std::random_access_iterator

 

C++

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Iterator library

Iterator concepts
indirectly_readable (C++20) indirectly_writable (C++20) weakly_incrementable (C++20) incrementable (C++20) input_or_output_iterator (C++20) sentinel_for (C++20) sized_sentinel_for (C++20)	input_iterator (C++20) output_iterator (C++20) forward_iterator (C++20) bidirectional_iterator (C++20) random_access_iterator (C++20) contiguous_iterator (C++20)
Iterator primitives
iterator_traits input_iterator_tagoutput_iterator_tagforward_iterator_tagbidirectional_iterator_tagrandom_access_iterator_tagcontiguous_iterator_tag (C++20) iterator (deprecated in C++17)	incrementable_traits (C++20) indirectly_readable_traits (C++20) iter_value_titer_difference_titer_reference_titer_const_reference_titer_rvalue_reference_titer_common_reference_t (C++20)(C++20)(C++20)(C++23)(C++20)(C++20)
Algorithm concepts and utilities
Indirect callable concepts
indirectly_unary_invocableindirectly_regular_unary_invocable (C++20)(C++20)   indirect_unary_predicate (C++20)	indirect_binary_predicate (C++20) indirect_equivalence_relation (C++20) indirect_strict_weak_order (C++20)
Common algorithm requirements
indirectly_movable (C++20) indirectly_movable_storable (C++20) indirectly_copyable (C++20) indirectly_copyable_storable (C++20)	indirectly_swappable (C++20) indirectly_comparable (C++20) permutable (C++20) mergeable (C++20) sortable (C++20)
Utilities
indirect_result_t (C++20)	projected (C++20)
Iterator adaptors
reverse_iterator move_iterator (C++11) move_sentinel (C++20) common_iterator (C++20) counted_iterator (C++20) basic_const_iterator (C++23) const_iterator (C++23) const_sentinel (C++23) back_insert_iterator front_insert_iterator insert_iterator	make_reverse_iterator (C++14) make_move_iterator (C++11) default_sentinel_tdefault_sentinel (C++20)(C++20) unreachable_sentinel_tunreachable_sentinel (C++20)(C++20) make_const_iterator (C++23) make_const_sentinel (C++23) back_inserter front_inserter inserter
Stream iterators
istream_iterator ostream_iterator	istreambuf_iterator ostreambuf_iterator
Iterator customization points
ranges::iter_move (C++20)
ranges::iter_swap (C++20)
Iterator operations
advance distance prev (C++11) next (C++11)	ranges::advance (C++20) ranges::distance (C++20) ranges::prev (C++20) ranges::next (C++20)
Range access
begincbegin (C++11)(C++14) endcend (C++11)(C++14) sizessize (C++17)(C++20)	rbegincrbegin (C++14)(C++14) rendcrend (C++14)(C++14) empty (C++17) data (C++17)

 

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) {       { 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>>;     };		(since C++20)

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. std::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). ^[1]
• (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). ^[1]
• (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.

Note that std::addressof returns the address of the iterator object, not the address of the object the iterator points to. I.e. operator+= and operator-= must return a reference to *this.

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

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
    (
        std::random_access_iterator<decltype(il.begin())>
        and
        cxx20::distance(il.begin(), il.end()) == 3
        and
        cxx20::distance(il.end(), il.begin()) == -3
    );
}

See also

bidirectional_iterator (C++20)	specifies that a forward_iterator is a bidirectional iterator, supporting movement backwards (concept)
contiguous_iterator (C++20)	specifies that a random_access_iterator is a contiguous iterator, referring to elements that are contiguous in memory (concept)