Repo created

TMessagesProj/jni/voip/webrtc/base/optional.h

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+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef BASE_OPTIONAL_H_
+#define BASE_OPTIONAL_H_
+
+#include <functional>
+#include <type_traits>
+#include <utility>
+
+#include "base/logging.h"
+#include "base/template_util.h"
+
+namespace base {
+
+// Specification:
+// http://en.cppreference.com/w/cpp/utility/optional/nullopt_t
+struct nullopt_t {
+  constexpr explicit nullopt_t(int) {}
+};
+
+// Specification:
+// http://en.cppreference.com/w/cpp/utility/optional/nullopt
+constexpr nullopt_t nullopt(0);
+
+// Forward declaration, which is refered by following helpers.
+template <typename T>
+class Optional;
+
+namespace internal {
+
+struct DummyUnionMember {};
+
+template <typename T, bool = std::is_trivially_destructible<T>::value>
+struct OptionalStorageBase {
+  // Provide non-defaulted default ctor to make sure it's not deleted by
+  // non-trivial T::T() in the union.
+  constexpr OptionalStorageBase() : dummy_() {}
+
+  template <class... Args>
+  constexpr explicit OptionalStorageBase(in_place_t, Args&&... args)
+      : is_populated_(true), value_(std::forward<Args>(args)...) {}
+
+  // When T is not trivially destructible we must call its
+  // destructor before deallocating its memory.
+  // Note that this hides the (implicitly declared) move constructor, which
+  // would be used for constexpr move constructor in OptionalStorage<T>.
+  // It is needed iff T is trivially move constructible. However, the current
+  // is_trivially_{copy,move}_constructible implementation requires
+  // is_trivially_destructible (which looks a bug, cf:
+  // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51452 and
+  // http://cplusplus.github.io/LWG/lwg-active.html#2116), so it is not
+  // necessary for this case at the moment. Please see also the destructor
+  // comment in "is_trivially_destructible = true" specialization below.
+  ~OptionalStorageBase() {
+    if (is_populated_)
+      value_.~T();
+  }
+
+  template <class... Args>
+  void Init(Args&&... args) {
+    DCHECK(!is_populated_);
+    ::new (&value_) T(std::forward<Args>(args)...);
+    is_populated_ = true;
+  }
+
+  bool is_populated_ = false;
+  union {
+    // |dummy_| exists so that the union will always be initialized, even when
+    // it doesn't contain a value. Union members must be initialized for the
+    // constructor to be 'constexpr'. Having a special trivial class for it is
+    // better than e.g. using char, because the latter will have to be
+    // zero-initialized, and the compiler can't optimize this write away, since
+    // it assumes this might be a programmer's invariant. This can also cause
+    // problems for conservative GC in Oilpan. Compiler is free to split shared
+    // and non-shared parts of the union in separate memory locations (or
+    // registers). If conservative GC is triggered at this moment, the stack
+    // scanning routine won't find the correct object pointed from
+    // Optional<HeapObject*>. This dummy valueless struct lets the compiler know
+    // that we don't care about the value of this union member.
+    DummyUnionMember dummy_;
+    T value_;
+  };
+};
+
+template <typename T>
+struct OptionalStorageBase<T, true /* trivially destructible */> {
+  // Provide non-defaulted default ctor to make sure it's not deleted by
+  // non-trivial T::T() in the union.
+  constexpr OptionalStorageBase() : dummy_() {}
+
+  template <class... Args>
+  constexpr explicit OptionalStorageBase(in_place_t, Args&&... args)
+      : is_populated_(true), value_(std::forward<Args>(args)...) {}
+
+  // When T is trivially destructible (i.e. its destructor does nothing) there
+  // is no need to call it. Implicitly defined destructor is trivial, because
+  // both members (bool and union containing only variants which are trivially
+  // destructible) are trivially destructible.
+  // Explicitly-defaulted destructor is also trivial, but do not use it here,
+  // because it hides the implicit move constructor. It is needed to implement
+  // constexpr move constructor in OptionalStorage iff T is trivially move
+  // constructible. Note that, if T is trivially move constructible, the move
+  // constructor of OptionalStorageBase<T> is also implicitly defined and it is
+  // trivially move constructor. If T is not trivially move constructible,
+  // "not declaring move constructor without destructor declaration" here means
+  // "delete move constructor", which works because any move constructor of
+  // OptionalStorage will not refer to it in that case.
