Alternative to friend functions?

[Adnan]

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;
}