Allow unknown described class members with a parse option

[kiwixz]

Hi, I'd like to make the parsing of my JSON config file more or less compatible across versions.

If a new field gets added I can easily add it if not present in the json::value before parsing it to make sure value_to has anything it needs. But a parse option to default construct in this case instead of erroring would not hurt.

In a more complex case, if the config gets updated before the application parsing it value_to will complain because of size mismatch (see here). I'd like to disable this check with a parse option, because it's much harder to iterate all the described classes to make sure there are no extra properties in the JSON.

[grisumbras]

So, you are suggesting a runtime option, not another type trait?

[kiwixz]

I think this makes more sense as a runtime option, it's not that the type is special but rather that I don't care when parsing it. Like allow_comments and friends.

[grisumbras]

Do I understand it correctly, that you want to both allow missing fields, and allow extra fields? Or just the latter?

[grisumbras]

To allow missing fields our standard solution is to use optional. Would this work for you?

[kiwixz]

Allowing extra fields is more important to me, because its much more complicated to do beforehand and doesn't raise any questions on boost::json side.

Allowing missing fields could be nice, but then you have to decide how to set them (letting them default-constructed seems an obvious solution).

I already use optionals for null values, I didn't know it worked with missing values too!