# C++ concepts: InputIterator

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< cpp‎ | concept

An `InputIterator` is an `Iterator` that can read from the pointed-to element. `InputIterator`s only guarantee validity for single pass algorithms: once an `InputIterator` i has been incremented, all copies of its previous value may be invalidated.

### Requirements

The type `It` satisfies `InputIterator` if

- The type `It` satisfies `Iterator`
- The type `It` satisfies `EqualityComparable`

And, given

- `i` and `j`, values of type It or const It
- `reference`, the type denoted by std::iterator_traits<It>::reference
- `value_type`, the type denoted by std::iterator_traits<It>::value_type

The following expressions must be valid and have their specified effects

Expression | Return | Equivalent expression | Notes
--- | --- | --- | ---
i != j | contextually convertible to bool | !(i == j) | Precondition: (i, j) is in the domain of ==.
*i | reference, convertible to value_type | If i == j and (i, j) is in the domain of == then this is equivalent to *j. | Precondition: i is dereferenceable. The expression (void)*i, *i is equivalent to *i.
i->m |  | (*i).m | Precondition: i is dereferenceable.
++i | It& |  | Precondition: i is dereferenceable. Postcondition: i is dereferenceable or i is past-the-end. Postcondition: Any copies of the previous value of i are no longer required to be either dereferenceable or to be in the domain of ==.
(void)i++ |  | (void)++i |
*i++ | convertible to value_type | value_type x = *i; ++i; return x; |

### Notes

"in the domain of `==`" means equality comparison is defined between the two iterator values. For input iterators, equality comparison does not need to be defined for all values, and the set of the values in the domain of `==` may change over time.

The reference type for an input iterator that is not also a `ForwardIterator` does not have to be a reference type: dereferencing an input iterator may return a proxy object or value_type itself by value (as in the case of `std::istreambuf_iterator`).

### See also

- Iterator library

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