std::contiguous_iterator - cppreference.com (original) (raw)
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 + [0, n), standard library functions can replace it with std::to_address(i) + [0, 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) |
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Contents
- 1 Iterator concept determination
- 2 Semantic requirements
- 3 Equality preservation
- 4 Implicit expression variations
- 5 Notes
- 6 Defect reports
- 7 See also
[edit] 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.
(That is, std::derived_from</*ITER_CONCEPT*/<I>, std::contiguous_iterator_tag> is assumed to be false.) - Otherwise, /*ITER_CONCEPT*/<I> 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<I>(b - a).
- std::to_address(c) == std::to_address(a) + std::iter_difference_t<I>(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 customranges::iter_move and ranges::iter_swap behaviors | prohibited |
| LWG 4170 | C++20 | a pair of value-initialized contiguous_iteratorsmight not be able to represent an empty range | guaranteed |