Alternative to friend functions?

[Adnan via Digitalmars-d-learn]

What is the alternative to C++'s friend functions in D?

module stable_matching;

alias FemaleID = int;
alias MaleID = int;

class Person {
    string name;
    int id;
}

class Male : Person {
    this(string name = "Unnamed Male") {
        static int nextID = 0;
        this.id = nextID++;
        this.name = name;
    }
}

class Female : Person {
    this(string name = "Unnamed Female") {
        static int nextID = 0;
        this.id = nextID++;
        this.name = name;
    }
}

class Husband(uint N) : Male {
    FemaleID engagedTo = -1;
    const FemaleID[N] preferences;

    this(FemaleID[N] preferences) {
        this.preferences = preferences;
    }
}

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
}

class MatchPool(uint N) {
    Husband!N[N] husbands;
    Wife!N[N] wives;
}