# C++ concepts: OutputIterator

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An `OutputIterator` is an `Iterator` that can write to the pointed-to element.

An example of a type that implements `OutputIterator` is std::ostream_iterator.

When `ForwardIterator`, `BidirectionalIterator`, or `RandomAccessIterator` satisfies the `OutputIterator` requirements in addition to its own requirements, it is described as mutable.

### Requirements

The type `X` satisfies `OutputIterator` if

- The type `X` satisfies `Iterator`
- `X` is a class type or a pointer type

And, given

- `o`, a value of some type that is writable to the output iterator (there may be multiple types that are writable, e.g. if `operator=` may be a template. There is no notion of `value_type` as for the input iterators)
- `r`, an lvalue of type `X`,

The following expressions must be valid and have their specified effects

Expression | Return | Equivalent expression | Pre-condition | Post-conditions | Notes
--- | --- | --- | --- | --- | ---
*r = o | (not used) |  | `r` is dereferencable | `r` is incrementable | After this operation `r` is not required to be dereferenceable and any copies of the previous value of `r` are no longer required to be dereferenceable or incrementable.
++r | X& |  | `r` is incrementable | &r == &++r, `r` is dereferencable or past-the-end | After this operation `r` is not required to be incrementable and any copies of the previous value of `r` are no longer required to be dereferenceable or incrementable.
r++ | convertible to const X& | X temp = r; ++r; return temp; |  |  |
*r++ = o | (not used) | *r = o; ++r; |  |  |

### Notes

The only valid use of `operator*` with an output iterator is on the left of an assignment: `operator*` may return a proxy object, which defines a member `operator=` (which may be a template)

Equality and inequality may not be defined for output iterators. Even if an `operator==` is defined, `x == y` need not imply `++x == ++y`.

Assignment through the same value of an output iterator happens only once: algorithms on output iterators must be single-pass algorithms.

Assignment through an output iterator is expected to alternate with incrementing. Double-increment is undefined behavior (C++ standard currently claims that double increment is supported, contrary to the STL documentation; this is LWG #2035)

Pure output-only iterator is allowed to declare its `iterator_traits<X>::value_type`, `iterator_traits<X>::difference_type`, `iterator_traits<X>::pointer`, and `iterator_traits<X>::reference` to be void (and iterators such as std::back_insert_iterator do just that).

### Standard library

The following standard library iterators are output iterators that are not forward iterators:

- std::ostream_iterator
- std::ostreambuf_iterator
- std::insert_iterator
- std::back_insert_iterator
- std::front_insert_iterator

### See also

- Iterator library

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