std::bidirectional_iterator

From cppreference.com

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros(C++20)
Language support library
Concepts library(C++20)
Diagnostics library
Memory management library
Metaprogramming library(C++11)
General utilities library
Containers library
Iterators library
Ranges library(C++20)
Algorithms library
Strings library
Text processing library
Numerics library
Date and time library
Input/output library
Filesystem library(C++17)
Concurrency support library(C++11)
Execution control library(C++26)
Technical specifications
Symbols index
External libraries

[edit]

Iterator library

Iterator concepts

indirectly_readable (C++20)
indirectly_writable (C++20)
weakly_incrementable (C++20)
incrementable (C++20)
is-integer-like is-signed-integer-like (C++20)(C++20)

sentinel_for (C++20)
sized_sentinel_for (C++20)
input_iterator (C++20)
output_iterator (C++20)
input_or_output_iterator (C++20)

forward_iterator (C++20)
bidirectional_iterator (C++20)
random_access_iterator (C++20)
contiguous_iterator (C++20)

Iterator primitives

input_iterator_tagoutput_iterator_tagforward_iterator_tagbidirectional_iterator_tagrandom_access_iterator_tagcontiguous_iterator_tag

(C++20)

iter_value_titer_difference_titer_reference_titer_const_reference_titer_rvalue_reference_titer_common_reference_t

(C++20)(C++20)(C++20)(C++23)(C++20)(C++20)

iterator (deprecated in C++17)
iterator_traits
incrementable_traits (C++20)
indirectly_readable_traits (C++20)

Algorithm concepts and utilities

Indirect callable concepts

indirectly_unary_invocableindirectly_regular_unary_invocable

(C++20)(C++20)

indirect_unary_predicate (C++20)
indirect_binary_predicate (C++20)

indirect_equivalence_relation (C++20)
indirect_strict_weak_order (C++20)

Common algorithm requirements

indirectly_movable (C++20)
indirectly_movable_storable (C++20)
indirectly_copyable (C++20)

indirectly_copyable_storable (C++20)
indirectly_swappable (C++20)
indirectly_comparable (C++20)

permutable (C++20)
mergeable (C++20)
sortable (C++20)

(C++20)

(C++20)

(C++26)

reverse_iterator
make_reverse_iterator (C++14)
move_iterator (C++11)
make_move_iterator (C++11)
default_sentinel_tdefault_sentinel (C++20)(C++20)
unreachable_sentinel_tunreachable_sentinel (C++20)(C++20)

front_insert_iterator
back_insert_iterator
inserter
insert_iterator
front_inserter
back_inserter
move_sentinel (C++20)

common_iterator (C++20)
counted_iterator (C++20)
basic_const_iterator (C++23)
const_iterator (C++23)
const_sentinel (C++23)
make_const_iterator (C++23)
make_const_sentinel (C++23)

Stream iterators

istream_iterator
ostream_iterator

istreambuf_iterator
ostreambuf_iterator

Iterator customization points
ranges::iter_move (C++20)
ranges::iter_swap (C++20)

Iterator operations

advance
distance
prev (C++11)
next (C++11)

ranges::advance (C++20)
ranges::distance (C++20)
ranges::prev (C++20)
ranges::next (C++20)

Range access

begincbegin (C++11)(C++14)
rbegincrbegin (C++14)(C++14)

endcend (C++11)(C++14)
rendcrend (C++14)(C++14)

sizessize (C++17)(C++20)
empty (C++17)
data (C++17)

[edit]

Defined in header`<iterator>`
template<class I> concept bidirectional_iterator= std::forward_iterator<I>&& std::derived_from</ITER_CONCEPT/<I>,std::bidirectional_iterator_tag>&& requires(I i){ {--i}->std::same_as<I&>; { i--}->std::same_as<I>; };		(since C++20)

The conceptbidirectional_iterator refinesforward_iterator by adding the ability to move an iterator backward.

[edit]Iterator concept determination

Definition of this concept is specified via an exposition-only alias template/*ITER_CONCEPT*/.

In order to determine/*ITER_CONCEPT*/, letITER_TRAITS denoteI if the specializationstd::iterator_traits is generated from the primary template, orstd::iterator_traits otherwise:

IfITER_TRAITS::iterator_concept is valid and names a type,/*ITER_CONCEPT*/ denotes the type.
Otherwise, ifITER_TRAITS::iterator_category is valid and names a type,/*ITER_CONCEPT*/ denotes the type.
Otherwise, ifstd::iterator_traits is generated from the primary template,/*ITER_CONCEPT*/ denotesstd::random_access_iterator_tag.
(That is,std::derived_from</*ITER_CONCEPT*/,std::bidirectional_iterator_tag> is assumed to betrue.)
Otherwise,/*ITER_CONCEPT*/ does not denote a type and results in a substitution failure.

[edit]Semantic requirements

A bidirectional iteratorr is said to bedecrementable if and only if there exists somes such that++s== r.

std::bidirectional_iterator is modeled only if all the concepts it subsumes are modeled, and given two objectsa andb of typeI:

Ifa is decrementable,a is in the domain of the expressions--a anda--.
Pre-decrement yields an lvalue that refers to the operand:std::addressof(--a)==std::addressof(a).
Post-decrement yields the previous value of the operand: ifbool(a== b), thenbool(a--== b).
Post-decrement and pre-decrement perform the same modification on its operand: Ifbool(a== b), then after evaluating botha-- and--b,bool(a== b) still holds.
Increment and decrement are inverses of each other:

Ifa is incrementable andbool(a== b), thenbool(--(++a)== b).
Ifa is decrementable andbool(a== b), thenbool(++(--a)== b).

[edit]Equality preservation

Expressions declared inrequires expressions of the standard library concepts are required to beequality-preserving (except where stated otherwise).

[edit]Notes

Unlike theLegacyBidirectionalIterator requirements, thebidirectional_iterator concept does not require dereference to return an lvalue.

[edit]Example

A minimum bidirectional iterator.

#include <cstddef>#include <iterator> class SimpleBidiIterator{public:using difference_type=std::ptrdiff_t;using value_type=int;     SimpleBidiIterator();    SimpleBidiIterator(const SimpleBidiIterator&);     SimpleBidiIterator& operator=(const SimpleBidiIterator&); int operator*()const;     SimpleBidiIterator& operator++();     SimpleBidiIterator operator++(int){auto tmp=*this;++*this;return tmp;}     SimpleBidiIterator& operator--();     SimpleBidiIterator operator--(int){auto tmp=*this;--*this;return tmp;} bool operator==(const SimpleBidiIterator&)const;}; static_assert(std::bidirectional_iterator<SimpleBidiIterator>);

[edit]See also

forward_iterator (C++20)	specifies that an`input_iterator` is a forward iterator, supporting equality comparison and multi-pass (concept)[edit]
random_access_iterator (C++20)	specifies that a`bidirectional_iterator` is a random-access iterator, supporting advancement in constant time and subscripting (concept)[edit]