I think we need more development in this area. I certainly have use to make some interfaces public to some things but not others, without making everything public.
And better observability without compromise would also help in things like unit tests.
Making public to things that meet certain requirements is an interesting idea.

________________________________
From: Std-Discussion <std-discussion-bounces_at_[hidden]> on behalf of SD SH via Std-Discussion <std-discussion_at_[hidden]>
Sent: Saturday, June 21, 2025 12:15:18 PM
To: std-discussion_at_[hidden] <std-discussion_at_[hidden]>
Cc: SD SH <Z5515zwy_at_[hidden]>
Subject: [std-discussion] A Idea: Friend with Conditions

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