Feature Request: Add support for serializing comments

[Lazrius]

Good evening.

I know such things have been discussed before, but I couldn't find in the issue a history a consensus on the idea of supporting comments. The main use case I see for this is large generated config files, where users might not be sure what various settings do.

While I am not entirely familiar with most of the formats reflect-cpp supports, I know from personal experience that both pugixml and yaml-cpp, which I understand this library uses, have support for both reading and writing of comments to a document. For this reason I see a lof of benefit if we had some kind of rfl::Comment processor that we could add to an rfl::Field to cause it to emit a comment (if the serialization format supports it, that is).

I don't know the support among other types, so I am unsure on the general feasibility, but I still think it would be quite helpful.
Thank you for your time and great work.

[liuzicheng1987]

@Lazrius it has come up before, but so far we haven't prioritised it, because it only affects two formats.

But basically my idea would be this: We could implement an rfl::Comment wrapper around std::string.

XML and YAML would serialise this as comments, all other formats would treat it like normal strings.

What are your thoughts?

[Lazrius]

I think that is a great idea. I would maybe add a prefix or something to non-XML/non-YAML like __comment or the like just to be very explicit about it, but otherwise I think that is very much the path of least resistance.

If this is something you need help with I should have some time tomorrow to take a swing at it, but if so I would like some directions of where to get started. It's been a minute since I looked at the code base and have forgotten how to add types in the year since I last tried 😄.

The only thing I am potentially concerned about, do you know how we can guarantee comments go in 'the right place', or is that just handled by how reflect cpp works? (I can't even think where to start with supporting yaml trailing comments)

In your mind, would the following make sense:

struct Data
{
    rfl::Comment timeDelayComment = "This is the time delay in milliseconds";
    int timeDelay = 100;
}

<Data>
    <!-- This is the time delay in Milliseconds -->
    <timeDelay>100</timeDelay>
</Data>