JSONStringValueSerializer: Constructor argument must not be null.

Previously, this class would fail gracefully if given a nullptr. Now it
has undefined behaviour. Dealing with null isn't necessary as no caller
passes nullptr here, and in general, pass-by-result style functions do
not expect null pointers. Dealing with nullptr makes my follow-up work
messy, so I am just removing it.

Also, moved constructor implementation to .cc file, and properly
documented the lifetime constraints on |json_string|.

BUG=455068

Review URL: https://codereview.chromium.org/889303004

Cr-Commit-Position: refs/heads/master@{#314703}

base/json/json_string_value_serializer.cc

@@ -10,6 +10,22 @@
 
 using base::Value;
 
+JSONStringValueSerializer::JSONStringValueSerializer(std::string* json_string)
+    : json_string_(json_string),
+      initialized_with_const_string_(false),
+      pretty_print_(false),
+      allow_trailing_comma_(false) {
+  DCHECK(json_string);
+}
+
+JSONStringValueSerializer::JSONStringValueSerializer(
+    const std::string& json_string)
+    : json_string_(&const_cast<std::string&>(json_string)),
+      initialized_with_const_string_(true),
+      pretty_print_(false),
+      allow_trailing_comma_(false) {
+}
+
 JSONStringValueSerializer::~JSONStringValueSerializer() {}
 
 bool JSONStringValueSerializer::Serialize(const Value& root) {
@@ -23,7 +39,7 @@ bool JSONStringValueSerializer::SerializeAndOmitBinaryValues(
 
 bool JSONStringValueSerializer::SerializeInternal(const Value& root,
                                                   bool omit_binary_values) {
-  if (!json_string_ || initialized_with_const_string_)
+  if (initialized_with_const_string_)
     return false;
 
   int options = 0;
@@ -37,9 +53,6 @@ bool JSONStringValueSerializer::SerializeInternal(const Value& root,
 
 Value* JSONStringValueSerializer::Deserialize(int* error_code,
                                               std::string* error_str) {
-  if (!json_string_)
-    return NULL;
-
   return base::JSONReader::ReadAndReturnError(*json_string_,
       allow_trailing_comma_ ? base::JSON_ALLOW_TRAILING_COMMAS :
           base::JSON_PARSE_RFC,

base/json/json_string_value_serializer.h

@@ -14,24 +14,15 @@
 
 class BASE_EXPORT JSONStringValueSerializer : public base::ValueSerializer {
  public:
-  // json_string is the string that will be source of the deserialization
+  // |json_string| is the string that will be source of the deserialization
   // or the destination of the serialization.  The caller of the constructor
-  // retains ownership of the string.
-  explicit JSONStringValueSerializer(std::string* json_string)
-      : json_string_(json_string),
-        initialized_with_const_string_(false),
-        pretty_print_(false),
-        allow_trailing_comma_(false) {
-  }
+  // retains ownership of the string. |json_string| must not be null.
+  explicit JSONStringValueSerializer(std::string* json_string);
 
   // This version allows initialization with a const string reference for
-  // deserialization only.
-  explicit JSONStringValueSerializer(const std::string& json_string)
-      : json_string_(&const_cast<std::string&>(json_string)),
-        initialized_with_const_string_(true),
-        pretty_print_(false),
-        allow_trailing_comma_(false) {
-  }
+  // deserialization only. Retains a reference to |json_string|, so the string
+  // argument must outlive the JSONStringValueSerializer.
+  explicit JSONStringValueSerializer(const std::string& json_string);
 
   ~JSONStringValueSerializer() override;
 
@@ -46,7 +37,7 @@ class BASE_EXPORT JSONStringValueSerializer : public base::ValueSerializer {
 
   // Attempt to deserialize the data structure encoded in the string passed
   // in to the constructor into a structure of Value objects.  If the return
-  // value is NULL, and if |error_code| is non-null, |error_code| will
+  // value is null, and if |error_code| is non-null, |error_code| will
   // contain an integer error code (a JsonParseError in this case).
   // If |error_message| is non-null, it will be filled in with a formatted
   // error message including the location of the error if appropriate.
@@ -64,7 +55,7 @@ class BASE_EXPORT JSONStringValueSerializer : public base::ValueSerializer {
  private:
   bool SerializeInternal(const base::Value& root, bool omit_binary_values);
 
-  std::string* json_string_;
+  std::string* json_string_;  // Not null.
   bool initialized_with_const_string_;
   bool pretty_print_;  // If true, serialization will span multiple lines.
   // If true, deserialization will allow trailing commas.