+
+  template <class... Args>
+  void Init(Args&&... args) {
+    DCHECK(!is_populated_);
+    ::new (&value_) T(std::forward<Args>(args)...);
+    is_populated_ = true;
+  }
+
+  bool is_populated_ = false;
+  union {
+    // |dummy_| exists so that the union will always be initialized, even when
+    // it doesn't contain a value. Union members must be initialized for the
+    // constructor to be 'constexpr'. Having a special trivial class for it is
+    // better than e.g. using char, because the latter will have to be
+    // zero-initialized, and the compiler can't optimize this write away, since
+    // it assumes this might be a programmer's invariant. This can also cause
+    // problems for conservative GC in Oilpan. Compiler is free to split shared
+    // and non-shared parts of the union in separate memory locations (or
+    // registers). If conservative GC is triggered at this moment, the stack
+    // scanning routine won't find the correct object pointed from
+    // Optional<HeapObject*>. This dummy valueless struct lets the compiler know
+    // that we don't care about the value of this union member.
+    DummyUnionMember dummy_;
+    T value_;
+  };
+};
+
+// Implement conditional constexpr copy and move constructors. These are
+// constexpr if is_trivially_{copy,move}_constructible<T>::value is true
+// respectively. If each is true, the corresponding constructor is defined as
+// "= default;", which generates a constexpr constructor (In this case,
+// the condition of constexpr-ness is satisfied because the base class also has
+// compiler generated constexpr {copy,move} constructors). Note that
+// placement-new is prohibited in constexpr.
+template <typename T,
+          bool = is_trivially_copy_constructible<T>::value,
+          bool = std::is_trivially_move_constructible<T>::value>
+struct OptionalStorage : OptionalStorageBase<T> {
+  // This is no trivially {copy,move} constructible case. Other cases are
+  // defined below as specializations.
+
+  // Accessing the members of template base class requires explicit
+  // declaration.
+  using OptionalStorageBase<T>::is_populated_;
+  using OptionalStorageBase<T>::value_;
+  using OptionalStorageBase<T>::Init;
+
+  // Inherit constructors (specifically, the in_place constructor).
+  using OptionalStorageBase<T>::OptionalStorageBase;
+
+  // User defined constructor deletes the default constructor.
+  // Define it explicitly.
+  OptionalStorage() = default;
+
+  OptionalStorage(const OptionalStorage& other) {
+    if (other.is_populated_)
+      Init(other.value_);
+  }
+
+  OptionalStorage(OptionalStorage&& other) noexcept(
+      std::is_nothrow_move_constructible<T>::value) {
+    if (other.is_populated_)
+      Init(std::move(other.value_));
+  }
+};
+
+template <typename T>
+struct OptionalStorage<T,
+                       true /* trivially copy constructible */,
+                       false /* trivially move constructible */>
+    : OptionalStorageBase<T> {
+  using OptionalStorageBase<T>::is_populated_;
+  using OptionalStorageBase<T>::value_;
+  using OptionalStorageBase<T>::Init;
+  using OptionalStorageBase<T>::OptionalStorageBase;
+
+  OptionalStorage() = default;
+  OptionalStorage(const OptionalStorage& other) = default;
+
+  OptionalStorage(OptionalStorage&& other) noexcept(
+      std::is_nothrow_move_constructible<T>::value) {
+    if (other.is_populated_)
+      Init(std::move(other.value_));
+  }
+};
+
+template <typename T>
+struct OptionalStorage<T,
+                       false /* trivially copy constructible */,
+                       true /* trivially move constructible */>
+    : OptionalStorageBase<T> {
+  using OptionalStorageBase<T>::is_populated_;
+  using OptionalStorageBase<T>::value_;
+  using OptionalStorageBase<T>::Init;
+  using OptionalStorageBase<T>::OptionalStorageBase;
+
+  OptionalStorage() = default;
+  OptionalStorage(OptionalStorage&& other) = default;
+
+  OptionalStorage(const OptionalStorage& other) {
+    if (other.is_populated_)
+      Init(other.value_);
+  }
+};
+
+template <typename T>
+struct OptionalStorage<T,
+                       true /* trivially copy constructible */,
+                       true /* trivially move constructible */>
+    : OptionalStorageBase<T> {
+  // If both trivially {copy,move} constructible are true, it is not necessary
+  // to use user-defined constructors. So, just inheriting constructors
+  // from the base class works.
+  using OptionalStorageBase<T>::OptionalStorageBase;
+};
+
+// Base class to support conditionally usable copy-/move- constructors
+// and assign operators.
