SV *buildA(A& a)
{
  SV *tmpSv, a_sv;
  I32 count;
  int i;
  dSP;
  ENTER;
  SAVETMPS;
  PUSHMARK;
  tmpSv = sv_2mortal(newSVpv("A", 0));
  XPUSHs(tmpSv);
  PUTBACK;
  count = call_method("new", G_SCALAR);
  // A::new {
  //   my $proto = shift;
  //   my $class = ref($proto) || $proto;
  //   my $this = { };
  //   bless($this, $class);
  //   $this->{B} = [ ];
  //   return $this;
  // }
  SPAGAIN;
  assert(1 == count);
  tmpSv = POPs;
  a_sv = newSVsv(tmpSv); // or use NEWSV/SvSetMagicSV
  PUTBACK;
  FREETMPS;
  LEAVE;

  std::vector<SV*> b_svs;
  for (i = 0; i < a.b_count; ++i) {
    tmpSv = buildB(a.b[i]);
    b_svs.push_back(tmpSv);
  }
  addBs(a_sv, b_svs);
  return a_sv;
}