr = launder(reinterpret_cast<T*>(
        reinterpret_cast<Replaceable<T>*>(this)->storage_));
    destruct_ptr->~T();
    ::new (reinterpret_cast<Replaceable<T>*>(this)->storage_)
        T(*std::move(reinterpret_cast<Replaceable<T>&>(other)));
    return *this;
  }
  move_assignment_mixin& operator=(move_assignment_mixin const&) = default;
};

template <class T, bool = std::is_copy_constructible<T>::value>
struct copy_ctor_mixin;

/* Not copy-constructible */
template <class T>
struct copy_ctor_mixin<T, false> {
  copy_ctor_mixin() = default;
  copy_ctor_mixin(copy_ctor_mixin&&) = default;
  copy_ctor_mixin(copy_ctor_mixin const&) = delete;
  copy_ctor_mixin& operator=(copy_ctor_mixin&&) = default;
  copy_ctor_mixin& operator=(copy_ctor_mixin const&) = delete;
};

/* Copy-constructible */
template <class T>
struct copy_ctor_mixin<T, true> {
  copy_ctor_mixin() = default;
  inline copy_ctor_mixin(copy_ctor_mixin const& other) noexcept(
      std::is_nothrow_constructible<T, T const&>::value) {
    ::new (reinterpret_cast<Replaceable<T>*>(this)->storage_)
        T(*reinterpret_cast<Replaceable<T> const&>(other));
  }
  copy_ctor_mixin(copy_ctor_mixin&&) = default;
  copy_ctor_mixin& operator=(copy_ctor_mixin&&) = default;
  copy_ctor_mixin& operator=(copy_ctor_mixin const&) = default;
};

template <
    class T,
    bool = (std::is_destructible<T>::value) &&
        (std::is_copy_constructible<T>::value)>
struct copy_assignment_mixin;

/* Not (destructible and copy-constructible) */
template <class T>
struct copy_assignment_mixin<T, false> {
  copy_assignment_mixin() = default;
  copy_assignment_mixin(copy_assignment_mixin&&) = default;
  copy_assignment_mixin(copy_assignment_mixin const&) = default;
  copy_assignment_mixin& operator=(copy_assignment_mixin&&) = default;
  copy_assignment_mixin& operator=(copy_assignment_mixin const&) = delete;
};

/* Both destructible and copy-constructible */
template <class T>
struct copy_assignment_mixin<T, true> {
  copy_assignment_mixin() = default;
  copy_assignment_mixin(copy_assignment_mixin&&) = default;
  copy_assignment_mixin(copy_assignment_mixin const&) = default;
  copy_assignment_mixin& operator=(copy_assignment_mixin&&) = default;
  inline copy_assignment_mixin&
  operator=(copy_assignment_mixin const& other) noexcept(
      std::is_nothrow_destructible<T>::value&&
          std::is_nothrow_copy_constructible<T>::value) {
    T* destruct_ptr = launder(reinterpret_cast<T*>(
        reinterpret_cast<Replaceable<T>*>(this)->storage_));
    destruct_ptr->~T();
    ::new (reinterpret_cast<Replaceable<T>*>(this)->storage_)
        T(*reinterpret_cast<Replaceable<T> const&>(other));
    return *this;
  }
};

template <typename T>
struct is_constructible_from_replaceable
    : bool_constant<
          std::is_constructible<T, Replaceable<T>&>::value ||
          std::is_constructible<T, Replaceable<T>&&>::value ||
          std::is_constructible<T, const Replaceable<T>&>::value ||
          std::is_constructible<T, const Replaceable<T>&&>::value> {};

template <typename T>
struct is_convertible_from_replaceable
    : bool_constant<
          std::is_convertible<Replaceable<T>&, T>::value ||
          std::is_convertible<Replaceable<T>&&, T>::value ||
          std::is_convertible<const Replaceable<T>&, T>::value ||
          std::is_convertible<const Replaceable<T>&&, T>::value> {};
} // namespace replaceable_detail

// Type trait template to statically test whether a type is a specialization of
// Replaceable
template <class T>
struct is_replaceable : std::false_type {};

template <class T>
struct is_replaceable<Replaceable<T>> : std::true_type {};