+template <typename T>
+class OptionalBase {
+  // This class provides implementation rather than public API, so everything
+  // should be hidden. Often we use composition, but we cannot in this case
+  // because of C++ language restriction.
+ protected:
+  constexpr OptionalBase() = default;
+  constexpr OptionalBase(const OptionalBase& other) = default;
+  constexpr OptionalBase(OptionalBase&& other) = default;
+
+  template <class... Args>
+  constexpr explicit OptionalBase(in_place_t, Args&&... args)
+      : storage_(in_place, std::forward<Args>(args)...) {}
+
+  // Implementation of converting constructors.
+  template <typename U>
+  explicit OptionalBase(const OptionalBase<U>& other) {
+    if (other.storage_.is_populated_)
+      storage_.Init(other.storage_.value_);
+  }
+
+  template <typename U>
+  explicit OptionalBase(OptionalBase<U>&& other) {
+    if (other.storage_.is_populated_)
+      storage_.Init(std::move(other.storage_.value_));
+  }
+
+  ~OptionalBase() = default;
+
+  OptionalBase& operator=(const OptionalBase& other) {
+    CopyAssign(other);
+    return *this;
+  }
+
+  OptionalBase& operator=(OptionalBase&& other) noexcept(
+      std::is_nothrow_move_assignable<T>::value&&
+          std::is_nothrow_move_constructible<T>::value) {
+    MoveAssign(std::move(other));
+    return *this;
+  }
+
+  template <typename U>
+  void CopyAssign(const OptionalBase<U>& other) {
+    if (other.storage_.is_populated_)
+      InitOrAssign(other.storage_.value_);
+    else
+      FreeIfNeeded();
+  }
+
+  template <typename U>
+  void MoveAssign(OptionalBase<U>&& other) {
+    if (other.storage_.is_populated_)
+      InitOrAssign(std::move(other.storage_.value_));
+    else
+      FreeIfNeeded();
+  }
+
+  template <typename U>
+  void InitOrAssign(U&& value) {
+    if (storage_.is_populated_)
+      storage_.value_ = std::forward<U>(value);
+    else
+      storage_.Init(std::forward<U>(value));
+  }
+
+  void FreeIfNeeded() {
+    if (!storage_.is_populated_)
+      return;
+    storage_.value_.~T();
+    storage_.is_populated_ = false;
+  }
+
+  // For implementing conversion, allow access to other typed OptionalBase
+  // class.
+  template <typename U>
+  friend class OptionalBase;
+
+  OptionalStorage<T> storage_;
+};
+
+// The following {Copy,Move}{Constructible,Assignable} structs are helpers to
+// implement constructor/assign-operator overloading. Specifically, if T is
+// is not movable but copyable, Optional<T>'s move constructor should not
+// participate in overload resolution. This inheritance trick implements that.
+template <bool is_copy_constructible>
+struct CopyConstructible {};
+
+template <>
+struct CopyConstructible<false> {
+  constexpr CopyConstructible() = default;
+  constexpr CopyConstructible(const CopyConstructible&) = delete;
+  constexpr CopyConstructible(CopyConstructible&&) = default;
+  CopyConstructible& operator=(const CopyConstructible&) = default;
+  CopyConstructible& operator=(CopyConstructible&&) = default;
+};
+
+template <bool is_move_constructible>
+struct MoveConstructible {};
+
+template <>
+struct MoveConstructible<false> {
+  constexpr MoveConstructible() = default;
+  constexpr MoveConstructible(const MoveConstructible&) = default;
+  constexpr MoveConstructible(MoveConstructible&&) = delete;
+  MoveConstructible& operator=(const MoveConstructible&) = default;
+  MoveConstructible& operator=(MoveConstructible&&) = default;
+};
+
+template <bool is_copy_assignable>
+struct CopyAssignable {};
+
+template <>
+struct CopyAssignable<false> {
+  constexpr CopyAssignable() = default;
+  constexpr CopyAssignable(const CopyAssignable&) = default;
+  constexpr CopyAssignable(CopyAssignable&&) = default;
+  CopyAssignable& operator=(const CopyAssignable&) = delete;
+  CopyAssignable& operator=(CopyAssignable&&) = default;
+};
+
+template <bool is_move_assignable>
+struct MoveAssignable {};
+
+template <>
+struct MoveAssignable<false> {
+  constexpr MoveAssignable() = default;
+  constexpr MoveAssignable(const MoveAssignable&) = default;
+  constexpr MoveAssignable(MoveAssignable&&) = default;
+  MoveAssignable& operator=(const MoveAssignable&) = default;
+  MoveAssignable& operator=(MoveAssignable&&) = delete;
+};
+
+// Helper to conditionally enable converting constructors and assign operators.
