Add stream logging support for base::Optional

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 <ostream>
 #include <type_traits>
 
 #include "base/logging.h"
 #include "base/memory/aligned_memory.h"
 #include "base/template_util.h"
 
 namespace base {
 
 // Specification:
 // http://en.cppreference.com/w/cpp/utility/optional/in_place_t
@@ skipping 411 matching lines @@
 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));
 }
 
+// For logging use only.
+template <class T>
+std::ostream& operator<<(std::ostream& os, const Optional<T>& opt) {
+  if (opt)
+    os << opt.value();
+  else
+    os << "(unset)";
+  return os;
+}
+
 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_