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;

[horo, 2016/03/24 06:25:58]

I think you need an explicit initializer to initialize 'in_place'.

constexpr in_place_t in_place {};

[mlamouri (slow - plz ping), 2016/03/24 09:20:44]

danajk@ was against using this.

[horo, 2016/03/24 09:47:22]

I'm not familiar with constexpr.
But when I added '#include "base/optional.h' in ipc/ipc_message_utils.h for
https://crrev.com/1731153005 I saw this error.

danajk@
Could you please let me know how to avoid this error?

---------------------
In file included from ../ipc/ipc_message_utils.h:25:
../base/optional.h:27:22: error: default initialization of an object of const
type 'const base::in_place_t' without a user-provided default constructor
constexpr in_place_t in_place;
                     ^
../base/optional.h:27:30: note: add an explicit initializer to initialize
'in_place'
constexpr in_place_t in_place;
                             ^
                             {}
1 error generated.
---------------------

[dcheng, 2016/03/24 10:20:51]

Some alternatives:
  constexpr in_place_t in_place = in_place_t();
  constexpr in_place_t in_place = {};

FWIW, I'd probably go with the latter: using {} like that to initialize PODs is
a common and accepted idiom. I believe danakj@'s comment was specifically about
the braced initialization syntax introduced in C++11 (which allows the '=' to be
omitted): http://en.cppreference.com/w/cpp/language/aggregate_initialization

[danakj, 2016/03/24 20:38:51]

= {} sounds good.

[danakj, 2016/04/12 23:37:53]

Let's do this ^ still.

+
+// 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:
+// http://en.cppreference.com/w/cpp/utility/optional
+// The following known differences apply:
+// - 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 when possible.

[danakj, 2016/03/21 21:49:38]

Can you show what the error is when you use it?

[mlamouri (slow - plz ping), 2016/03/24 09:20:44]

Sure:
In file included from ../../base/optional_unittest.cc:5:
../../base/optional.h:71:14: error: no template named
'is_trivially_destructible' in namespace 'std'; did you mean
'has_trivial_destructor'?
    if (std::is_trivially_destructible<T>::value)
        ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~
             has_trivial_destructor
../../build/linux/debian_wheezy_amd64-sysroot/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../../include/c++/4.6/type_traits:731:12:
note: 'has_trivial_destructor' declared here
    struct has_trivial_destructor
           ^

[danakj, 2016/03/24 20:38:51]

Oh, gcc is missing that, ok. We should add it to base/template_util.h then at
some point to use it.

+    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): switch to is_copy_convertible when possible.

[danakj, 2016/03/21 21:49:37]

It should assert both when possible, not switch to. So just mention what it
should be also asserting when is_copy_constructible exists.

[mlamouri (slow - plz ping), 2016/03/24 09:20:44]

Done.

+    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): switch to is_move_convertible when possible.

[danakj, 2016/03/21 21:49:38]

ditto

[mlamouri (slow - plz ping), 2016/03/24 09:20:44]

Done.

+    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;

[danakj, 2016/03/21 21:49:37]

can you use () around the nested expression?

[mlamouri (slow - plz ping), 2016/03/24 09:20:45]

If I understood correctly, done.

+}
+
+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 != nullopt ? value == *opt : false;
+}
+
+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 : true;
+}
+
+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 opt != nullopt ? value < *opt : false;
+}
+
+template <class T>
+constexpr bool operator>(const T& value, const Optional<T>& opt) {
+  return opt != nullopt ? *opt < value : true;
+}
+
+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_