C++ Input Iterator -

i'm reading c++ standard (section input iterator) , i'm having hard time visualize text in bold:

note: input iterators, == b not imply ++a == ++b. (equality not guarantee substitution property or referential transparency.) algorithms on input iterators should never attempt pass through same iterator twice. should single pass algorithms.

i understand input iterators single pass, can't seem visualize in c++ code. can please show me in example on how make iterator become single pass?

suppose want read integers standard input. 1 way using

#include <iterator> #include <iostream> #include <vector>  using namespace std;  const vector<int> v{istream_iterator<int>{cin}, istream_iterator<int>{}}; 

this read integers v (see istream_iterator).

in case, makes lot of sense iterators single pass, since range created on fly, user types in things, , each element consumed , disappeared. can't reiterate range.

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other that, not sure mean

how make iterator become single pass?

• if mean "how signify iterator class you're writing has restriction?", set iterator_category input_iterator_tag

  struct my_iterator {     using iterator_category = input_iterator_tag; }; 

• if want query (at compile time) whether iterator class has restriction, use iterator_traits.