[RFC] A comment in string::_S_construct(input_iterator)

[Paolo Carlini]

Hi,

there is a long standing comment in the code special cased for
Input Iterators that I have trouble understanding completely:

  // NB: this loop looks precisely this way because
  // it avoids comparing __beg != __end any more
  // than strictly necessary; != might be expensive!

Of course, in the loop __beg == __end appears.

So, sorry for my naive question: when Input Iterators are
involved is != in any non-trivial way different from ==?

Or the comment is really about _comparing_ Input Iterators?

In that case, the chosen loop is really able to reduce the
number of comparisons? Which are the (so bad) alternatives?

Thanks for any help,
Paolo.