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

• 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;	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 (since C++20)

Click to expand

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 LegacyIterator#Concept.

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]

↑ [1] LWG issue 299 was reopened after this resolution.