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C++ named requirements: LegacyRandomAccessIterator

A LegacyRandomAccessIterator is a LegacyBidirectionalIterator that can be moved to point to any element in constant time.

If a LegacyRandomAccessIterator it originates from a Container, then it's value_type is the same as the container's, so dereferencing (*it) obtains the container's value_type.

A pointer to an element of an array satisfies all requirements of LegacyRandomAccessIterator.

Requirements

The type It satisfies LegacyRandomAccessIterator if

And, given

  • value_type, the type denoted by std::iterator_traits<It>::value_type
  • difference_type, the type denoted by std::iterator_traits<It>::difference_type
  • reference, the type denoted by std::iterator_traits<It>::reference
  • i, a, b, objects of type It or const It
  • r, an lvalue of type It
  • n, an integer of type difference_type

The following expressions must be valid and have their specified effects:

Expression Return type Operational semantics Notes
r += n It& difference_type m = n; if (m >= 0) while (m--) ++r; else while (m++) --r; return r;
  • n can be both positive or negative
  • The complexity is constant (that is, the implementation cannot actually execute the while loop shown in operational semantics)
a + n

n + a

 It It temp = a; return temp += n;
  • n can be both positive or negative
  • a + n == n + a
r -= n It& return r += -n; The absolute value of n must be within the range of representable values of difference_type.
i - n It It temp = i; return temp -= n;
b - a difference_type return n;
(see the precondition)

Precondition:

  • there exists a value n of type difference_type such that a + n == b

Postcondition:

  • b == a + (b - a).
i[n] convertible to reference *(i + n)
a < b contextually convertible to bool Equivalent to return b - a > 0; Precondition:
  • same as of b - a

Strict total ordering relation:

  • !(a < a)
  • if a < b then !(b < a)
  • if a < b and b < c then a < c
  • a < b or b < a or a == b
    (exactly one of the expressions is true)
a > b contextually convertible to bool b < a Total ordering relation opposite to a < b
a >= b contextually convertible to bool !(a < b)
a <= b contextually convertible to bool !(a > b)

The above rules imply that LegacyRandomAccessIterator also implements LessThanComparable.

A mutable LegacyRandomAccessIterator is a LegacyRandomAccessIterator that additionally satisfies the LegacyOutputIterator requirements.

Concept

For the definition of std::iterator_traits, the following exposition-only concept is defined.

template<class I>
concept __LegacyRandomAccessIterator =
    __LegacyBidirectionalIterator<I> && std::totally_ordered<I> &&
        requires(I i, typename std::incrementable_traits<I>::difference_type n)
        {
            { i += n } -> std::same_as<I&>;
            { i -= n } -> std::same_as<I&>;
            { i +  n } -> std::same_as<I>;
            { n +  i } -> std::same_as<I>;
            { i -  n } -> std::same_as<I>;
            { i -  i } -> std::same_as<decltype(n)>;
            {  i[n]  } -> std::convertible_to<std::iter_reference_t<I>>;
        };

where the exposition-only concept __LegacyBidirectionalIterator is described in LegacyBidirectionalIterator#Concept.

 (since C++20)

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 299
(N3066)		 C++98 the return type of a[n] was required
to be convertible to const value_type&		 the return type is required to
be convertible to reference
LWG 448 C++98 the return type of a[n] was required
to be convertible to value_type		 the return type is required to be
convertible to const value_type&[1]
LWG 1079 C++98 b - a was defined using a < b,
resulted in circular definition		 removed a < b from the definition
  1. LWG issue 299 was reopened after this resolution.

See also

(C++20)
specifies that a bidirectional_iterator is a random-access iterator, supporting advancement in constant time and subscripting
(concept)
Iterator library provides definitions for iterators, iterator traits, adaptors, and utility functions

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