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std::contiguous_iterator - cppreference.com

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

Given an iterator i of a type that models contiguous_iterator, a sentinel s and a non-negative integer n:

For any range [i, s), standard library functions can replace it with [std::to_address(i), std::to_address(i + ranges::distance(i, s))).
For any range i + [â0â, n), standard library functions can replace it with std::to_address(i) + [â0â, std::to_address(i + n)).

This means a program cannot rely on any side effects of dereferencing, incrementing or decrementing a contiguous iterator i, because standard library functions might operate on pointers obtained by std::to_address(i) instead of operating on i directly.

(since C++26) [edit] Iterator concept determination

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

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

If ITER_TRAITS::iterator_concept is valid and names a type, /*ITER_CONCEPT*/ denotes the type.
Otherwise, if ITER_TRAITS::iterator_category is valid and names a type, /*ITER_CONCEPT*/ denotes the type.
Otherwise, if std::iterator_traits is generated from the primary template, /*ITER_CONCEPT*/ denotes std::random_access_iterator_tag.
(That is, std::derived_from</*ITER_CONCEPT*/, std::contiguous_iterator_tag> is assumed to be false.)
Otherwise, /*ITER_CONCEPT*/ does not denote a type and results in a substitution failure.

[edit] 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 all following conditions are satisfied:

std::to_address(a) == std::addressof(*a).
std::to_address(b) == std::to_address(a) + std::iter_difference_t(b - a).
std::to_address(c) == std::to_address(a) + std::iter_difference_t(c - a).
std::to_address(I{}) is well-defined.
ranges::iter_move(a) has the same type, value category, and effects as std::move(*a).
If ranges::iter_swap(a, b) is well-formed, it has effects equivalent to ranges::swap(*a, *b).

[edit] Equality preservation

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

[edit] Implicit expression variations

A requires expression that uses an expression that is non-modifying for some constant lvalue operand also requires implicit expression variations.

[edit] 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.

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior LWG 3607 C++20 contiguous_iterator could have custom
ranges::iter_move and ranges::iter_swap behaviors prohibited LWG 4170 C++20 a pair of value-initialized contiguous_iterators
might not be able to represent an empty range guaranteed [edit] See also

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