Making a member public exposes it to all callers, while inheritable
friendship grants access to only a specified class and its derived classes.
The purpose is to preserve encapsulation while allowing an
implementation hierarchy to access internals without requiring the granting
class to enumerate every derived class.

On Thu, Oct 22, 2026 at 8:53 PM Bo Persson via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> 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.
>
>
>
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