Re: A most vexing issues with the "search" algorithm and the "istream_iterator"
On Monday, 9 February 2015 14:40:11 UTC, Michael wrote:
{ edited by mod to shorten lines to ~70 characters. -mod }
<snip>
I found this unexpected behaviour particular vexing until I discovered
how search was behaving using stream iterators --it works as expected
with normal std container iterators.
Something for others to be wary of when using stream iterators with std
algorithms...
<snip>
##########################################################################
LISTING 1:
##########################################################################
int main(int argc, char **argv)
{
std::ifstream is("test.binary", std::ifstream::binary);
const auto SYNC = "PKT:";
std::istream_iterator<char> eos;
std::istream_iterator<char> sos(is);
auto sf = std::search(sos, eos, SYNC, SYNC+4);
for (auto i = 0u; i < 30u; i++)
std::cout << *++sf;
std::cout << std::endl;
return EXIT_SUCCESS;
}
<snip>
Your problem is that std::search() requires Forward Iterators
as its parameters. istream_iterators are only Input Iterators
and therefore do not meet these requirements. Failing to meet
the documented requirements probably invokes Undefined Behaviour.
Forward Iterators have the useful quality of being copyable and,
when one copy has been advanced so many times, the other copy
can also be advanced to the point where they compare equal again.
Input Iterators do not support this, but I suspect that algorithms
such as std::search() may rely on this behaviour.
Also, Input Iterators often cache part of the underlying data
stream (commonly, a one-element lookahead) in order to detect
end-of-sequence without having yet been incremented. This means
that you cannot predict the state of the underlying data source
(e.g. your std::ifstream object) after having created iterators
over it and handed them over to some algorithm that will manipulate
them.
HTH,
Tony.
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