Use a Myers Singleton for null

Avoid some static initialization problems.

From @marklakata
See #488

include/json/value.h

@@ -190,6 +190,8 @@ public:
 
   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.
+
   /// Minimum signed integer value that can be stored in a Json::Value.
   static const LargestInt minLargestInt;
   /// Maximum signed integer value that can be stored in a Json::Value.

src/lib_json/json_value.cpp

@@ -31,10 +31,22 @@ namespace Json {
 #else
 #define ALIGNAS(byte_alignment)
 #endif
-static const unsigned char ALIGNAS(8) kNull[sizeof(Value)] = { 0 };
+//static const unsigned char ALIGNAS(8) kNull[sizeof(Value)] = { 0 };
-const unsigned char& kNullRef = kNull[0];
+//const unsigned char& kNullRef = kNull[0];
-const Value& Value::null = reinterpret_cast<const Value&>(kNullRef);
+//const Value& Value::null = reinterpret_cast<const Value&>(kNullRef);
-const Value& Value::nullRef = null;
+//const Value& Value::nullRef = null;
+
+// static
+Value const& Value::nullSingleton()
+{
+ static Value const* nullStatic = new Value;
+ return *nullStatic;
+}
+
+// for backwards compatibility, we'll leave these global references around, but DO NOT
+// use them in JSONCPP library code any more!
+Value const& Value::null = Value::nullSingleton();
+Value const& Value::nullRef = Value::nullSingleton();
 
 const Int Value::minInt = Int(~(UInt(-1) / 2));
 const Int Value::maxInt = Int(UInt(-1) / 2);
@@ -972,7 +984,7 @@ Value& Value::operator[](ArrayIndex index) {
   if (it != value_.map_->end() && (*it).first == key)
     return (*it).second;
 
-  ObjectValues::value_type defaultValue(key, nullRef);
+  ObjectValues::value_type defaultValue(key, nullSingleton());
   it = value_.map_->insert(it, defaultValue);
   return (*it).second;
 }
@@ -989,11 +1001,11 @@ const Value& Value::operator[](ArrayIndex index) const {
       type_ == nullValue || type_ == arrayValue,
       "in Json::Value::operator[](ArrayIndex)const: requires arrayValue");
   if (type_ == nullValue)
-    return nullRef;
+    return nullSingleton();
   CZString key(index);
   ObjectValues::const_iterator it = value_.map_->find(key);
   if (it == value_.map_->end())
-    return nullRef;
+    return nullSingleton();
   return (*it).second;
 }
 
@@ -1027,7 +1039,7 @@ Value& Value::resolveReference(const char* key) {
   if (it != value_.map_->end() && (*it).first == actualKey)
     return (*it).second;
 
-  ObjectValues::value_type defaultValue(actualKey, nullRef);
+  ObjectValues::value_type defaultValue(actualKey, nullSingleton());
   it = value_.map_->insert(it, defaultValue);
   Value& value = (*it).second;
   return value;
@@ -1047,7 +1059,7 @@ Value& Value::resolveReference(char const* key, char const* cend)
   if (it != value_.map_->end() && (*it).first == actualKey)
     return (*it).second;
 
-  ObjectValues::value_type defaultValue(actualKey, nullRef);
+  ObjectValues::value_type defaultValue(actualKey, nullSingleton());
   it = value_.map_->insert(it, defaultValue);
   Value& value = (*it).second;
   return value;
@@ -1055,7 +1067,7 @@ Value& Value::resolveReference(char const* key, char const* cend)
 
 Value Value::get(ArrayIndex index, const Value& defaultValue) const {
   const Value* value = &((*this)[index]);
-  return value == &nullRef ? defaultValue : *value;
+  return value == &nullSingleton() ? defaultValue : *value;
 }
 
 bool Value::isValidIndex(ArrayIndex index) const { return index < size(); }
@@ -1074,13 +1086,13 @@ Value const* Value::find(char const* key, char const* cend) const
 const Value& Value::operator[](const char* key) const
 {
   Value const* found = find(key, key + strlen(key));
-  if (!found) return nullRef;
+  if (!found) return nullSingleton();
   return *found;
 }
 Value const& Value::operator[](JSONCPP_STRING const& key) const
 {
   Value const* found = find(key.data(), key.data() + key.length());
-  if (!found) return nullRef;
+  if (!found) return nullSingleton();
   return *found;
 }
 
@@ -1103,7 +1115,7 @@ Value& Value::operator[](const CppTL::ConstString& key) {
 Value const& Value::operator[](CppTL::ConstString const& key) const
 {
   Value const* found = find(key.c_str(), key.end_c_str());
-  if (!found) return nullRef;
+  if (!found) return nullSingleton();
   return *found;
 }
 #endif
@@ -1151,7 +1163,7 @@ Value Value::removeMember(const char* key)
   JSON_ASSERT_MESSAGE(type_ == nullValue || type_ == objectValue,
                       "in Json::Value::removeMember(): requires objectValue");
   if (type_ == nullValue)
-    return nullRef;
+    return nullSingleton();
 
   Value removed;  // null
   removeMember(key, key + strlen(key), &removed);
@@ -1538,7 +1550,7 @@ const Value& Path::resolve(const Value& root) const {
         // Error: unable to resolve path (object value expected at position...)
       }
       node = &((*node)[arg.key_]);
-      if (node == &Value::nullRef) {
+      if (node == &Value::nullSingleton()) {
         // Error: unable to resolve path (object has no member named '' at
         // position...)
       }
@@ -1559,7 +1571,7 @@ Value Path::resolve(const Value& root, const Value& defaultValue) const {
       if (!node->isObject())
         return defaultValue;
       node = &((*node)[arg.key_]);
-      if (node == &Value::nullRef)
+      if (node == &Value::nullSingleton())
         return defaultValue;
     }
   }