Add type_trait for IDL types, so as to get PubSubType from idl types.

[wjswjq]

Now it is easy to get idl type from PubSubType by calling PubSubType::type. I suggest to add some method like type_trait so that we can get PubSubType from idl types. This might simplify the usage as the user does not need to know about PubSubType any more.

• common header

template <typename MessageT>
struct type_traits {
  static ::eprosima::fastdds::dds::TopicDataType* GetPubSubType() {
    return nullptr;
  }
};

• codes generated by fastddsgen

template <>
struct type_traits<test> {
  static ::eprosima::fastdds::dds::TopicDataType* GetPubSubType() {
    return new testPubsubTypes();
  }
};

[Mario-DL]

Hi @wjswjq

Thanks for the suggestion ! While it could be a good idea to isolate the <idltype>PubSubType() from the user, I can come with some debatable points:

• Method's name: since Get..() (for me) its declaring the intention of retrieving some internal object, not creating a new one every time.
• Given the case, it could be arguable if constructor for the PubSubType() should be hidden so that creation is managed from only one place, which I don`t know if it would be standard-compliant right now.

[wjswjq]

• the method name could be discussed.
• Later maybe the TypeSupport class can be constructed by idl type， rather than PubSubType？

[Mario-DL]

I don't think that the TypeSupport should be constructed directly from the idl_type but from a TopicDataType since (among other things) the implementation for methods like serialize() or deserialize() for the eProsima's types (SerializedPayload_t) must be provided.
The idea is that the user-defined types should be kept agnostic of the vendor-specific types which is the reasoning for using an intermediate type to accommodate both (...PusSubTypes()))

[wjswjq]

And maybe method name could be something like NewPubSubType() ?

• common header

template <typename MessageT>
struct type_traits {
  static ::eprosima::fastdds::dds::TopicDataType* NewPubSubType() {
    return nullptr;
  }
};

• codes generated by fastddsgen

template <>
struct type_traits<test> {
  static ::eprosima::fastdds::dds::TopicDataType* NewPubSubType() {
    return new testPubsubTypes();
  }
};