# std::contiguous_iterator

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Defined in header `<iterator>` |  |
template<class I> concept contiguous_iterator = std::random_access_iterator<I> && std::derived_from</*ITER_CONCEPT*/<I>, std::contiguous_iterator_tag> && std::is_lvalue_reference_v<std::iter_reference_t<I>> && std::same_as< std::iter_value_t<I>, std::remove_cvref_t<std::iter_reference_t<I>> > && requires(const I& i) { { std::to_address(i) } -> std::same_as<std::add_pointer_t<std::iter_reference_t<I>>>; }; |  | (since C++20)

The `contiguous_iterator` concept refines `random_access_iterator` by providing a guarantee the denoted elements are stored contiguously in the memory.

### Iterator concept determination

Definition of this concept is specified via an exposition-only alias template /*ITER_CONCEPT*/.

In order to determine /*ITER_CONCEPT*/<I>, let ITER_TRAITS<I> denote I if the specialization std::iterator_traits<I> is generated from the primary template, or std::iterator_traits<I> otherwise:

- If ITER_TRAITS<I>::iterator_concept is valid and names a type, /*ITER_CONCEPT*/<I> denotes the type.
- Otherwise, if ITER_TRAITS<I>::iterator_category is valid and names a type, /*ITER_CONCEPT*/<I> denotes the type.
- Otherwise, if std::iterator_traits<I> is generated from the primary template, /*ITER_CONCEPT*/<I> denotes std::random_access_iterator_tag.
- Otherwise, /*ITER_CONCEPT*/<I> does not denote a type and results in a substitution failure.

### Semantic requirements

Let `a` and `b` be dereferenceable iterators and `c` be a non-dereferenceable iterator of type `I` such that `b` is reachable from `a` and `c` is reachable from `b`. The type `I` models `contiguous_iterator` only if all the concepts it subsumes are modeled and:

- std::to_address(a) == std::addressof(*a),
- std::to_address(b) == std::to_address(a) + std::iter_difference_t<I>(b - a), and
- std::to_address(c) == std::to_address(a) + std::iter_difference_t<I>(c - a).

### Equality preservation

Expressions declared in requires-expressions of the standard library concepts are required to be equality-preserving (except where stated otherwise).

### Implicit expression variations

A requires-expression that uses an expression that is non-modifying for some constant lvalue operand also implicitly requires additional variations of that expression that accept a non-constant lvalue or (possibly constant) rvalue for the given operand unless such an expression variation is explicitly required with differing semantics. These implicit expression variations must meet the same semantic requirements of the declared expression. The extent to which an implementation validates the syntax of the variations is unspecified.

### Notes

`contiguous_iterator` is modeled by every pointer type to complete object type.

Iterator types in the standard library that are required to satisfy the LegacyContiguousIterator requirements in C++17 are also required to model `contiguous_iterator` in C++20.

### See also

random_access_iterator (C++20) | specifies that a `bidirectional_iterator` is a random-access iterator, supporting advancement in constant time and subscripting (concept)

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