Add base::Reversed() as an adapter for range-based for loops in reverse

base/containers/adapters.h

Index: base/containers/adapters.h
diff --git a/base/containers/adapters.h b/base/containers/adapters.h
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index 0000000000000000000000000000000000000000..01545a68dec74b4aee95da8c0e9b36000ff8bfd8
--- /dev/null
+++ b/base/containers/adapters.h
@@ -0,0 +1,49 @@
+// Copyright 2014 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.
+
+// This file contains adapters to iterate containers in alternative orders.

[brettw, 2014/09/29 16:55:14]

I'd just remove this line. We could potentially add more variety of stuff here.
And then I'd add a more specific comment (and move the example) to right before
the definition of "reversed".

[mdempsky, 2014/09/29 17:47:40]

Done.

+//
+// Example:
+//
+//   std::vector<int> v = ...;
+//   for (int i : base::reversed(v)) {
+//     // iterates through v in reverse order
+//   }
+
+#ifndef BASE_CONTAINERS_ADAPTERS_H_
+#define BASE_CONTAINERS_ADAPTERS_H_
+
+#include "base/macros.h"
+
+namespace base {
+
+namespace internal {
+
+template <typename T>

[brettw, 2014/09/29 16:55:14]

Can you include an example here?

[mdempsky, 2014/09/29 17:47:40]

This class isn't intended for end users. I've added a comment to try to further
clarify that it's intended as an implementation detail only.

+class ReversedAdapter {
+ public:
+  typedef decltype(static_cast<T*>(nullptr)->rbegin()) Iterator;
+
+  explicit ReversedAdapter(T& t) : t_(t) {}
+  ReversedAdapter(const ReversedAdapter& ra) : t_(ra.t_) {}
+
+  friend Iterator begin(const ReversedAdapter& ra) { return ra.t_.rbegin(); }

[brettw, 2014/09/29 16:55:14]

I don't understand what "friend" does in this context. If it's really necessary,
can you add a comment?

[mdempsky, 2014/09/29 17:47:41]

Turns out it's not necessary; I was looking at an old C++11 draft instead of the
actual C++11 spec.

(Earlier C++11 drafts declared that range-based 'for' loops only checked for
"begin(__range)", and this code was providing inline friend function definitions
to satisfy that API; however, the actual C++11 spec says "__range.begin()" is
also supported, so I've simplified these functions to take advantage of that.)

+  friend Iterator end(const ReversedAdapter& ra) { return ra.t_.rend(); }
+
+ private:
+  T& t_;
+
+  DISALLOW_ASSIGN(ReversedAdapter);
+};
+
+}  // namespace internal
+
+template <typename T>
+internal::ReversedAdapter<T> reversed(T& t) {

[brettw, 2014/09/29 16:55:14]

Google-style would say this should be capitalized. It's a bit weird since
sty-style says lower-cased. I'm inclined to side with Google-style here, though.

[mdempsky, 2014/09/29 17:47:41]

Done.

+  return internal::ReversedAdapter<T>(t);
+}
+
+}  // namespace base
+
+#endif  // BASE_CONTAINERS_ADAPTERS_H_