C++ named requirements: LegacyRandomAccessIterator

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C++ named requirements

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

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


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, a value 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;


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


  • b == a + (b - a).
i[n] convertible to reference *(i + n)
a < b contextually convertible to bool b - a > 0 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.


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

template<class I>

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


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

(since C++20)

See also

specifies that a BidirectionalIterator is a random-access iterator, supporting advancement in constant time and subscripting