headless: Add a Maybe-type

headless/public/util/maybe.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 HEADLESS_PUBLIC_UTIL_MAYBE_H_
+#define HEADLESS_PUBLIC_UTIL_MAYBE_H_
+
+#include <algorithm>
+
+#include "base/logging.h"
+#include "base/macros.h"
+
+namespace headless {
+
+// A simple Maybe which may or may not have a value. Based on v8::Maybe.
+template <typename T>
+class Maybe {
+ public:
+  Maybe() : has_value_(false) {}
+
+  bool IsNothing() const { return !has_value_; }
+  bool IsJust() const { return has_value_; }
+
+  // Will crash if the Maybe<> is nothing.
+  T& FromJust() {
+    DCHECK(IsJust());
+    return value_;
+  }
+  const T& FromJust() const {
+    DCHECK(IsJust());
+    return value_;
+  }
+
+  T FromMaybe(const T& default_value) const {
+    return has_value_ ? value_ : default_value;
+  }
+
+  bool operator==(const Maybe& other) const {
+    return (IsJust() == other.IsJust()) &&
+           (!IsJust() || FromJust() == other.FromJust());
+  }
+
+  bool operator!=(const Maybe& other) const { return !operator==(other); }
+
+  Maybe& operator=(Maybe&& other) {
+    has_value_ = other.has_value_;
+    std::swap(value_, other.value_);

[alex clarke (OOO till 29th), 2016/04/07 11:45:32]

shouldn't this be value_ = std::move(other.value_);

[Sami, 2016/04/07 12:00:43]

Good point, done. I've also added a question to
https://sites.google.com/a/chromium.org/dev/rvalue-references?pli=1

+    return *this;
+  }
+
+  Maybe& operator=(const Maybe& other) {
+    has_value_ = other.has_value_;
+    value_ = other.value_;
+    return *this;
+  }
+
+  Maybe(const Maybe& other) { *this = other; }

[alex clarke (OOO till 29th), 2016/04/07 11:45:32]

I wonder if this and the one below could be = default?

[Sami, 2016/04/07 12:00:43]

Yep, they can, fixed. If you're wondering why we need to define them explicitly,
it's because:

"copy constructor is implicitly deleted because 'Maybe<headless::(anonymous
namespace)::MoveOnlyType>' has a user-declared move assignment operator"

+
+  Maybe(Maybe&& other) { *this = std::move(other); }
+
+ private:
+  template <class U>
+  friend Maybe<U> Nothing();
+  template <class U>
+  friend Maybe<U> Just(const U& u);
+  template <class U>
+  friend Maybe<typename std::remove_reference<U>::type> Just(U&& u);
+
+  explicit Maybe(const T& t) : has_value_(true), value_(t) {}
+  explicit Maybe(T&& t) : has_value_(true), value_(std::move(t)) {}
+
+  bool has_value_;
+  T value_;
+};
+
+template <class T>
+Maybe<T> Nothing() {
+  return Maybe<T>();
+}
+
+template <class T>
+Maybe<T> Just(const T& t) {
+  return Maybe<T>(t);
+}
+
+template <class T>
+Maybe<typename std::remove_reference<T>::type> Just(T&& t) {
+  return Maybe<typename std::remove_reference<T>::type>(std::move(t));
+}
+
+}  // namespace headless
+
+#endif  // HEADLESS_PUBLIC_UTIL_MAYBE_H_