+template <typename T, typename U>
+using IsConvertibleFromOptional =
+    disjunction<std::is_constructible<T, Optional<U>&>,
+                std::is_constructible<T, const Optional<U>&>,
+                std::is_constructible<T, Optional<U>&&>,
+                std::is_constructible<T, const Optional<U>&&>,
+                std::is_convertible<Optional<U>&, T>,
+                std::is_convertible<const Optional<U>&, T>,
+                std::is_convertible<Optional<U>&&, T>,
+                std::is_convertible<const Optional<U>&&, T>>;
+
+template <typename T, typename U>
+using IsAssignableFromOptional =
+    disjunction<IsConvertibleFromOptional<T, U>,
+                std::is_assignable<T&, Optional<U>&>,
+                std::is_assignable<T&, const Optional<U>&>,
+                std::is_assignable<T&, Optional<U>&&>,
+                std::is_assignable<T&, const Optional<U>&&>>;
+
+// Forward compatibility for C++17.
+// Introduce one more deeper nested namespace to avoid leaking using std::swap.
+namespace swappable_impl {
+using std::swap;
+
+struct IsSwappableImpl {
+  // Tests if swap can be called. Check<T&>(0) returns true_type iff swap
+  // is available for T. Otherwise, Check's overload resolution falls back
+  // to Check(...) declared below thanks to SFINAE, so returns false_type.
+  template <typename T>
+  static auto Check(int)
+      -> decltype(swap(std::declval<T>(), std::declval<T>()), std::true_type());
+
+  template <typename T>
+  static std::false_type Check(...);
+};
+}  // namespace swappable_impl
+
+template <typename T>
+struct IsSwappable : decltype(swappable_impl::IsSwappableImpl::Check<T&>(0)) {};
+
+// Forward compatibility for C++20.
+template <typename T>
+using RemoveCvRefT = std::remove_cv_t<std::remove_reference_t<T>>;
+
+}  // namespace internal
+
+// On Windows, by default, empty-base class optimization does not work,
+// which means even if the base class is empty struct, it still consumes one
+// byte for its body. __declspec(empty_bases) enables the optimization.
+// cf)
+// https://blogs.msdn.microsoft.com/vcblog/2016/03/30/optimizing-the-layout-of-empty-base-classes-in-vs2015-update-2-3/
+#ifdef OS_WIN
+#define OPTIONAL_DECLSPEC_EMPTY_BASES __declspec(empty_bases)
+#else
+#define OPTIONAL_DECLSPEC_EMPTY_BASES
+#endif
+
+// base::Optional is a Chromium version of the C++17 optional class:
+// std::optional documentation:
+// http://en.cppreference.com/w/cpp/utility/optional
+// Chromium documentation:
+// https://chromium.googlesource.com/chromium/src/+/master/docs/optional.md
+//
+// These are the differences between the specification and the implementation:
+// - Constructors do not use 'constexpr' as it is a C++14 extension.
+// - 'constexpr' might be missing in some places for reasons specified locally.
+// - No exceptions are thrown, because they are banned from Chromium.
+//   Marked noexcept for only move constructor and move assign operators.
+// - All the non-members are in the 'base' namespace instead of 'std'.
+//
+// Note that T cannot have a constructor T(Optional<T>) etc. Optional<T> checks
+// T's constructor (specifically via IsConvertibleFromOptional), and in the
+// check whether T can be constructible from Optional<T>, which is recursive
+// so it does not work. As of Feb 2018, std::optional C++17 implementation in
+// both clang and gcc has same limitation. MSVC SFINAE looks to have different
+// behavior, but anyway it reports an error, too.
+template <typename T>
+class OPTIONAL_DECLSPEC_EMPTY_BASES Optional
+    : public internal::OptionalBase<T>,
+      public internal::CopyConstructible<std::is_copy_constructible<T>::value>,
+      public internal::MoveConstructible<std::is_move_constructible<T>::value>,
+      public internal::CopyAssignable<std::is_copy_constructible<T>::value &&
+                                      std::is_copy_assignable<T>::value>,
+      public internal::MoveAssignable<std::is_move_constructible<T>::value &&
+                                      std::is_move_assignable<T>::value> {
+ private:
+  // Disable some versions of T that are ill-formed.
