25 Iterators library [iterators]

25.3 Iterator requirements [iterator.requirements]

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

In the following sections, a and b denote values of type X or const X, difference_type and reference refer to the types iterator_traits<X>​::​difference_type and iterator_traits<X>​::​reference, respectively, n denotes a value of difference_type, u, tmp, and m denote identifiers, r denotes a value of X&, t denotes a value of value type T, o denotes a value of some type that is writable to the output iterator.
[Note 1: 
For an iterator type X there must be an instantiation of iterator_traits<X> ([iterator.traits]).
— end note]