On 2026-10-22 at 21:20, Theoden Cook via Std-Proposals wrote:
> Hello,
>
> 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; private: void
> internal_operation() {} }; class B {}; class C : public B { public: void
> run(A& a) { a.internal_operation(); // Would be permitted } };|
>
> ```
>
> The [[inheritable]] syntax is illustrative. I'm open to alternatives :).
>
> The intended semantics would be that explicitly granting inheritable
> friendship to B also grants the same access privileges to classes
> derived from B, potentially across multiple levels of inheritance.
> Ordinary friendship would retain its current behavior unless explicitly
> marked as inheritable.
>
> The motivation is to make friendship compose more naturally with
> inheritance, without requiring the granting class to explicitly friend
> every derived class, or expose additional public/protected interface
> solely to accommodate those classes.
>
How is this different from making the member public?

class unrelated : B
{ };

will get access, just like if internal_operation() were no longer private.