25 Iterators library [iterators]

25.3 Iterator requirements [iterator.requirements]

25.3.5 C++17 iterator requirements [iterator.cpp17]

25.3.5.7 Random access iterators [random.access.iterators]

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A class or pointer type X meets the requirements of a random access iterator if, in addition to meeting the Cpp17BidirectionalIterator requirements, the following expressions are valid as shown in Table 91.
Table 91: Cpp17RandomAccessIterator requirements (in addition to Cpp17BidirectionalIterator) [tab:randomaccessiterator]
Expression
Return type
Operational
Assertion/note
semantics
pre-/post-condition
r += n
X&
{ difference_type m = n;
if (m >= 0)
while (m--)
++r;
else
while (m++)
--r;
return r; }
a + n
n + a
X
{ X tmp = a;
return tmp += n; }
a + n == n + a.
r -= n
X&
return r += -n;
Preconditions: the absolute value of n is in the range of representable values of difference_type.
a - n
X
{ X tmp = a;
return tmp -= n; }
b - a
difference_type
return n;
Preconditions: there exists a value n of type difference_type such that a + n == b.

b == a + (b - a).
a[n]
convertible to reference
*(a + n)
a < b
decltype(a < b) models boolean-testable
Effects: Equivalent to: return b - a > 0;
< is a total ordering relation
a > b
decltype(a > b) models boolean-testable
b < a
> is a total ordering relation opposite to <.
a >= b
decltype(a >= b) models boolean-testable
!(a < b)
a <= b
decltype(a <= b) models boolean-testable
!(a > b)