+  // See: https://timsong-cpp.github.io/cppwp/n4659/optional#syn-1
+  static_assert(
+      !std::is_same<internal::RemoveCvRefT<T>, in_place_t>::value,
+      "instantiation of base::Optional with in_place_t is ill-formed");
+  static_assert(!std::is_same<internal::RemoveCvRefT<T>, nullopt_t>::value,
+                "instantiation of base::Optional with nullopt_t is ill-formed");
+  static_assert(
+      !std::is_reference<T>::value,
+      "instantiation of base::Optional with a reference type is ill-formed");
+  // See: https://timsong-cpp.github.io/cppwp/n4659/optional#optional-3
+  static_assert(std::is_destructible<T>::value,
+                "instantiation of base::Optional with a non-destructible type "
+                "is ill-formed");
+  // Arrays are explicitly disallowed because for arrays of known bound
+  // is_destructible is of undefined value.
+  // See: https://en.cppreference.com/w/cpp/types/is_destructible
+  static_assert(
+      !std::is_array<T>::value,
+      "instantiation of base::Optional with an array type is ill-formed");
+
+ public:
+#undef OPTIONAL_DECLSPEC_EMPTY_BASES
+  using value_type = T;
+
+  // Defer default/copy/move constructor implementation to OptionalBase.
+  constexpr Optional() = default;
+  constexpr Optional(const Optional& other) = default;
+  constexpr Optional(Optional&& other) noexcept(
+      std::is_nothrow_move_constructible<T>::value) = default;
+
+  constexpr Optional(nullopt_t) {}  // NOLINT(runtime/explicit)
+
+  // Converting copy constructor. "explicit" only if
+  // std::is_convertible<const U&, T>::value is false. It is implemented by
+  // declaring two almost same constructors, but that condition in enable_if_t
+  // is different, so that either one is chosen, thanks to SFINAE.
+  template <
+      typename U,
+      std::enable_if_t<std::is_constructible<T, const U&>::value &&
+                           !internal::IsConvertibleFromOptional<T, U>::value &&
+                           std::is_convertible<const U&, T>::value,
+                       bool> = false>
+  Optional(const Optional<U>& other) : internal::OptionalBase<T>(other) {}
+
+  template <
+      typename U,
+      std::enable_if_t<std::is_constructible<T, const U&>::value &&
+                           !internal::IsConvertibleFromOptional<T, U>::value &&
+                           !std::is_convertible<const U&, T>::value,
+                       bool> = false>
+  explicit Optional(const Optional<U>& other)
+      : internal::OptionalBase<T>(other) {}
+
+  // Converting move constructor. Similar to converting copy constructor,
+  // declaring two (explicit and non-explicit) constructors.
+  template <
+      typename U,
+      std::enable_if_t<std::is_constructible<T, U&&>::value &&
+                           !internal::IsConvertibleFromOptional<T, U>::value &&
+                           std::is_convertible<U&&, T>::value,
+                       bool> = false>
+  Optional(Optional<U>&& other) : internal::OptionalBase<T>(std::move(other)) {}
+
+  template <
+      typename U,
+      std::enable_if_t<std::is_constructible<T, U&&>::value &&
+                           !internal::IsConvertibleFromOptional<T, U>::value &&
+                           !std::is_convertible<U&&, T>::value,
+                       bool> = false>
+  explicit Optional(Optional<U>&& other)
+      : internal::OptionalBase<T>(std::move(other)) {}
+
+  template <class... Args>
+  constexpr explicit Optional(in_place_t, Args&&... args)
+      : internal::OptionalBase<T>(in_place, std::forward<Args>(args)...) {}
+
+  template <
+      class U,
+      class... Args,
+      class = std::enable_if_t<std::is_constructible<value_type,
+                                                     std::initializer_list<U>&,
+                                                     Args...>::value>>
+  constexpr explicit Optional(in_place_t,
+                              std::initializer_list<U> il,
+                              Args&&... args)
+      : internal::OptionalBase<T>(in_place, il, std::forward<Args>(args)...) {}
+
+  // Forward value constructor. Similar to converting constructors,
+  // conditionally explicit.
