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To my knowledge, the hierarchy of iterator categories goes like this: Random access -> Bi-directional -> Forward -> Input -> Output Correct? I always thought there was a rule, that if an algorithm expects a particular type of iterator, you can provide iterators of categories up the chain, but not down. So I was reading this answer , where ildjarn suggests suggested (then later corrected himself) using std::ifstream with std::istream_iterator and std::search to find data in a in a file. I was about to comment that you can't do that, because search expects Forward iterators, and istream_iterator is an Input iterator. But just to make sure, I tried this: std::istringstream iss("Elephant hats for sale."); std::istream_iterator<char> begin(iss), end; std::string sub("hat"); auto i = std::search(begin, end, sub.begin(), sub.end()); I didn't expect it to compile, but it did. However, the results seem to be useless because if I follow it with this: while(i != end) { std::cout << *i; ++i; } There is no output. So, my question is this: Is my compiler in error for allowing my call to search using istream_iterator ? Or are there no rules preventing this sort of thing?

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