// Function to make a Replaceable with a type deduced from its input
template <class T>
constexpr Replaceable<std::decay_t<T>> make_replaceable(T&& t) {
  return Replaceable<std::decay_t<T>>(std::forward<T>(t));
}

template <class T, class... Args>
constexpr Replaceable<T> make_replaceable(Args&&... args) {
  return Replaceable<T>(in_place, std::forward<Args>(args)...);
}

template <class T, class U, class... Args>
constexpr Replaceable<T> make_replaceable(
    std::initializer_list<U> il,
    Args&&... args) {
  return Replaceable<T>(in_place, il, std::forward<Args>(args)...);
}

template <class T>
class alignas(T) Replaceable
    : public replaceable_detail::dtor_mixin<T>,
      public replaceable_detail::default_and_move_ctor_mixin<T>,
      public replaceable_detail::copy_ctor_mixin<T>,
      public replaceable_detail::move_assignment_mixin<T>,
      public replaceable_detail::copy_assignment_mixin<T> {
  using ctor_base = replaceable_detail::default_and_move_ctor_mixin<T>;

 public:
  using value_type = T;

  /* Rule-of-zero default- copy- and move- constructors. The ugly code to make
   * these work are above, in namespace folly::replaceable_detail.
   */
  constexpr Replaceable() = default;
  constexpr Replaceable(const Replaceable&) = default;
  constexpr Replaceable(Replaceable&&) = default;

  /* Rule-of-zero copy- and move- assignment operators. The ugly code to make
   * these work are above, in namespace folly::replaceable_detail.
   *
   * Note - these destruct the `T` and then in-place construct a new one based
   * on what is in the other replaceable; they do not invoke the assignment
   * operator of `T`.
   */
  Replaceable& operator=(const Replaceable&) = default;
  Replaceable& operator=(Replaceable&&) = default;

  /* Rule-of-zero destructor. The ugly code to make this work is above, in
   * namespace folly::replaceable_detail.
   */
  ~Replaceable() = default;

  template <
      class... Args,
      std::enable_if_t<std::is_constructible<T, Args&&...>::value, int> = 0>
  FOLLY_CPP14_CONSTEXPR explicit Replaceable(in_place_t, Args&&... args)
      // clang-format off
      noexcept(std::is_nothrow_constructible<T, Args&&...>::value)
      // clang-format on
      : ctor_base(0) {
    ::new (storage_) T(std::forward<Args>(args)...);
  }

  template <
      class U,
      class... Args,
      std::enable_if_t<
          std::is_constructible<T, std::initializer_list<U>, Args&&...>::value,
          int> = 0>
  FOLLY_CPP14_CONSTEXPR explicit Replaceable(
      in_place_t,
      std::initializer_list<U> il,
      Args&&... args)
      // clang-format off
      noexcept(std::is_nothrow_constructible<
          T,
          std::initializer_list<U>,
          Args&&...>::value)
      // clang-format on
      : ctor_base(0) {
    ::new (storage_) T(il, std::forward<Args>(args)...);
  }

  template <
      class U = T,
      std::enable_if_t<
          std::is_constructible<T, U&&>::value &&
              !std::is_same<std::decay_t<U>, in_place_t>::value &&
              !std::is_same<Replaceable<T>, std::decay_t<U>>::value &&
              std::is_convertible<U&&, T>::value,
          int> = 0>
  FOLLY_CPP14_CONSTEXPR /* implicit */ Replaceable(U&& other)
      // clang-format off
      noexcept(std::is_nothrow_constructible<T, U&&>::value)
      // clang-format on
      : ctor_base(0) {
    ::new (storage_) T(std::forward<U>(other));
  }

  template <
      class U = T,
      std::enable_if_t<
          std::is_constructible<T, U&&>::value &&
              !std::is_same<std::decay_t<U>, in_place_t>::value &&
              !std::is_same<Replaceable<T>, std::decay_t<U>>::value &&
              !std::is_convertible<U&&, T>::value,
          int> = 0>
  FOLLY_CPP14_CONSTEXPR explicit Replaceable(U&& other)
      // clang-format off
      noexcept(std::is_nothrow_constructible<T, U&&>::value)
      // clang-format on
      : ctor_base(0) {
    ::new (storage_) T(std::forward<U>(other));
  }

