Revert of Base: add Optional<T>.

base/optional.h

-// 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 <type_traits>
-
-#include "base/logging.h"
-#include "base/memory/aligned_memory.h"
-
-namespace base {
-
-// Specification:
-// http://en.cppreference.com/w/cpp/utility/optional/in_place_t
-struct in_place_t {};
-
-// 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/in_place
-constexpr in_place_t in_place = {};
-
-// Specification:
-// http://en.cppreference.com/w/cpp/utility/optional/nullopt
-constexpr nullopt_t nullopt(0);
-
-// 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:
-// - The constructor and emplace method using initializer_list are not
-//   implemented because 'initializer_list' is banned from Chromium.
-// - 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.
-// - All the non-members are in the 'base' namespace instead of 'std'.
-template <typename T>
-class Optional {
- public:
-  constexpr Optional() = default;
-  Optional(base::nullopt_t) : Optional() {}
-
-  Optional(const Optional& other) {
-    if (!other.is_null_)
-      Init(other.value());
-  }
-
-  Optional(Optional&& other) {
-    if (!other.is_null_)
-      Init(std::move(other.value()));
-  }
-
-  Optional(const T& value) { Init(value); }
-
-  Optional(T&& value) { Init(std::move(value)); }
-
-  template <class... Args>
-  explicit Optional(base::in_place_t, Args&&... args) {
-    emplace(std::forward<Args>(args)...);
-  }
-
-  ~Optional() {
-    // TODO(mlamouri): use is_trivially_destructible<T>::value when possible.
-    FreeIfNeeded();
-  }
-
-  Optional& operator=(base::nullopt_t) {
-    FreeIfNeeded();
-    return *this;
-  }
-
-  Optional& operator=(const Optional& other) {
-    if (other.is_null_) {
-      FreeIfNeeded();
-      return *this;
-    }
-
-    InitOrAssign(other.value());
-    return *this;
-  }
-
-  Optional& operator=(Optional&& other) {
-    if (other.is_null_) {
-      FreeIfNeeded();
-      return *this;
-    }
-
-    InitOrAssign(std::move(other.value()));
-    return *this;
-  }
-
-  template <class U>
-  typename std::enable_if<std::is_same<std::decay<U>, T>::value,
-                          Optional&>::type
-  operator=(U&& value) {
-    InitOrAssign(std::forward<U>(value));
-    return *this;
-  }
-
-  // TODO(mlamouri): can't use 'constexpr' with DCHECK.
-  const T* operator->() const {
-    DCHECK(!is_null_);
-    return &value();
-  }
-
-  // TODO(mlamouri): using 'constexpr' here breaks compiler that assume it was
-  // meant to be 'constexpr const'.
-  T* operator->() {
-    DCHECK(!is_null_);
-    return &value();
-  }
-
-  constexpr const T& operator*() const& { return value(); }
-
-  // TODO(mlamouri): using 'constexpr' here breaks compiler that assume it was
-  // meant to be 'constexpr const'.
-  T& operator*() & { return value(); }
-
-  constexpr const T&& operator*() const&& { return std::move(value()); }
-
-  // TODO(mlamouri): using 'constexpr' here breaks compiler that assume it was
-  // meant to be 'constexpr const'.
-  T&& operator*() && { return std::move(value()); }
-
-  constexpr explicit operator bool() const { return !is_null_; }
-
-  // TODO(mlamouri): using 'constexpr' here breaks compiler that assume it was
-  // meant to be 'constexpr const'.
-  T& value() & {
-    DCHECK(!is_null_);
-    return *buffer_.template data_as<T>();
-  }
-
-  // TODO(mlamouri): can't use 'constexpr' with DCHECK.
-  const T& value() const& {
-    DCHECK(!is_null_);
-    return *buffer_.template data_as<T>();
-  }
-
-  // TODO(mlamouri): using 'constexpr' here breaks compiler that assume it was
-  // meant to be 'constexpr const'.
-  T&& value() && {
-    DCHECK(!is_null_);
-    return std::move(*buffer_.template data_as<T>());
-  }
-
-  // TODO(mlamouri): can't use 'constexpr' with DCHECK.
