eclipse-aascw / aas-core-codegen / 35789133600

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Compose the C++ JSON de-serialization (#747)

A class found each of its properties by name, and spelled out a
procedure around every one of them.

We change the de-serialization process and introduce simpler building
boxes to make it easier to understand.

**A class walks its keys once.** A name-to-literal map and a ``switch``
replace asking, per declared property, whether it is there and then
fetching it -- two tree walks each, nlohmann's object being a
``std::map``. An unknown key falls out of the lookup.

    // before, per property, in each of the ``Deserialize{Cls}``
    if (!additional_properties) {
      for (const auto& key_val : json.items()) {
        auto it(kPropertiesInExtension.find(key_val.key()));
        if (it == kPropertiesInExtension.end()) { ...18 lines... }
      }
    }
    ...
    if (json.contains("semanticId")) {
      std::tie(
        the_semantic_id,
        error
      ) = DeserializeReference(
        json["semanticId"],
        additional_properties
      );

      if (error.has_value()) {
        error->path.segments.emplace_front(
          common::make_unique<PropertySegment>(
            L"semanticId"
          )
        );

        return std::make_pair<
          common::optional<std::shared_ptr<T> >,
          common::optional<DeserializationError>
        >(
          common::nullopt,
          std::move(error)
        );
      }
    }

    // after
    common::optional<DeserializationError> error(
      ParseProperties(
        json,
        properties::kMapOfExtension,
        additional_properties,
        [&](
          properties::OfExtension property,
          const nlohmann::json& value
        ) -> common::optional<DeserializationError> {
          switch (property) {
            case properties::OfExtension::kSemanticId:
              return ParseInto(
                the_semantic_id,
                DeserializeReference(
                  value,
                  additional_properties
                ... (continued)

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