C++ concepts: RandomAccessIterator

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Revision as of 21:43, 14 September 2013 by Lichray (Talk | contribs)


A RandomAccessIterator is a BidirectionalIterator that can be moved to point to any element in constant time.

A standard pointer is an example of a type that satisfies this concept.


In addition to the above requirement, for a type It to be an RandomAccessIterator, instances a, b, i, and r of It must:

Expression Return Equivalent expression Notes
r += n It& if ( n >= 0 )

   while(n--) ++r;
   while(n++) --r;
return r;

  • n can be both positive or negative
  • Constant complexity (that is, the equivalent expression cannot be used as implementation)
i + n It It temp = i;

return temp += n;

n + i It i + n
r -= n It& return r += -n;
i - n It It temp = i;

return temp -= n;

b - a difference n returns n such that a + n == b, where 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)

Table Notes

  • It is the type implementing this concept
  • T is the type std::iterator_traits<It>::value_type
  • reference is the type std::iterator_traits<It>::reference
  • difference is the type std::iterator_traits<It>::difference_type
  • i, a, b are objects of type It or const It
  • r is a value of type It&
  • n is an integer of type difference

The above rules imply that RandomAccessIterator also implements LessThanComparable.

A mutable RandomAccessIterator is a RandomAccessIterator that additionally satisfies the OutputIterator requirements.

See also