Why are iostream sentry objects called "cerberos" in the standard?

Question

I was wondering what __cerb means in the libstdc++ of g++, and I found out that this seems to be a short name for cerberos as named in the official C++ standard document.

§ 22.3.1 / 3:

[ Example: An iostream operator<< might be implemented as:

template <class charT, class traits>
basic_ostream<charT,traits>&
operator<< (basic_ostream<charT,traits>& s, Date d) {
                                                            //      !!!!!!!!!!
  typename basic_ostream<charT,traits>::sentry cerberos(s); // <--- !! HERE !!
                                                            //      !!!!!!!!!!
  if (cerberos) {
    ios_base::iostate err = ios_base::iostate::goodbit;
    tm tmbuf; d.extract(tmbuf);
    use_facet<time_put<charT,ostreambuf_iterator<charT,traits> > >(
      s.getloc()).put(s, s, s.fill(), err, &tmbuf, ’x’);
    s.setstate(err);            // might throw
  }
  return s;
}

— end example ]

Why are iostream sentry objects called cerberos, and what does cerberos actually mean?

Answer 1

Cerberus was the three headed dog¹ who guarded the gates of hell, keeping those trapped within from crossing back over the river Styx. Whomever wrote that section fancied Greek and/or Roman mythology and decided it would be an applicable name for a local variable sentry.

As to why it ends in os, one can imagine three scenarios:

1. They completely forgot the rules for Greek and Latin noun declinations and thought that was how it is spelled
2. I completely forgot the rules for Greek and Latin noun declinations and am pointing out a non-issue.
3. They conflated the spellings of Cerberus and Kerberos, giving birth to Cerberos the basic_stream sentry of Greek and Roman descent.

_{1. Apparently the number of heads on Cerberus is variable. Perhaps this is better expressed as the "N-headed dog, where N is greater than or equal to 1."}