Serialization: Correctly encode structs with no fields

Empty structs were being serialized as `null`, when they should have been serialized as `{}`.

This was due to the type inference on the serializer - where no calls to `field()` would result in the default `null` type.

To solve this, the `serialize(const void* object, const std::initializer_list<Field>&)` inline helper has been promoted to a virtual function (and renamed to `fields()`).
The JSON serializer implementation of this now first sets the object type to `object`, even if there are no fields to serialize.

Added test for this.

Fixes: #10

src/json_serializer.cpp

@@ -224,6 +224,19 @@ bool Serializer::array(size_t count,
   return true;
 }
 
+bool Serializer::fields(const void* object,
+                        const std::initializer_list<Field>& fields) {
+  *json = nlohmann::json({}, false, nlohmann::json::value_t::object);
+  for (auto const& f : fields) {
+    if (!field(f.name, [&](dap::Serializer* d) {
+          auto ptr = reinterpret_cast<const uint8_t*>(object) + f.offset;
+          return f.type->serialize(d, ptr);
+        }))
+      return false;
+  }
+  return true;
+}
+
 bool Serializer::field(const std::string& name,
                        const std::function<bool(dap::Serializer*)>& cb) {
   Serializer s(&(*json)[name]);

src/json_serializer.h

@@ -98,6 +98,8 @@ struct Serializer : public dap::Serializer {
   bool serialize(const any& v) override;
   bool array(size_t count,
              const std::function<bool(dap::Serializer*)>&) override;
+  bool fields(const void* object,
+              const std::initializer_list<Field>& fields) override;
   bool field(const std::string& name, const FieldSerializer&) override;
   void remove() override;
 
@@ -130,10 +132,6 @@ struct Serializer : public dap::Serializer {
     return dap::Serializer::serialize(v);
   }
 
-  inline bool serialize(const void* o, const std::initializer_list<Field>& f) {
-    return dap::Serializer::serialize(o, f);
-  }
-
   inline bool serialize(const char* v) { return dap::Serializer::serialize(v); }
 
  private:

src/json_serializer_test.cpp

@@ -56,7 +56,9 @@ DAP_STRUCT_TYPEINFO(JSONTestObject,
                     DAP_FIELD(o2, "o2"),
                     DAP_FIELD(inner, "inner"));
 
-TEST(JSONSerializer, Decode) {}
+struct JSONObjectNoFields {};
+
+DAP_STRUCT_TYPEINFO(JSONObjectNoFields, "json-object-no-fields");
 
 }  // namespace dap
 
@@ -91,3 +93,10 @@ TEST(JSONSerializer, SerializeDeserialize) {
   ASSERT_EQ(encoded.o2, decoded.o2);
   ASSERT_EQ(encoded.inner.i, decoded.inner.i);
 }
+
+TEST(JSONSerializer, SerializeObjectNoFields) {
+  dap::JSONObjectNoFields obj;
+  dap::json::Serializer s;
+  ASSERT_TRUE(s.serialize(obj));
+  ASSERT_EQ(s.dump(), "{}");
+}