On Sat, Jun 21, 2025 at 6:15 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.

OK: *how* would it be useful? Why would you want a class to be a
friend just because that class has a default constructor, or is a base
class of something? Where would this stuff actually be useful?

Also, what is `T` in the above example? Was there supposed to be a
template header somewhere?