Allow std::unordered_*.

base/strings/string16.h

 // Copyright 2013 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_STRINGS_STRING16_H_
 #define BASE_STRINGS_STRING16_H_
 
 // WHAT:
 // A version of std::basic_string that provides 2-byte characters even when
 // wchar_t is not implemented as a 2-byte type. You can access this class as
@@ skipping 11 matching lines @@
 // entirely improper on those systems where the encoding of wchar_t is defined
 // as UTF-32.
 //
 // Here, we define string16, which is similar to std::wstring but replaces all
 // libc functions with custom, 2-byte-char compatible routines. It is capable
 // of carrying UTF-16-encoded data.
 
 #include <stddef.h>
 #include <stdint.h>
 #include <stdio.h>
+
+#include <functional>
 #include <string>
 
 #include "base/base_export.h"
 #include "build/build_config.h"
 
 #if defined(WCHAR_T_IS_UTF16)
 
 namespace base {
 
 typedef wchar_t char16;
@@ skipping 133 matching lines @@
 // in which the linker does not fully coalesce symbols when dead code
 // stripping is enabled.  This bug causes the memory errors described above
 // to occur even when a std::basic_string<> does not cross shared library
 // boundaries, such as in statically-linked executables.
 //
 // TODO(mark): File this bug with Apple and update this note with a bug number.
 
 extern template
 class BASE_EXPORT std::basic_string<base::char16, base::string16_char_traits>;
 
+// Specialize std::hash for base::string16. Although the style guide forbids
+// this in general, it is necessary for consistency with WCHAR_T_IS_UTF16
+// platforms, where base::string16 is a type alias for std::wstring.
+namespace std {
+template<>
+struct hash<base::string16> {
+  std::size_t operator()(const base::string16& s) const {
+    std::size_t result = 0;
+    for (base::char16 c : s)
+      result = (result * 131) + c;
+    return result;
+  }
+};
+}  // namespace std
+
 #endif  // WCHAR_T_IS_UTF32
 
 #endif  // BASE_STRINGS_STRING16_H_