  template <
      class U,
      std::enable_if_t<
          std::is_constructible<T, const U&>::value &&
              !replaceable_detail::is_constructible_from_replaceable<
                  T>::value &&
              !replaceable_detail::is_convertible_from_replaceable<T>::value &&
              std::is_convertible<const U&, T>::value,
          int> = 0>
  /* implicit */ Replaceable(const Replaceable<U>& other)
      // clang-format off
      noexcept(std::is_nothrow_constructible<T, U const&>::value)
      // clang-format on
      : ctor_base(0) {
    ::new (storage_) T(*other);
  }

  template <
      class U,
      std::enable_if_t<
          std::is_constructible<T, const U&>::value &&
              !replaceable_detail::is_constructible_from_replaceable<
                  T>::value &&
              !replaceable_detail::is_convertible_from_replaceable<T>::value &&
              !std::is_convertible<const U&, T>::value,
          int> = 0>
  explicit Replaceable(const Replaceable<U>& other)
      // clang-format off
      noexcept(std::is_nothrow_constructible<T, U const&>::value)
      // clang-format on
      : ctor_base(0) {
    ::new (storage_) T(*other);
  }

  template <
      class U,
      std::enable_if_t<
          std::is_constructible<T, U&&>::value &&
              !replaceable_detail::is_constructible_from_replaceable<
                  T>::value &&
              !replaceable_detail::is_convertible_from_replaceable<T>::value &&
              std::is_convertible<U&&, T>::value,
          int> = 0>
  /* implicit */ Replaceable(Replaceable<U>&& other)
      // clang-format off
      noexcept(std::is_nothrow_constructible<T, U&&>::value)
      // clang-format on
      : ctor_base(0) {
    ::new (storage_) T(std::move(*other));
  }

  template <
      class U,
      std::enable_if_t<
          std::is_constructible<T, U&&>::value &&
              !replaceable_detail::is_constructible_from_replaceable<
                  T>::value &&
              !replaceable_detail::is_convertible_from_replaceable<T>::value &&
              !std::is_convertible<U&&, T>::value,
          int> = 0>
  explicit Replaceable(Replaceable<U>&& other)
      // clang-format off
      noexcept(std::is_nothrow_constructible<T, U&&>::value)
      // clang-format on
      : ctor_base(0) {
    ::new (storage_) T(std::move(*other));
  }

  template <class... Args>
  T& emplace(Args&&... args) noexcept {
    T* destruct_ptr = launder(reinterpret_cast<T*>(storage_));
    destruct_ptr->~T();
    return *::new (storage_) T(std::forward<Args>(args)...);
  }

  template <class U, class... Args>
  T& emplace(std::initializer_list<U> il, Args&&... args) noexcept {
    T* destruct_ptr = launder(reinterpret_cast<T*>(storage_));
    destruct_ptr->~T();
    return *::new (storage_) T(il, std::forward<Args>(args)...);
  }

  void swap(Replaceable& other) {
    using std::swap;
    swap(*(*this), *other);
  }

  constexpr const T* operator->() const {
    return launder(reinterpret_cast<T const*>(storage_));
  }

  FOLLY_CPP14_CONSTEXPR T* operator->() {
    return launder(reinterpret_cast<T*>(storage_));
  }

  constexpr const T& operator*() const& {
    return *launder(reinterpret_cast<T const*>(storage_));
  }

  FOLLY_CPP14_CONSTEXPR T& operator*() & {
    return *launder(reinterpret_cast<T*>(storage_));
  }

  FOLLY_CPP14_CONSTEXPR T&& operator*() && {
    return std::move(*launder(reinterpret_cast<T*>(storage_)));
  }

  constexpr const T&& operator*() const&& {
    return std::move(*launder(reinterpret_cast<T const*>(storage_)));
  }

 private:
  friend struct replaceable_detail::dtor_mixin<T>;
  friend struct replaceable_detail::default_and_move_ctor_mixin<T>;
  friend struct replaceable_detail::copy_ctor_mixin<T>;
  friend struct replaceable_detail::move_assignment_mixin<T>;
  friend struct replaceable_detail::copy_assignment_mixin<T>;
  std::aligned_storage_t<sizeof(T), alignof(T)> storage_[1];
};

#if __cplusplus > 201402L
// C++17 allows us to define a deduction guide:
template <class T>
Replaceable(T)->Replaceable<T>;
#endif
} // namespace folly