Add new JSON_USE_NULLREF flag

This patch adds a new flag, JSON_USE_NULLREF, which removes
the legacy singletons null, nullRef for consumers that require not
having static initialized globals, like Chromium.

include/json/config.h

@@ -30,6 +30,11 @@
 #define JSON_USE_EXCEPTION 1
 #endif
 
+// Temporary, tracked for removal with issue #982.
+#ifndef JSON_USE_NULLREF
+#define JSON_USE_NULLREF 1
+#endif
+
 /// If defined, indicates that the source file is amalgamated
 /// to prevent private header inclusion.
 /// Remarks: it is automatically defined in the generated amalgamated header.

include/json/value.h

@@ -193,11 +193,14 @@ public:
   // Required for boost integration, e. g. BOOST_TEST
   typedef std::string value_type;
 
-  static const Value& null; ///< We regret this reference to a global instance;
-                            ///< prefer the simpler Value().
-  static const Value& nullRef; ///< just a kludge for binary-compatibility; same
-                               ///< as null
-  static Value const& nullSingleton(); ///< Prefer this to null or nullRef.
+#if JSON_USE_NULLREF
+  // Binary compatibility kludges, do not use.
+  static const Value& null;
+  static const Value& nullRef;
+#endif
+
+  // null and nullRef are deprecated, use this instead.
+  static Value const& nullSingleton();
 
   /// Minimum signed integer value that can be stored in a Json::Value.
   static const LargestInt minLargestInt;