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DescriptionAn Input Iterator is an iterator that may be dereferenced to refer to some object, and that may be incremented to obtain the next iterator in a sequence. Input Iterators are not required to be mutable.
Refinement ofTrivial iterator.
DefinitionsAn iterator is past-the-end if it points beyond the last element of a container. Past-the-end values are nonsingular and nondereferenceable.
An iterator is valid if it is dereferenceable or past-the-end.
An iterator i is incrementable if there is a "next" iterator, that is, if ++i is well-defined. Past-the-end iterators are not incrementable.
An Input Iterator j is reachable from an Input Iterator i if, after applying operator++ to i a finite number of times, i == j. 
The notation [i,j) refers to a range of iterators beginning with i and up to but not including j.
The range [i,j) is a valid range if both i and j are valid iterators, and j is reachable from i .
Valid expressionsIn addition to the expressions defined in Trivial Iterator, the following expressions must be valid.
Complexity guaranteesAll operations are amortized constant time.
 i == j does not imply ++i == ++j.
 Every iterator in a valid range [i, j) is dereferenceable, and j is either dereferenceable or past-the-end. The fact that every iterator in the range is dereferenceable follows from the fact that incrementable iterators must be deferenceable.
 After executing ++i, it is not required that copies of the old value of i be dereferenceable or that they be in the domain of operator==.
 It is not guaranteed that it is possible to pass through the same input iterator twice.
See alsoOutput Iterator, Iterator overview
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