Feature Request: Optional Attributes

[Ryanf55]

DDS supports the optional attribute in IDL 4.2 specification attribute section 8.3.1.3. In C++, now that we have C++17, we can use std::optional.

A prior dicussion is here: https://discourse.ros.org/t/optional-fields-in-message/991/16

Consider adding a @optional specifier to rosidl that translates to the DDS optional .

Amend the ROS 2 design documentation for Standardized Annotations to add optional:
https://design.ros2.org/articles/idl_interface_definition.html

It's supported by:

• FastDDS
• RTI
• CycloneDDS
• Zenoh

Workarounds: Bounded array with a max length of 1, but the syntax is funky.

[alsora]

Hi, we discussed this during the ROS waffle meeting and while we agree that this would be a useful feature, we also recognize that it will require a relatively large amount of work.
It would be great to get some support from the community.

[Ryanf55]

Thanks. If I end up getting time allocated at work, I'll request being assigned, but otherwise anyone is welcome.

Would you accept a merge with opt-in support of @optional if I got it working only with the following:

• FastDDS
• rosidl_generator_cpp

We want this feature to integrate with a non-ROS-based software. The only incompatibility is the use of optional.
Getting this working on all RMW's and all languages is unlikely to be approved.