Add scoped_ptr-safe base::Value to Dictionary/List conversion functions.

This change adds two static From() functions to the DictionaryValue and
ListValue classes which take a scoped_ptr to a Value and either convert
it to a scoped_ptr to a DictionaryValue or ListValue, or return nullptr.
These are intended to replace the existing pattern,

  make_scoped_ptr(static_cast<base::DictionaryValue*>(value.release()))

with the shorter and safer alternative,

  base::DictionaryValue::From(value.Pass())

Instances of this pattern in //extensions have been converted as
examples.

Review URL: https://codereview.chromium.org/1308013005

Cr-Commit-Position: refs/heads/master@{#348294}

2 files changed, 26 insertions, 0 deletions

diff --git a/base/values.cc b/base/values.cc
index c829a7f..9b2483e 100644
--- a/base/values.cc
+++ b/base/values.cc
@@ -351,6 +351,16 @@ bool BinaryValue::Equals(const Value* other) const {
 
 ///////////////////// DictionaryValue ////////////////////
 
+// static
+scoped_ptr<DictionaryValue> DictionaryValue::From(scoped_ptr<Value> value) {
+  DictionaryValue* out;
+  if (value && value->GetAsDictionary(&out)) {
+    ignore_result(value.release());
+    return make_scoped_ptr(out);
+  }
+  return nullptr;
+}
+
 DictionaryValue::DictionaryValue()
     : Value(TYPE_DICTIONARY) {
 }
@@ -870,6 +880,16 @@ bool DictionaryValue::Equals(const Value* other) const {
 
 ///////////////////// ListValue ////////////////////
 
+// static
+scoped_ptr<ListValue> ListValue::From(scoped_ptr<Value> value) {
+  ListValue* out;
+  if (value && value->GetAsList(&out)) {
+    ignore_result(value.release());
+    return make_scoped_ptr(out);
+  }
+  return nullptr;
+}
+
 ListValue::ListValue() : Value(TYPE_LIST) {
 }
 
diff --git a/base/values.h b/base/values.h
index 8756deb..56be542 100644
--- a/base/values.h
+++ b/base/values.h
@@ -209,6 +209,9 @@ class BASE_EXPORT BinaryValue: public Value {
 // are |std::string|s and should be UTF-8 encoded.
 class BASE_EXPORT DictionaryValue : public Value {
  public:
+  // Returns |value| if it is a dictionary, nullptr otherwise.
+  static scoped_ptr<DictionaryValue> From(scoped_ptr<Value> value);
+
   DictionaryValue();
   ~DictionaryValue() override;
 
@@ -387,6 +390,9 @@ class BASE_EXPORT ListValue : public Value {
   typedef ValueVector::iterator iterator;
   typedef ValueVector::const_iterator const_iterator;
 
+  // Returns |value| if it is a list, nullptr otherwise.
+  static scoped_ptr<ListValue> From(scoped_ptr<Value> value);
+
   ListValue();
   ~ListValue() override;