Though about the cross

gistfile1.d

import std.stdio;

interface CrossDef{
  abstract int init();
  abstract int emit();
}

class RIL : CrossDef{
  int init(){ return 1; }
  int emit(){ return 2; }
}

class BC : CrossDef{
  int init(){ return 1; }
  int emit(){ return 2; }
}

class preCC : CrossDef{
  this(bool phase = true){ _phase = phase; }
  int init(){ if(_phase){ return 1; }else{ return 2;} }
  int emit(){ return 2; }
  
  bool _phase;
}

class Cross(T : CrossDef) : CrossDef{
  this(T clazz){ _clazz = clazz; }
  
  int init(){ return _clazz.init(); }
  int emit(){ return _clazz.emit(); }
  T _clazz;
}

CrossDef create_cross(string type){
  switch(type){
    case "BC": return(new Cross!(BC)(new BC()));
    case "RIL": return(new Cross!(RIL)(new RIL()));
    case "preCC_PK": return(new Cross!(preCC)(new preCC()));
    case "preCC_PU": return(new Cross!(preCC)(new preCC(false)));
  }
}

int hmm_fun(CrossDef c){
  //Now we can call any function CrossDef has defined, 
  //and the compiler forces any implementation to be valid
  c.init();
}

void main(){
    CrossDef backcross = create_cross("BC");
    CrossDef rilcross = create_cross("RIL");
}