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 @@ class JSON_API Value {
   // 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;

src/lib_json/json_value.cpp

@@ -54,7 +54,6 @@ int JSON_API msvc_pre1900_c99_snprintf(char* outBuf,
 #define JSON_ASSERT_UNREACHABLE assert(false)
 
 namespace Json {
-
 template <typename T>
 static std::unique_ptr<T> cloneUnique(const std::unique_ptr<T>& p) {
   std::unique_ptr<T> r;
@@ -72,21 +71,22 @@ static std::unique_ptr<T> cloneUnique(const std::unique_ptr<T>& p) {
 #else
 #define ALIGNAS(byte_alignment)
 #endif
-// static const unsigned char ALIGNAS(8) kNull[sizeof(Value)] = { 0 };
-// const unsigned char& kNullRef = kNull[0];
-// const Value& Value::null = reinterpret_cast<const Value&>(kNullRef);
-// const Value& Value::nullRef = null;
 
 // static
 Value const& Value::nullSingleton() {
   static Value const nullStatic;
   return nullStatic;
 }
 
+#if JSON_USE_NULLREF
 // for backwards compatibility, we'll leave these global references around, but
 // DO NOT use them in JSONCPP library code any more!
+// static
 Value const& Value::null = Value::nullSingleton();
+
+// static
 Value const& Value::nullRef = Value::nullSingleton();
+#endif
 
 const Int Value::minInt = Int(~(UInt(-1) / 2));
 const Int Value::maxInt = Int(UInt(-1) / 2);
@@ -1648,20 +1648,20 @@ const Value& Path::resolve(const Value& root) const {
   for (const auto& arg : args_) {
     if (arg.kind_ == PathArgument::kindIndex) {
       if (!node->isArray() || !node->isValidIndex(arg.index_)) {
-        // Error: unable to resolve path (array value expected at position...
-        return Value::null;
+        // Error: unable to resolve path (array value expected at position... )
+        return Value::nullSingleton();
       }
       node = &((*node)[arg.index_]);
     } else if (arg.kind_ == PathArgument::kindKey) {
       if (!node->isObject()) {
         // Error: unable to resolve path (object value expected at position...)
-        return Value::null;
+        return Value::nullSingleton();
       }
       node = &((*node)[arg.key_]);
       if (node == &Value::nullSingleton()) {
         // Error: unable to resolve path (object has no member named '' at
         // position...)
-        return Value::null;
+        return Value::nullSingleton();
       }
     }
   }