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C++ named requirements: LegacyRandomAccessIterator - cppreference.com

From cppreference.com

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

meets BooleanTestable

(until C++20)

models boolean-testable

(since C++20)
 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

meets BooleanTestable

(until C++20)

models boolean-testable

(since C++20)
 b < a Total ordering relation opposite to a < b
a >= b

meets BooleanTestable

(until C++20)

models boolean-testable

(since C++20)
 !(a < b)
a <= b

meets BooleanTestable

(until C++20)

models boolean-testable

(since C++20)
 !(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.

(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
LWG 2114
(P2167R3) 		C++98 convertibility to bool was too weak to reflect the expectation of implementations requirements strengthened
  1. ↑ LWG issue 299 was reopened after this resolution.

See also