# C++ named requirements: LegacyRandomAccessIterator

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< cpp‎ | named req

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

- The type `It` satisfies LegacyBidirectionalIterator

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

random_access_iterator (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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