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

include/dap/serialization.h

@@ -166,6 +166,10 @@ class Serializer {
   // Serializer that should be used to encode the n'th array element's data.
   virtual bool array(size_t count, const std::function<bool(Serializer*)>&) = 0;
 
+  // fields() encodes all the provided fields of the given object.
+  virtual bool fields(const void* object,
+                      const std::initializer_list<Field>&) = 0;
+
   // field() encodes a field to the struct object referenced by this Serializer.
   // The FieldSerializer will be called with a Serializer used to encode the
   // field's data.
@@ -192,10 +196,6 @@ class Serializer {
   template <typename T0, typename... Types>
   inline bool serialize(const dap::variant<T0, Types...>&);
 
-  // serialize() encodes all the provided fields of the given object.
-  inline bool serialize(const void* object,
-                        const std::initializer_list<Field>&);
-
   // deserialize() encodes the given string.
   inline bool serialize(const char* v);
 
@@ -231,18 +231,6 @@ bool Serializer::serialize(const dap::variant<T0, Types...>& var) {
   return serialize(var.value);
 }
 
-bool Serializer::serialize(const void* object,
-                           const std::initializer_list<Field>& fields) {
-  for (auto const& f : fields) {
-    if (!field(f.name, [&](Serializer* d) {
-          auto ptr = reinterpret_cast<const uint8_t*>(object) + f.offset;
-          return f.type->serialize(d, ptr);
-        }))
-      return false;
-  }
-  return true;
-}
-
 bool Serializer::serialize(const char* v) {
   return serialize(std::string(v));
 }

include/dap/typeof.h

@@ -164,7 +164,7 @@ M member_type(M T::*);
         return d->deserialize(ptr, {__VA_ARGS__});                        \
       }                                                                   \
       bool serialize(Serializer* s, const void* ptr) const override {     \
-        return s->serialize(ptr, {__VA_ARGS__});                          \
+        return s->fields(ptr, {__VA_ARGS__});                             \
       }                                                                   \
     };                                                                    \
     static TI typeinfo;                                                   \