-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On Friday 14 May 2010, Boris Schaeling wrote:
A reader of my book noticed that the sample program at http://en.highscore.de/cpp/boost/src/4.2.8/main.cpp which uses a combiner to return the smallest value of all slot values is buggy. The combiner's implementation is rather simple as iterators are simply forwarded to std::min_element():
min_element requires a forward iterator. The slot iterators are only input iterators.
Then I was trying to understand what's special about the iterators. Please look at the following code and guess whether the values printed will be different or not:
T operator()(InputIterator first, InputIterator last) const { InputIterator temp = first; ++first; std::cout << *first << std::endl; std::cout << *temp << std::endl; }
If you think you get two different values then you'll be as surprised as I am: Both iterators temp and first return the second value in the range [first, last).
temp no longer has to be valid after first is incremented (see definition of input iterator). -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.9 (GNU/Linux) iEYEARECAAYFAkvtUZcACgkQ5vihyNWuA4VAlgCeI5+8kRxe0YOj0m9Pu1tBMgBJ BvgAnApE7eG+SDyD0mZ9Ucca6oIvU9Zg =0lZi -----END PGP SIGNATURE-----