Use base::flat_map for base::Value dictionary storage.

base/values.h

 // Copyright (c) 2012 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 specifies a recursive data storage class called Value intended for
 // storing settings and other persistable data.
 //
 // A Value represents something that can be stored in JSON or passed to/from
 // JavaScript. As such, it is NOT a generalized variant type, since only the
 // types supported by JavaScript/JSON are supported.
@@ skipping 11 matching lines @@
 
 #include <iosfwd>
 #include <map>
 #include <memory>
 #include <string>
 #include <utility>
 #include <vector>
 
 #include "base/base_export.h"
 #include "base/compiler_specific.h"
+#include "base/containers/flat_map.h"
 #include "base/macros.h"
 #include "base/memory/manual_constructor.h"
 #include "base/strings/string16.h"
 #include "base/strings/string_piece.h"
 
 namespace base {
 
 class DictionaryValue;
 class ListValue;
 class Value;
 using BinaryValue = Value;
 
 // The Value class is the base class for Values. A Value can be instantiated
 // via the Create*Value() factory methods, or by directly creating instances of
 // the subclasses.
 //
 // See the file-level comment above for more information.
 class BASE_EXPORT Value {
  public:
-  using DictStorage = std::map<std::string, std::unique_ptr<Value>>;
+  using DictStorage = base::flat_map<std::string, std::unique_ptr<Value>>;
   using ListStorage = std::vector<std::unique_ptr<Value>>;
 
   enum class Type {
     NONE = 0,
     BOOLEAN,
     INTEGER,
     DOUBLE,
     STRING,
     BINARY,
     DICTIONARY,
@@ skipping 23 matching lines @@
   // Value(std::string&&) allow for efficient move construction.
   // Value(StringPiece) exists due to many callsites passing StringPieces as
   // arguments.
   explicit Value(const char* in_string);
   explicit Value(const std::string& in_string);
   explicit Value(std::string&& in_string) noexcept;
   explicit Value(const char16* in_string);
   explicit Value(const string16& in_string);
   explicit Value(StringPiece in_string);
 
+  explicit Value(DictStorage&& in_dict) noexcept;

[jdoerrie, 2017/04/10 11:44:57]

nit: Consider moving this constructor down after the blob constructors. So far I
was following the order of Types defined in line 55 when declaring these
constructors.

+
   explicit Value(const std::vector<char>& in_blob);
   explicit Value(std::vector<char>&& in_blob) noexcept;
 
   Value& operator=(const Value& that);
   Value& operator=(Value&& that) noexcept;
 
   ~Value();
 
   // Returns the name for a given |type|.
   static const char* GetTypeName(Type type);
@@ skipping 83 matching lines @@
 
   union {
     bool bool_value_;
     int int_value_;
     double double_value_;
     ManualConstructor<std::string> string_value_;
     ManualConstructor<std::vector<char>> binary_value_;
     // For current gcc and clang sizeof(DictStorage) = 48, which would result
     // in sizeof(Value) = 56 if DictStorage was stack allocated. Allocating it
     // on the heap results in sizeof(Value) = 40 for all of gcc, clang and MSVC.
     ManualConstructor<std::unique_ptr<DictStorage>> dict_ptr_;

[jdoerrie, 2017/04/10 11:44:57]

Now that DictStorage is effectively a vector there is no need to wrap it in a
unique_ptr anymore. Dropping the wrapper should increase performance as well
with no increase in memory usage. I am fine with both doing this in this CL or
in a follow up CL authored by you or me. What do you prefer?

     ManualConstructor<ListStorage> list_;
   };
 
  private:
   void InternalCopyFundamentalValue(const Value& that);
   void InternalCopyConstructFrom(const Value& that);
   void InternalMoveConstructFrom(Value&& that);
   void InternalCopyAssignFromSameType(const Value& that);
   void InternalCleanup();
 };
@@ skipping 321 matching lines @@
   return out << static_cast<const Value&>(value);
 }
 
 // Stream operator so that enum class Types can be used in log statements.
 BASE_EXPORT std::ostream& operator<<(std::ostream& out,
                                      const Value::Type& type);
 
 }  // namespace base
 
 #endif  // BASE_VALUES_H_