libaddressinput - reduce number of copies in storage class by 2

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 73 matching lines @@
 
   // These methods allow the convenient retrieval of the contents of the Value.
   // If the current object can be converted into the given type, the value is
   // returned through the |out_value| parameter and true is returned;
   // otherwise, false is returned and |out_value| is unchanged.
   virtual bool GetAsBoolean(bool* out_value) const;
   virtual bool GetAsInteger(int* out_value) const;
   virtual bool GetAsDouble(double* out_value) const;
   virtual bool GetAsString(std::string* out_value) const;
   virtual bool GetAsString(string16* out_value) const;
+  virtual bool GetAsStringValue(const StringValue** out_value) const;

[brettw, 2014/01/28 03:00:24]

This new name seems good but it doesn't follow the pattern of the rest of the
functions. I think for consistency this should be just "GetAsString" (or feel
free to change the other ones!).

   virtual bool GetAsList(ListValue** out_value);
   virtual bool GetAsList(const ListValue** out_value) const;
   virtual bool GetAsDictionary(DictionaryValue** out_value);
   virtual bool GetAsDictionary(const DictionaryValue** out_value) const;
   // Note: Do not add more types. See the file-level comment above for why.
 
   // This creates a deep copy of the entire Value tree, and returns a pointer
   // to the copy.  The caller gets ownership of the copy, of course.
   //
   // Subclasses return their own type directly in their overrides;
@@ skipping 41 matching lines @@
 };
 
 class BASE_EXPORT StringValue : public Value {
  public:
   // Initializes a StringValue with a UTF-8 narrow character string.
   explicit StringValue(const std::string& in_value);
 
   // Initializes a StringValue with a string16.
   explicit StringValue(const string16& in_value);
 
+  // Creates a string value that takes ownership of a string.
+  explicit StringValue(scoped_ptr<std::string> in_value);
+
   virtual ~StringValue();
 
   // Overridden from Value:
   virtual bool GetAsString(std::string* out_value) const OVERRIDE;
   virtual bool GetAsString(string16* out_value) const OVERRIDE;
+  virtual bool GetAsStringValue(const StringValue** out_value) const OVERRIDE;
   virtual StringValue* DeepCopy() const OVERRIDE;
   virtual bool Equals(const Value* other) const OVERRIDE;
 
+  const std::string& value() const { return *value_; }
+
  private:
-  std::string value_;
+  scoped_ptr<std::string> value_;

[brettw, 2014/01/28 03:00:24]

I assume you did this to avoid the extra copy when constructing from a passed-in
string for your use-case. But this adds an extra malloc for every existing user
of this class, and generally malloc will be much more expensive than copying a
random string. Are we sure that this is the correct tradeoff?

[Evan Stade, 2014/01/28 18:37:36]

yes
hmm. Do you think I should create a second StringValue class instead?
HeapStringValue or some such?

I've also noticed there's a JSONStringValue class and I'd like to be able to
access its payload without copying, just as I've done here. But GetAsStringValue
won't work for that, so I guess I need GetAsStringRef(std::string** out_value)
instead of GetAsStringValue/value().

[Evan Stade, 2014/01/28 18:43:35]

Actually, it looks like StringPiece can't return a reference to itself as a
string. Let me rethink this.

[Evan Stade, 2014/01/28 18:48:45]

Well, even in lieu of that other optimization, I still think the one  in this
patch is helfpul. So the question, again, is do you want me to make a
HeapStringValue instead?

 };
 
 class BASE_EXPORT BinaryValue: public Value {
  public:
   // Creates a BinaryValue with a null buffer and size of 0.
   BinaryValue();
 
   // Creates a BinaryValue, taking ownership of the bytes pointed to by
   // |buffer|.
   BinaryValue(scoped_ptr<char[]> buffer, size_t size);
@@ skipping 342 matching lines @@
 }
 
 BASE_EXPORT inline std::ostream& operator<<(std::ostream& out,
                                             const ListValue& value) {
   return out << static_cast<const Value&>(value);
 }
 
 }  // namespace base
 
 #endif  // BASE_VALUES_H_