-  const T&& value() const&& {
-    DCHECK(!is_null_);
-    return std::move(*buffer_.template data_as<T>());
-  }
-
-  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 is_null_ ? static_cast<T>(std::forward<U>(default_value)) : value();
-  }
-
-  template <class U>
-  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 is_null_ ? static_cast<T>(std::forward<U>(default_value))
-                    : std::move(value());
-  }
-
-  void swap(Optional& other) {
-    if (is_null_ && other.is_null_)
-      return;
-
-    if (is_null_ != other.is_null_) {
-      if (is_null_) {
-        Init(std::move(*other.buffer_.template data_as<T>()));
-        other.FreeIfNeeded();
-      } else {
-        other.Init(std::move(*buffer_.template data_as<T>()));
-        FreeIfNeeded();
-      }
-      return;
-    }
-
-    DCHECK(!is_null_ && !other.is_null_);
-    using std::swap;
-    swap(**this, *other);
-  }
-
-  template <class... Args>
-  void emplace(Args&&... args) {
-    FreeIfNeeded();
-    Init(std::forward<Args>(args)...);
-  }
-
- private:
-  void Init(const T& value) {
-    DCHECK(is_null_);
-    new (buffer_.template data_as<T>()) T(value);
-    is_null_ = false;
-  }
-
-  void Init(T&& value) {
-    DCHECK(is_null_);
-    new (buffer_.template data_as<T>()) T(std::move(value));
-    is_null_ = false;
-  }
-
-  template <class... Args>
-  void Init(Args&&... args) {
-    DCHECK(is_null_);
-    new (buffer_.template data_as<T>()) T(std::forward<Args>(args)...);
-    is_null_ = false;
-  }
-
-  void InitOrAssign(const T& value) {
-    if (is_null_)
-      Init(value);
-    else
-      *buffer_.template data_as<T>() = value;
-  }
-
-  void InitOrAssign(T&& value) {
-    if (is_null_)
-      Init(std::move(value));
-    else
-      *buffer_.template data_as<T>() = std::move(value);
-  }
-
-  void FreeIfNeeded() {
-    if (is_null_)
-      return;
-    buffer_.template data_as<T>()->~T();
-    is_null_ = true;
-  }
-
-  bool is_null_ = true;
-  base::AlignedMemory<sizeof(T), ALIGNOF(T)> buffer_;
-};
-
-template <class T>
-constexpr bool operator==(const Optional<T>& lhs, const Optional<T>& rhs) {
-  return !!lhs != !!rhs ? false : lhs == nullopt || (*lhs == *rhs);
-}
-
-template <class T>
-constexpr bool operator!=(const Optional<T>& lhs, const Optional<T>& rhs) {
-  return !(lhs == rhs);
-}
-
-template <class T>
-constexpr bool operator<(const Optional<T>& lhs, const Optional<T>& rhs) {
-  return rhs == nullopt ? false : (lhs == nullopt ? true : *lhs < *rhs);
-}
-
-template <class T>
-constexpr bool operator<=(const Optional<T>& lhs, const Optional<T>& rhs) {
-  return !(rhs < lhs);
-}
-
-template <class T>
-constexpr bool operator>(const Optional<T>& lhs, const Optional<T>& rhs) {
-  return rhs < lhs;
-}
-
-template <class T>
-constexpr bool operator>=(const Optional<T>& lhs, const Optional<T>& rhs) {
-  return !(lhs < rhs);
-}
-
-template <class T>
-constexpr bool operator==(const Optional<T>& opt, base::nullopt_t) {
-  return !opt;
-}
-
-template <class T>
-constexpr bool operator==(base::nullopt_t, const Optional<T>& opt) {
-  return !opt;
-}
-
-template <class T>
-constexpr bool operator!=(const Optional<T>& opt, base::nullopt_t) {
-  return !!opt;
-}
-
-template <class T>
-constexpr bool operator!=(base::nullopt_t, const Optional<T>& opt) {
-  return !!opt;
-}
-
-template <class T>
-constexpr bool operator<(const Optional<T>& opt, base::nullopt_t) {
-  return false;
-}
-
-template <class T>
-constexpr bool operator<(base::nullopt_t, const Optional<T>& opt) {
-  return !!opt;
-}
-
-template <class T>
-constexpr bool operator<=(const Optional<T>& opt, base::nullopt_t) {
-  return !opt;
-}
-
-template <class T>
-constexpr bool operator<=(base::nullopt_t, const Optional<T>& opt) {
-  return true;
-}
-
-template <class T>
-constexpr bool operator>(const Optional<T>& opt, base::nullopt_t) {
-  return !!opt;
-}
-
-template <class T>
-constexpr bool operator>(base::nullopt_t, const Optional<T>& opt) {
-  return false;
-}
-
-template <class T>
-constexpr bool operator>=(const Optional<T>& opt, base::nullopt_t) {
-  return true;
-}
-
-template <class T>
-constexpr bool operator>=(base::nullopt_t, const Optional<T>& opt) {
-  return !opt;
-}
-
-template <class T>
-constexpr bool operator==(const Optional<T>& opt, const T& value) {
-  return opt != nullopt ? *opt == value : false;
-}
-
-template <class T>
-constexpr bool operator==(const T& value, const Optional<T>& opt) {
-  return opt == value;
-}
-
-template <class T>
-constexpr bool operator!=(const Optional<T>& opt, const T& value) {
-  return !(opt == value);
-}
-
-template <class T>
-constexpr bool operator!=(const T& value, const Optional<T>& opt) {
-  return !(opt == value);
-}
-
-template <class T>
-constexpr bool operator<(const Optional<T>& opt, const T& value) {
-  return opt != nullopt ? *opt < value : true;
-}
-
-template <class T>
-constexpr bool operator<(const T& value, const Optional<T>& opt) {
-  return opt != nullopt ? value < *opt : false;
-}
-
-template <class T>
-constexpr bool operator<=(const Optional<T>& opt, const T& value) {
-  return !(opt > value);
-}
-
-template <class T>
-constexpr bool operator<=(const T& value, const Optional<T>& opt) {
-  return !(value > opt);
-}
-
-template <class T>
-constexpr bool operator>(const Optional<T>& opt, const T& value) {
-  return value < opt;
-}
-
-template <class T>
-constexpr bool operator>(const T& value, const Optional<T>& opt) {
-  return opt < value;
-}
-
-template <class T>
-constexpr bool operator>=(const Optional<T>& opt, const T& value) {
-  return !(opt < value);
-}
-
-template <class T>
-constexpr bool operator>=(const T& value, const Optional<T>& opt) {
-  return !(value < opt);
-}
-
-template <class T>
-constexpr Optional<typename std::decay<T>::type> make_optional(T&& value) {
-  return Optional<typename std::decay<T>::type>(std::forward<T>(value));
-}
-
-template <class T>
-void 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_