[forward.iterators]

# 24 Iterators library [[iterators]](./#iterators)

## 24.3 Iterator requirements [[iterator.requirements]](iterator.requirements#forward.iterators)

### 24.3.5 C++17 iterator requirements [[iterator.cpp17]](iterator.cpp17#forward.iterators)

#### 24.3.5.5 Forward iterators [forward.iterators]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L2230)

A class or pointer typeX meets the *Cpp17ForwardIterator* requirements if

- [(1.1)](#1.1)

X meets the *Cpp17InputIterator* requirements ([[input.iterators]](input.iterators "24.3.5.3 Input iterators")),

- [(1.2)](#1.2)

X meets the *Cpp17DefaultConstructible* requirements ([[utility.arg.requirements]](utility.arg.requirements "16.4.4.2 Template argument requirements")),

- [(1.3)](#1.3)

if X is a mutable iterator, reference is a reference to T;
if X is a constant iterator, reference is a reference to const T,

- [(1.4)](#1.4)

the expressions in Table [81](#tab:forwarditerator "Table 81: Cpp17ForwardIterator requirements (in addition to Cpp17InputIterator)") are valid and have the indicated semantics, and

- [(1.5)](#1.5)

objects of type X offer the multi-pass guarantee, described below[.](#1.sentence-1)

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L2249)

The domain of == for forward iterators is that of iterators over the same
underlying sequence[.](#2.sentence-1)

However, value-initialized iterators may be compared and
shall compare equal to other value-initialized iterators of the same type[.](#2.sentence-2)

[*Note [1](#note-1)*:

Value-initialized iterators behave as if they refer past the end of
the same empty sequence[.](#2.sentence-3)

— *end note*]

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L2258)

Two dereferenceable iterators a and b of type X offer the[*multi-pass guarantee*](#def:multi-pass_guarantee "24.3.5.5 Forward iterators [forward.iterators]") if

- [(3.1)](#3.1)

a == b implies ++a == ++b and

- [(3.2)](#3.2)

X is a pointer type or the expression(void)++X(a), *a is equivalent to the expression *a[.](#3.sentence-1)

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L2267)

[*Note [2](#note-2)*:

The requirement thata == b implies++a == ++b (which is not true for input and output iterators)
and the removal of the restrictions on the number of the assignments through
a mutable iterator
(which applies to output iterators)
allows the use of multi-pass one-directional algorithms with forward iterators[.](#4.sentence-1)

— *end note*]

Table [81](#tab:forwarditerator) — *Cpp17ForwardIterator* requirements (in addition to *Cpp17InputIterator*) [[tab:forwarditerator]](./tab:forwarditerator)  

| [🔗](#tab:forwarditerator-row-1)<br>**Expression** | **Return type** | **Operational** | **Assertion/note** |
| --- | --- | --- | --- |
| [🔗](#tab:forwarditerator-row-2) |  | **semantics** | **pre-/post-condition** |
| [🔗](#tab:forwarditerator-row-3)<br>r++ | convertible to const X& | { X tmp = r;    ++r;    return tmp; } |  |
| [🔗](#tab:forwarditerator-row-4)<br>*r++ | reference |  |  |

[5](#5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L2297)

If a and b are equal, then either a and b are both dereferenceable
or else neither is dereferenceable[.](#5.sentence-1)

[6](#6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L2302)

If a and b are both dereferenceable, then a == b if and only if*a and *b are bound to the same object[.](#6.sentence-1)