+  template <
+      typename U = value_type,
+      std::enable_if_t<
+          std::is_constructible<T, U&&>::value &&
+              !std::is_same<internal::RemoveCvRefT<U>, in_place_t>::value &&
+              !std::is_same<internal::RemoveCvRefT<U>, Optional<T>>::value &&
+              std::is_convertible<U&&, T>::value,
+          bool> = false>
+  constexpr Optional(U&& value)
+      : internal::OptionalBase<T>(in_place, std::forward<U>(value)) {}
+
+  template <
+      typename U = value_type,
+      std::enable_if_t<
+          std::is_constructible<T, U&&>::value &&
+              !std::is_same<internal::RemoveCvRefT<U>, in_place_t>::value &&
+              !std::is_same<internal::RemoveCvRefT<U>, Optional<T>>::value &&
+              !std::is_convertible<U&&, T>::value,
+          bool> = false>
+  constexpr explicit Optional(U&& value)
+      : internal::OptionalBase<T>(in_place, std::forward<U>(value)) {}
+
+  ~Optional() = default;
+
+  // Defer copy-/move- assign operator implementation to OptionalBase.
+  Optional& operator=(const Optional& other) = default;
+  Optional& operator=(Optional&& other) noexcept(
+      std::is_nothrow_move_assignable<T>::value&&
+          std::is_nothrow_move_constructible<T>::value) = default;
+
+  Optional& operator=(nullopt_t) {
+    FreeIfNeeded();
+    return *this;
+  }
+
+  // Perfect-forwarded assignment.
+  template <typename U>
+  std::enable_if_t<
+      !std::is_same<internal::RemoveCvRefT<U>, Optional<T>>::value &&
+          std::is_constructible<T, U>::value &&
+          std::is_assignable<T&, U>::value &&
+          (!std::is_scalar<T>::value ||
+           !std::is_same<std::decay_t<U>, T>::value),
+      Optional&>
+  operator=(U&& value) {
+    InitOrAssign(std::forward<U>(value));
+    return *this;
+  }
+
+  // Copy assign the state of other.
+  template <typename U>
+  std::enable_if_t<!internal::IsAssignableFromOptional<T, U>::value &&
+                       std::is_constructible<T, const U&>::value &&
+                       std::is_assignable<T&, const U&>::value,
+                   Optional&>
+  operator=(const Optional<U>& other) {
+    CopyAssign(other);
+    return *this;
+  }
+
+  // Move assign the state of other.
+  template <typename U>
+  std::enable_if_t<!internal::IsAssignableFromOptional<T, U>::value &&
+                       std::is_constructible<T, U>::value &&
+                       std::is_assignable<T&, U>::value,
+                   Optional&>
+  operator=(Optional<U>&& other) {
+    MoveAssign(std::move(other));
+    return *this;
+  }
+
+  constexpr const T* operator->() const {
+    CHECK(storage_.is_populated_);
+    return &storage_.value_;
+  }
+
+  constexpr T* operator->() {
+    CHECK(storage_.is_populated_);
+    return &storage_.value_;
+  }
+
+  constexpr const T& operator*() const & {
+    CHECK(storage_.is_populated_);
+    return storage_.value_;
+  }
+
+  constexpr T& operator*() & {
+    CHECK(storage_.is_populated_);
+    return storage_.value_;
+  }
+
+  constexpr const T&& operator*() const && {
+    CHECK(storage_.is_populated_);
+    return std::move(storage_.value_);
+  }
+
+  constexpr T&& operator*() && {
+    CHECK(storage_.is_populated_);
+    return std::move(storage_.value_);
+  }
+
+  constexpr explicit operator bool() const { return storage_.is_populated_; }
+
+  constexpr bool has_value() const { return storage_.is_populated_; }
+
+  constexpr T& value() & {
+    CHECK(storage_.is_populated_);
+    return storage_.value_;
+  }
+
+  constexpr const T& value() const & {
+    CHECK(storage_.is_populated_);
+    return storage_.value_;
+  }
+
+  constexpr T&& value() && {
+    CHECK(storage_.is_populated_);
+    return std::move(storage_.value_);
+  }
+
+  constexpr const T&& value() const && {
+    CHECK(storage_.is_populated_);
+    return std::move(storage_.value_);
+  }
+
+  template <class U>
+  constexpr T value_or(U&& default_value) const& {
+    // TODO(mlamouri): add the following assert when possible:
+    // static_assert(std::is_copy_constructible<T>::value,
+    //               "T must be copy constructible");
