NAME

std::random_access_iterator - std::random_access_iterator

Synopsis

Defined in header <iterator>
template<class I>

concept random_access_iterator =
std::bidirectional_iterator<I> &&
std::derived_from</*ITER_CONCEPT*/<I>,
std::random_access_iterator_tag> &&
std::totally_ordered<I> &&
std::sized_sentinel_for<I, I> &&
requires(I i, const I j, const std::iter_difference_t<I> n) { (since C++20)
{ i += n } -> std::same_as<I&>;
{ j + n } -> std::same_as<I>;
{ n + j } -> std::same_as<I>;
{ i -= n } -> std::same_as<I&>;
{ j - n } -> std::same_as<I>;
{ j[n] } -> std::same_as<std::iter_reference_t<I>>;

};

The concept random_access_iterator refines bidirectional_iterator by adding support
for constant time advancement with the +=, +, -=, and - operators, constant time
computation of distance with -, and array notation with subscripting.

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 valid iterators of type I such that b is reachable from a, and let n
be a value of type std::iter_difference_t<I> equal to b - a.
random_access_iterator<I> is modeled only if all the concepts it subsumes are
modeled and:

* (a += n) is equal to b.
* std::addressof(a += n) is equal to std::addressof(a).
* (a + n) is equal to (a += n).
* (a + n) is equal to (n + a).
* For any two positive integers x and y, if a + (x + y) is valid, then a + (x + y)
is equal to (a + x) + y.
* a + 0 is equal to a.
* If (a + (n - 1)) is valid, then --b is equal to (a + (n - 1)).
* (b += -n) and (b -= n) are both equal to a.
* std::addressof(b -= n) is equal to std::addressof(b).
* (b - n) is equal to (b -= n).
* If b is dereferenceable, then a[n] is valid and is equal to *b.
* bool(a <= b) is true.
* Every required operation has constant time complexity.

Equality preservation

An expression is equality preserving if it results in equal outputs given equal
inputs.

* The inputs to an expression consist of its operands.
* The outputs of an expression consist of its result and all operands modified by
the expression (if any).

In specification of standard concepts, operands are defined as the largest
subexpressions that include only:

* an id-expression, and
* invocations of std::move, std::forward, and std::declval.

The cv-qualification and value category of each operand is determined by assuming
that each template type parameter denotes a cv-unqualified complete non-array object
type.

Every expression required to be equality preserving is further required to be
stable: two evaluations of such an expression with the same input objects must have
equal outputs absent any explicit intervening modification of those input objects.

Unless noted otherwise, every expression used in a requires-expression is required
to be equality preserving and stable, and the evaluation of the expression may
modify only its non-constant operands. Operands that are constant must not be
modified.

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

Unlike the LegacyRandomAccessIterator requirements, the random_access_iterator
concept does not require dereference to return an lvalue.

Example

Demonstrates a possible implementation of std::distance via C++20 concepts.

// Run this code

#include <iterator>

namespace cxx20 {
template<std::input_or_output_iterator Iter>
constexpr std::iter_difference_t<Iter> distance(Iter first, Iter last)
{
if constexpr(std::random_access_iterator<Iter>)
return last - first;
else
{
std::iter_difference_t<Iter> result{};
for (;first != last;++first)
++result;
return result;
}
}
}

int main() {
static constexpr auto il = { 3, 1, 4 };
static_assert(cxx20::distance(il.begin(), il.end()) == 3);
static_assert(cxx20::distance(il.end(), il.begin()) == -3);
}

See also

bidirectional_iterator specifies that a forward_iterator is a bidirectional
(C++20) iterator, supporting movement backwards
(concept)