Not in response to just this post but all trying to modify friends & access.

The idea is we should be making the language simpler, not more complex.
If you are trying to give partial access to part of your class, you have a
design problem.
Too many divergent features in one class (failed cohesiveness). Break it
up. Move it around.

In general, using 'friend' is a code smell for anti-enpasilization and a
weak design, (you left the backdoor open)
IMHO, One of the few valid uses is for hidden friends (ADL operator
overloading).


On Sat, Jun 21, 2025 at 3:15 AM SD SH via Std-Discussion <
std-discussion_at_[hidden]> wrote:

> How about adding conditions for friend?
> Like this (or other?):
>
> class B { };
> class E
> {
> int i;
> E(int i) : i(i) { }
> };
> class A
> {
> static int i = 0;
> friend typename T if(std::is_base_of<B, T>::value); // if a class
> derived from B, it is a friend class of A
> friend E if(std::is_default_constructible<E>::value); // E is not a
> friend class of A because std::is_default_constructible<E>::value == false
> };
> class C : B
> {
> public:
> int geti()
> {
> return A::i; // OK, because std::is_base_of<B, C>::value == true
> }
> };
> class D
> {
> public:
> int geti()
> {
> return A::i; // Error, because std::is_base_of<B, D>::value ==
> false
> }
> };
>
> A Probably Syntax:
> friend <type-id> if/*constexpr*/(<constexpr-bool-expression>);
>
> It may be a useful syntax feature.
>
> E. S. Himers
> 2025/06
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