There is CardDAV sync built in to iOS Contacts, but I don't think the person who programmed it actually tried using it. When you create a contact on the iPhone, there's no reasonable UI for putting it in a group (it can be done, but it's ridiculously hard); and only contacts in groups are synced via CardDAV. So in essence you can't create contacts on the phone.
leokennis|4 years ago
I can very easily, using only built in apps and set up, share a calendar, a todo list, a photo album, a music playlist, a note, an e-mail account etc. with my wife.
But I cannot share a contact list with her without resorting to complicated CardDAV setups.