+    static_assert(std::is_convertible<U, T>::value,
+                  "U must be convertible to T");
+    return storage_.is_populated_
+               ? storage_.value_
+               : static_cast<T>(std::forward<U>(default_value));
+  }
+
+  template <class U>
+  constexpr T value_or(U&& default_value) && {
+    // TODO(mlamouri): add the following assert when possible:
+    // static_assert(std::is_move_constructible<T>::value,
+    //               "T must be move constructible");
+    static_assert(std::is_convertible<U, T>::value,
+                  "U must be convertible to T");
+    return storage_.is_populated_
+               ? std::move(storage_.value_)
+               : static_cast<T>(std::forward<U>(default_value));
+  }
+
+  void swap(Optional& other) {
+    if (!storage_.is_populated_ && !other.storage_.is_populated_)
+      return;
+
+    if (storage_.is_populated_ != other.storage_.is_populated_) {
+      if (storage_.is_populated_) {
+        other.storage_.Init(std::move(storage_.value_));
+        FreeIfNeeded();
+      } else {
+        storage_.Init(std::move(other.storage_.value_));
+        other.FreeIfNeeded();
+      }
+      return;
+    }
+
+    DCHECK(storage_.is_populated_ && other.storage_.is_populated_);
+    using std::swap;
+    swap(**this, *other);
+  }
+
+  void reset() { FreeIfNeeded(); }
+
+  template <class... Args>
+  T& emplace(Args&&... args) {
+    FreeIfNeeded();
+    storage_.Init(std::forward<Args>(args)...);
+    return storage_.value_;
+  }
+
+  template <class U, class... Args>
+  std::enable_if_t<
+      std::is_constructible<T, std::initializer_list<U>&, Args&&...>::value,
+      T&>
+  emplace(std::initializer_list<U> il, Args&&... args) {
+    FreeIfNeeded();
+    storage_.Init(il, std::forward<Args>(args)...);
+    return storage_.value_;
+  }
+
+ private:
+  // Accessing template base class's protected member needs explicit
+  // declaration to do so.
+  using internal::OptionalBase<T>::CopyAssign;
+  using internal::OptionalBase<T>::FreeIfNeeded;
+  using internal::OptionalBase<T>::InitOrAssign;
+  using internal::OptionalBase<T>::MoveAssign;
+  using internal::OptionalBase<T>::storage_;
+};
+
+// Here after defines comparation operators. The definition follows
+// http://en.cppreference.com/w/cpp/utility/optional/operator_cmp
+// while bool() casting is replaced by has_value() to meet the chromium
+// style guide.
+template <class T, class U>
+constexpr bool operator==(const Optional<T>& lhs, const Optional<U>& rhs) {
+  if (lhs.has_value() != rhs.has_value())
+    return false;
+  if (!lhs.has_value())
+    return true;
+  return *lhs == *rhs;
+}
+
+template <class T, class U>
+constexpr bool operator!=(const Optional<T>& lhs, const Optional<U>& rhs) {
+  if (lhs.has_value() != rhs.has_value())
+    return true;
+  if (!lhs.has_value())
+    return false;
+  return *lhs != *rhs;
+}
+
+template <class T, class U>
+constexpr bool operator<(const Optional<T>& lhs, const Optional<U>& rhs) {
+  if (!rhs.has_value())
+    return false;
+  if (!lhs.has_value())
+    return true;
+  return *lhs < *rhs;
+}
+
+template <class T, class U>
+constexpr bool operator<=(const Optional<T>& lhs, const Optional<U>& rhs) {
+  if (!lhs.has_value())
+    return true;
+  if (!rhs.has_value())
+    return false;
+  return *lhs <= *rhs;
+}
+
+template <class T, class U>
+constexpr bool operator>(const Optional<T>& lhs, const Optional<U>& rhs) {
+  if (!lhs.has_value())
+    return false;
+  if (!rhs.has_value())
+    return true;
+  return *lhs > *rhs;
+}
+
+template <class T, class U>
+constexpr bool operator>=(const Optional<T>& lhs, const Optional<U>& rhs) {
+  if (!rhs.has_value())
+    return true;
+  if (!lhs.has_value())
+    return false;
+  return *lhs >= *rhs;
+}
+
+template <class T>
+constexpr bool operator==(const Optional<T>& opt, nullopt_t) {
+  return !opt;
+}
+
+template <class T>
+constexpr bool operator==(nullopt_t, const Optional<T>& opt) {
+  return !opt;
+}
+
+template <class T>
+constexpr bool operator!=(const Optional<T>& opt, nullopt_t) {
+  return opt.has_value();
