# std::input_or_output_iterator

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< cpp‎ | iterator

Defined in header `<iterator>` |  |
template< class I > concept input_or_output_iterator = requires(I i) { { *i } -> /*can-reference*/; } && std::weakly_incrementable <I>; |  | (since C++20)

The `input_or_output_iterator` concept forms the basis of the iterator concept taxonomy; every iterator type satisfies the `input_or_output_iterator` requirements.

The exposition-only concept /*can-reference*/ is satisfied if and only if the type is referenceable.

### Notes

`input_or_output_iterator` itself only specifies operations for dereferencing and incrementing an iterator. Most algorithms will require additional operations, for example:

- comparing iterators with sentinels (see `sentinel_for`);
- reading values from an iterator (see `indirectly_readable` and `input_iterator`);
- writing values to an iterator (see `indirectly_writable` and `output_iterator`);
- a richer set of iterator movements (see `forward_iterator`, `bidirectional_iterator`, `random_access_iterator`).

Unlike the LegacyIterator requirements, the `input_or_output_iterator` concept does not require copyability.

*i is required to be equality-preserving, although the increment operations required by `weakly_incrementable` are not.

### Example

A minimum iterator.

    #include <cstddef>
    #include <iterator>

    struct SimpleIterator
    {
        using difference_type = std::ptrdiff_t;

        int operator*();

        SimpleIterator& operator++();
        void operator++(int) { ++*this; }
    };

    static_assert(std::input_or_output_iterator<SimpleIterator>);

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