Base: add Optional<T>.

base/optional.h

+// Copyright 2015 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 "base/logging.h"
+#include "base/memory/aligned_memory.h"
+
+namespace base {
+
+// Specification:
+// http://en.cppreference.com/w/cpp/experimental/optional/nullopt_t
+struct nullopt_t {
+  explicit nullopt_t(int _) { }

[danakj, 2015/11/19 20:55:46]

i recently discovered the style guide now says you can omit names of unused
variables, so no _ needed

[mlamouri (slow - plz ping), 2016/02/29 22:31:27]

Done.

+};
+
+// Specification:
+// http://en.cppreference.com/w/cpp/experimental/optional/nullopt
+const nullopt_t nullopt(0);
+
+// base::Optional is a Chromium version of the C++ library experimental optional
+// class: http://en.cppreference.com/w/cpp/experimental/optional/optional
+// The following methods are not implemented:
+// - All methods using rvalue reference (ie. && operator).

[danakj, 2015/11/19 20:55:46]

I think that I'd like to wait on this until we allow doing &&, which shouldn't
be long. We should be able to implement everything at that point I think?

[mlamouri (slow - plz ping), 2016/02/29 22:31:26]

Kind of. I've updated the list. Let me know how that sounds.

+//   Thus, instead of operator=(U&&), operator=(const T&) is implemented.
+// - Two constructor forms are not implemented.
+// - swap() and emplace() are not implemented.
+// - Private member |val| is not implemented.
+// - Some non-member functions, classes and helper objects are implemented but
+//   not all.
+template <typename T>
+class Optional {
+ public:
+  Optional() : is_null_(true) { }
+  Optional(base::nullopt_t opt) : Optional() { }
+
+  Optional(const Optional& other)

[danakj, 2015/12/10 23:45:45]

Can you add a Optional(Optional&&) constructor?

[mlamouri (slow - plz ping), 2016/02/29 22:31:26]

Done.

+      : Optional() {
+    if (!other.is_null_)
+      init(other.value());
+  }
+
+  Optional(const T& value)

[danakj, 2015/12/10 23:45:46]

And an Optional(T&&) constructor

[mlamouri (slow - plz ping), 2016/02/29 22:31:27]

Done.

+      : Optional() {
+    init(value);
+  }
+
+  // The difference between the specification and this method is that this
+  // implementation doesn't use std::is_trivially_destructible.

[mlamouri (slow - plz ping), 2016/02/29 22:31:27]

Is it okay to use std::is_trivially_destructible?

+  ~Optional() {
+    free_if_needed();
+  }
+
+  Optional& operator=(base::nullopt_t opt) {
+    free_if_needed();
+    return *this;
+  }
+
+  Optional& operator=(const Optional& other) {

[danakj, 2015/12/10 23:45:45]

can you add operator=(Optional&&)

[mlamouri (slow - plz ping), 2016/02/29 22:31:26]

Done.

+    if (other.is_null_) {
+      free_if_needed();
+      return *this;
+    }
+
+    init_or_assign(other.value());
+    return *this;
+  }
+
+  // This method is not part of the current C++ standard. It is instead defined
+  // as:
+  // template <class U>
+  // optional& operator=(U&& value);
+  Optional& operator=(const T& value) {

[danakj, 2015/12/10 23:45:45]

Let's make this take a T&&

[mlamouri (slow - plz ping), 2016/02/29 22:31:26]

Done. I've included the per-spec std::enable_if.

+    init_or_assign(value);
+    return *this;
+  }
+
+  const T* operator->() const {
+    DCHECK(!is_null_);
+    return buffer_.template data_as<T>();
+  }
+
+  T* operator->() {
+    DCHECK(!is_null_);
+    return buffer_.template data_as<T>();
+  }
+
+  const T& operator*() const {
+    return value();
+  }
+
+  T& operator*() {
+    return value();
+  }
+
+  explicit operator bool() const { return !is_null_; }

[danakj, 2015/12/10 23:45:46]

explicit operator is banned: http://chromium-cpp.appspot.com/#core-blacklist

you'll have to do a testable type trick instead (see scoped_ptr for example)

[mlamouri (slow - plz ping), 2016/02/29 22:31:26]

I couldn't get that to work. The issue is that I can't access the raw data
directly (ie. buffer_.data_). I'm sure there is some C++ magic I'm missing.

+
+  T& value() {

[danakj, 2015/12/10 23:45:45]

can you ref-quality this with &

[mlamouri (slow - plz ping), 2016/02/29 22:31:27]

Done.

+    DCHECK(!is_null_);
+    return *buffer_.template data_as<T>();
+  }
+
+  const T& value() const {

[danakj, 2015/12/10 23:45:45]

and this with &,

and add the T&& versions of value() and operator* from
http://en.cppreference.com/w/cpp/experimental/optional?

[mlamouri (slow - plz ping), 2016/02/29 22:31:26]

Done.

+    DCHECK(!is_null_);
+    return *buffer_.template data_as<T>();
+  }
+
+  template <class U>
+  T value_or(U&& default_value) const {

[danakj, 2015/12/10 23:45:46]

can add the non-const version of this method too

[mlamouri (slow - plz ping), 2016/02/29 22:31:26]

Done.

+    return is_null_ ? default_value : value();
+  }
+
+  bool operator==(const Optional& other) const {
+    return (is_null_ && other.is_null_) ||
+           (!is_null_ && !other.is_null_ && value() == other.value());
+  }
+
+  bool operator!=(const Optional& other) const {
+    return !this->operator==(other);
+  }
+
+ private:
+  void init(const T& value) {
+    DCHECK(is_null_);
+    new (buffer_.template data_as<T>()) T(value);
+    is_null_ = false;
+  }
+
+  void init_or_assign(const T& value) {
+    if (is_null_)
+      init(value);
+    else
+      *buffer_.template data_as<T>() = value;
+  }
+
+  void free_if_needed() {
+    if (is_null_)
+      return;
+    buffer_.template data_as<T>()->~T();
+    is_null_ = true;
+  }
+
+  bool is_null_;
+  base::AlignedMemory<sizeof(T), ALIGNOF(T)> buffer_;
+};
+
+}  // namespace base
+
+#endif // BASE_OPTIONAL_H_