Within a C++ class declaration, you can include the declaration of friend subprograms or classes, which are subprograms or classes that have full access to private data and operations of the class.
I’ve known of and used C++ for long enough but this is definitely the first time I’m hearing of such constructs as “friends“.
This only goes to show that I have a lot to learn about a lot of things. Which kinda reminds me of that quote I read in a notebook:
The more I study, the more I realize how little I know.
From Understanding Programming Languages by M. Ben-Ari