+}
+
+template <class T>
+constexpr bool operator!=(nullopt_t, const Optional<T>& opt) {
+  return opt.has_value();
+}
+
+template <class T>
+constexpr bool operator<(const Optional<T>& opt, nullopt_t) {
+  return false;
+}
+
+template <class T>
+constexpr bool operator<(nullopt_t, const Optional<T>& opt) {
+  return opt.has_value();
+}
+
+template <class T>
+constexpr bool operator<=(const Optional<T>& opt, nullopt_t) {
+  return !opt;
+}
+
+template <class T>
+constexpr bool operator<=(nullopt_t, const Optional<T>& opt) {
+  return true;
+}
+
+template <class T>
+constexpr bool operator>(const Optional<T>& opt, nullopt_t) {
+  return opt.has_value();
+}
+
+template <class T>
+constexpr bool operator>(nullopt_t, const Optional<T>& opt) {
+  return false;
+}
+
+template <class T>
+constexpr bool operator>=(const Optional<T>& opt, nullopt_t) {
+  return true;
+}
+
+template <class T>
+constexpr bool operator>=(nullopt_t, const Optional<T>& opt) {
+  return !opt;
+}
+
+template <class T, class U>
+constexpr bool operator==(const Optional<T>& opt, const U& value) {
+  return opt.has_value() ? *opt == value : false;
+}
+
+template <class T, class U>
+constexpr bool operator==(const U& value, const Optional<T>& opt) {
+  return opt.has_value() ? value == *opt : false;
+}
+
+template <class T, class U>
+constexpr bool operator!=(const Optional<T>& opt, const U& value) {
+  return opt.has_value() ? *opt != value : true;
+}
+
+template <class T, class U>
+constexpr bool operator!=(const U& value, const Optional<T>& opt) {
+  return opt.has_value() ? value != *opt : true;
+}
+
+template <class T, class U>
+constexpr bool operator<(const Optional<T>& opt, const U& value) {
+  return opt.has_value() ? *opt < value : true;
+}
+
+template <class T, class U>
+constexpr bool operator<(const U& value, const Optional<T>& opt) {
+  return opt.has_value() ? value < *opt : false;
+}
+
+template <class T, class U>
+constexpr bool operator<=(const Optional<T>& opt, const U& value) {
+  return opt.has_value() ? *opt <= value : true;
+}
+
+template <class T, class U>
+constexpr bool operator<=(const U& value, const Optional<T>& opt) {
+  return opt.has_value() ? value <= *opt : false;
+}
+
+template <class T, class U>
+constexpr bool operator>(const Optional<T>& opt, const U& value) {
+  return opt.has_value() ? *opt > value : false;
+}
+
+template <class T, class U>
+constexpr bool operator>(const U& value, const Optional<T>& opt) {
+  return opt.has_value() ? value > *opt : true;
+}
+
+template <class T, class U>
+constexpr bool operator>=(const Optional<T>& opt, const U& value) {
+  return opt.has_value() ? *opt >= value : false;
+}
+
+template <class T, class U>
+constexpr bool operator>=(const U& value, const Optional<T>& opt) {
+  return opt.has_value() ? value >= *opt : true;
+}
+
+template <class T>
+constexpr Optional<std::decay_t<T>> make_optional(T&& value) {
+  return Optional<std::decay_t<T>>(std::forward<T>(value));
+}
+
+template <class T, class... Args>
+constexpr Optional<T> make_optional(Args&&... args) {
+  return Optional<T>(in_place, std::forward<Args>(args)...);
+}
+
+template <class T, class U, class... Args>
+constexpr Optional<T> make_optional(std::initializer_list<U> il,
+                                    Args&&... args) {
+  return Optional<T>(in_place, il, std::forward<Args>(args)...);
+}
+
+// Partial specialization for a function template is not allowed. Also, it is
+// not allowed to add overload function to std namespace, while it is allowed
+// to specialize the template in std. Thus, swap() (kind of) overloading is
+// defined in base namespace, instead.
+template <class T>
+std::enable_if_t<std::is_move_constructible<T>::value &&
+                 internal::IsSwappable<T>::value>
+swap(Optional<T>& lhs, Optional<T>& rhs) {
+  lhs.swap(rhs);
+}
+
+}  // namespace base
+
+namespace std {
+
+template <class T>
+struct hash<base::Optional<T>> {
+  size_t operator()(const base::Optional<T>& opt) const {
+    return opt == base::nullopt ? 0 : std::hash<T>()(*opt);
+  }
+};
+
+}  // namespace std
+
+#endif  // BASE_OPTIONAL_H_