I have an idea for a potential C++ language feature. I'd appreciate feedback on whether it would be worth pursuing as a proposal.
The idea: opt-in inheritable friendship
Currently in C++, friendship is neither inherited nor transitive. If a class is declared as a friend of another class, its derived classes do not inherit that friendship.
I'd like to propose an opt-in mechanism that allows a class to explicitly declare a friendship relationship as inheritable.
For example, the syntax could look similar to this:
class A {
[[inheritable]] friend class B;
(1) Personally, I don't think this idea has the requisite "bang for the buck." I've never wanted such a feature myself; in fact I tend to say that the `friend` keyword in general is a code smell, except when it's used specifically for the hidden friend idiom.
(Note that despite that position, I was a coauthor/supporter on, although not the originator of,
P2893 Variadic Friends, adopted for C++26. I do like consistency in the language, even in parts you probably shouldn't use.)
(2) This definitely isn't the sort of thing you should express with an [[attribute]].
(3) There's nothing particularly special about inheritance relationships in particular. It looks to me like what you really want is more like
class A {
fantasy-template<class T> requires std::derived_from<T, B> friend T; // not a friend, but a template for stamping out friends
};
But that's the kind of syntax you want, so that you can use this same syntax to express things unrelated to inheritance, like
fantasy-template<class T> requires requires { typename T::enable_friendship; } friend T; // befriend any class type that supplies a member typedef named `enable_friendship`
and
fantasy-template<class T> friend std::vector<T*>; // befriend vector<U> when U is a pointer type, and otherwise don't
and so on.
my $.02,
Arthur