[JSONSerialization] Add ability to emit 'null' value

Author: rintaro

include/swift/Basic/JSONSerialization.h

@@ -110,6 +110,30 @@ struct ScalarTraits {
 };
 
 
+/// This class should be specialized by any type that can be 'null' in JSON.
+/// For example:
+///
+///    template<>
+///    struct NullableTraits<MyType *> > {
+///      static bool isNull(MyType *&ptr) {
+///        return !ptr;
+///      }
+///      static MyType &get(MyType *&ptr) {
+///        return *ptr;
+///      }
+///    };
+template<typename T>
+struct NullableTraits {
+  // Must provide:
+  //
+  // Function to return true if the value is 'null'.
+  // static bool isNull(const T &Val);
+  //
+  // Function to return a reference to the unwrapped value.
+  // static T::value_type &get(const T &Val);
+};
+
+
 /// This class should be specialized by any type that needs to be converted
 /// to/from a JSON array.  For example:
 ///
@@ -249,6 +273,23 @@ template<typename T>
 struct has_ArrayTraits : public std::integral_constant<bool,
     has_ArrayMethodTraits<T>::value > { };
 
+// Test if NullableTraits<T> is defined on type T.
+template <class T>
+struct has_NullableTraits
+{
+  typedef bool (*Signature_isNull)(T&);
+
+  template <typename U>
+  static char test(SameType<Signature_isNull, &U::isNull> *);
+
+  template <typename U>
+  static double test(...);
+
+public:
+  static bool const value =
+  (sizeof(test<NullableTraits<T>>(nullptr)) == 1);
+};
+
 inline bool isNumber(StringRef S) {
   static const char DecChars[] = "0123456789";
   if (S.find_first_not_of(DecChars) == StringRef::npos)
@@ -285,6 +326,7 @@ struct missingTraits : public std::integral_constant<bool,
     !has_ScalarEnumerationTraits<T>::value
  && !has_ScalarBitSetTraits<T>::value
  && !has_ScalarTraits<T>::value
+ && !has_NullableTraits<T>::value
  && !has_ObjectTraits<T>::value
  && !has_ArrayTraits<T>::value> {};
 
@@ -338,6 +380,7 @@ class Output {
   void endBitSetScalar();
 
   void scalarString(StringRef &, bool);
+  void null();
 
   template <typename T>
   void enumCase(T &Val, const char* Str, const T ConstVal) {
@@ -579,6 +622,16 @@ jsonize(Output &out, T &Val, bool) {
 }
 
 
+template<typename T>
+typename std::enable_if<has_NullableTraits<T>::value,void>::type
+jsonize(Output &out, T &Obj, bool) {
+  if (NullableTraits<T>::isNull(Obj))
+    out.null();
+  else
+    jsonize(out, NullableTraits<T>::get(Obj), true);
+}
+
+
 template<typename T>
 typename std::enable_if<validatedObjectTraits<T>::value, void>::type
 jsonize(Output &out, T &Val, bool) {

lib/Basic/JSONSerialization.cpp

@@ -222,6 +222,10 @@ void Output::scalarString(StringRef &S, bool MustQuote) {
     Stream << S;
 }
 
+void Output::null() {
+  Stream << "null";
+}
+
 void Output::indent() {
   Stream.indent(StateStack.size() * 2);
 }