Alternative to friend functions?

[Simen Kjærås]

On Tuesday, 18 February 2020 at 12:43:22 UTC, Adnan wrote:
> class Wife(uint N) : Female {
>     FemaleID engagedTo = -1;
>     const MaleID[N] preferences;
>
>     this(MaleID[N] preferences) {
>         this.preferences = preferences;
>     }
> }
>
> void engage(N)(ref Wife!N, wife, ref Husband!N husband) {
>     // Here, I want to access both husband and wife's engaged_to
> }

Petar's answer covers your question, so I won't elaborate on 
that, but I'd like to point out that as Wife and Husband are 
classes, you probably don't intend to take them by ref - classes 
are always by ref in D, so you're effectively passing a reference 
to a reference to a class in `engage`.

Basically:

class Foo {
     int n;
}

void fun(Foo f) {
     f.n = 3;
     // Local copy of the reference - does not modify other 
references.
     f = null;
}

void gun(ref Foo f) {
     f = null;
}

unittest {
     Foo f = new Foo();
     Foo g = f;
     f.n = 17;
     // f and g point to the same object:
     assert(f.n == 17);
     assert(g.n == 17);

     fun(f);
     // fun() changed the object that both f and g point to:
     assert(f.n == 3);
     assert(g.n == 3);

     gun(f);
     // gun() changed f to no longer point at the same object, but 
left g untouched:
     assert(f is null);
     assert(g !is null);
     assert(g.n == 3);
}

--
   Simen