base: Add three helper functions to Values API.

base/values.h

 // Copyright (c) 2011 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 setting and other persistable data.  It includes the ability to
 // specify (recursive) lists and dictionaries, so it's fairly expressive.
 // However, the API is optimized for the common case, namely storing a
 // hierarchical tree of simple values.  Given a DictionaryValue root, you can
 // easily do things like:
@@ skipping 52 matching lines @@
     TYPE_BINARY,
     TYPE_DICTIONARY,
     TYPE_LIST
   };
 
   virtual ~Value();
 
   // Convenience methods for creating Value objects for various
   // kinds of values without thinking about which class implements them.
   // These can always be expected to return a valid Value*.
-  static Value* CreateNullValue();
   static FundamentalValue* CreateBooleanValue(bool in_value);
   static FundamentalValue* CreateIntegerValue(int in_value);
   static FundamentalValue* CreateDoubleValue(double in_value);
   static StringValue* CreateStringValue(const std::string& in_value);
   static StringValue* CreateStringValue(const string16& in_value);
 
   // Returns the type of the value stored by the current Value object.
   // Each type will be implemented by only one subclass of Value, so it's
   // safe to use the Type to determine whether you can cast from
   // Value* to (Implementing Class)*.  Also, a Value object never changes
@@ skipping 19 matching lines @@
   // to the copy.  The caller gets ownership of the copy, of course.
   //
   // Subclasses return their own type directly in their overrides;
   // this works because C++ supports covariant return types.
   virtual Value* DeepCopy() const;
 
   // Compares if two Value objects have equal contents.
   virtual bool Equals(const Value* other) const;
 
   // Compares if two Value objects have equal contents. Can handle NULLs.
-  // NULLs are considered equal but different from Value::CreateNullValue().
+  // NULLs are considered equal but different from NullValue().
   static bool Equals(const Value* a, const Value* b);
 
  protected:
   // This isn't safe for end-users (they should use the Create*Value()
   // static methods above), but it's useful for subclasses.
   explicit Value(Type type);
 
  private:
   Value();
 
   Type type_;
 
   DISALLOW_COPY_AND_ASSIGN(Value);
 };
 
-// FundamentalValue represents the simple fundamental types of values.
+// Helper functions. Use TrueValue() and FalseValue() instead of:
+// new FundamentalValue() and CreateBooleanValue().
+BASE_EXPORT Value* NullValue();
+BASE_EXPORT FundamentalValue* TrueValue();
+BASE_EXPORT FundamentalValue* FalseValue();
+
+// FundamentalValue represents one of the fundamental values:
+// bool, int or double.
 class BASE_EXPORT FundamentalValue : public Value {
  public:
   explicit FundamentalValue(bool in_value);
   explicit FundamentalValue(int in_value);
   explicit FundamentalValue(double in_value);
   virtual ~FundamentalValue();
 
   // Overridden from Value:
   virtual bool GetAsBoolean(bool* out_value) const OVERRIDE;
   virtual bool GetAsInteger(int* out_value) const OVERRIDE;
@@ skipping 299 matching lines @@
  private:
   ValueVector list_;
 
   DISALLOW_COPY_AND_ASSIGN(ListValue);
 };
 
 // This interface is implemented by classes that know how to serialize and
 // deserialize Value objects.
 class BASE_EXPORT ValueSerializer {
  public:
-  virtual ~ValueSerializer();
+  virtual ~ValueSerializer() {}
 
   virtual bool Serialize(const Value& root) = 0;
 
   // This method deserializes the subclass-specific format into a Value object.
   // If the return value is non-NULL, the caller takes ownership of returned
   // Value. If the return value is NULL, and if error_code is non-NULL,
   // error_code will be set with the underlying error.
   // If |error_message| is non-null, it will be filled in with a formatted
   // error message including the location of the error if appropriate.
   virtual Value* Deserialize(int* error_code, std::string* error_str) = 0;
 };
 
 }  // namespace base
 
 // http://crbug.com/88666
 using base::DictionaryValue;
 using base::ListValue;
 using base::StringValue;
 using base::Value;
 
 #endif  // BASE_VALUES_H_