Introduce rtc::Maybe<T>, which either contains a T or not.

webrtc/base/maybe.h

+/*
+ *  Copyright 2015 The WebRTC Project Authors. All rights reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef WEBRTC_BASE_MAYBE_H_
+#define WEBRTC_BASE_MAYBE_H_
+
+#include <algorithm>
+#include <utility>
+
+#include "webrtc/base/checks.h"
+
+namespace rtc {
+
+// Simple std::experimental::optional-wannabe. It either contains a T or not.
+// In order to keep the implementation simple and portable, this implementation
+// actually contains a (default-constructed) T even when it supposedly doesn't
+// contain a value; use e.g. rtc::scoped_ptr<T> instead if that's too
+// expensive.
+//
+// A moved-from Maybe<T> may only be destroyed, and assigned to if T allows
+// being assigned to after having been moved from. Specifically, you may not
+// assume that it just doesn't contain a value anymore.
+//
+// TODO(kwiberg): Get rid of this class when the standard library has
+// std::optional (and we're allowed to use it).
+template <typename T>
+class Maybe final {
+ public:
+  // Construct an empty Maybe.
+  Maybe() : has_value_(false) {}
+
+  // Construct a Maybe that contains a value. Note: These are non-explicit, so
+  // that a T will implicitly convert to Maybe<T>.
+  Maybe(const T& val) : value_(val), has_value_(true) {}
+  Maybe(T&& val) : value_(static_cast<T&&>(val)), has_value_(true) {}
+
+  // Copy and move constructors.
+  // TODO(kwiberg): =default the move constructor when MSVC supports it.
+  Maybe(const Maybe&) = default;
+  Maybe(Maybe&& m)
+      : value_(static_cast<T&&>(m.value_)), has_value_(m.has_value_) {}
+
+  // Assignment. Note that we allow assignment from either Maybe<T> or plain T.
+  // TODO(kwiberg): =default the move assignment op when MSVC supports it.
+  Maybe& operator=(const Maybe&) = default;
+  Maybe& operator=(Maybe&& m) {
+    value_ = static_cast<T&&>(m.value_);
+    has_value_ = m.has_value_;
+    return *this;
+  }
+  Maybe& operator=(const T& val) {
+    value_ = val;
+    has_value_ = true;
+    return *this;
+  }
+  Maybe& operator=(T&& val) {
+    value_ = static_cast<T&&>(val);
+    has_value_ = true;
+    return *this;
+  }
+
+  friend void swap(Maybe& m1, Maybe& m2) {
+    using std::swap;
+    swap(m1.value_, m2.value_);
+    swap(m1.has_value_, m2.has_value_);
+  }
+
+  // Conversion to bool to test if we have a value.
+  explicit operator bool() const { return has_value_; }
+
+  // Dereferencing. Only allowed if we have a value.
+  const T* operator->() const {
+    RTC_DCHECK(has_value_);
+    return &value_;
+  }
+  T* operator->() {
+    RTC_DCHECK(has_value_);
+    return &value_;
+  }
+  const T& operator*() const {
+    RTC_DCHECK(has_value_);
+    return value_;
+  }
+  T& operator*() {
+    RTC_DCHECK(has_value_);
+    return value_;
+  }
+
+  // Dereference with a default value in case we don't have a value.
+  const T& value_or(const T& default_val) const {
+    return has_value_ ? value_ : default_val;
+  }
+
+  // Equality tests. Two Maybes are equal if they contain equivalent values, or
+  // if they're both empty.
+  friend bool operator==(const Maybe& m1, const Maybe& m2) {
+    return m1.has_value_ && m2.has_value_ ? m1.value_ == m2.value_
+                                          : m1.has_value_ == m2.has_value_;
+  }
+  friend bool operator!=(const Maybe& m1, const Maybe& m2) {
+    return m1.has_value_ && m2.has_value_ ? m1.value_ != m2.value_
+                                          : m1.has_value_ != m2.has_value_;
+  }
+
+ private:
+  // Invariant: Unless *this has been moved from, value_ is default-initialized
+  // (or copied or moved from a default-initialized T) if !has_value_.
+  T value_;
+  bool has_value_;
+};
+
+}  // namespace rtc
+
+#endif  // WEBRTC_BASE_MAYBE_H_