[forward.list.overview]

# 23 Containers library [[containers]](./#containers)

## 23.3 Sequence containers [[sequences]](sequences#forward.list.overview)

### 23.3.7 Class template forward_list [[forward.list]](forward.list#overview)

#### 23.3.7.1 Overview [forward.list.overview]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L6962)

A forward_list is a container that supports forward iterators and allows
constant time insert and erase operations anywhere within the sequence, with storage
management handled automatically[.](#1.sentence-1)

Fast random access to list elements is not supported[.](#1.sentence-2)

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

It is intended that forward_list have zero space or time overhead
relative to a hand-written C-style singly linked list[.](#1.sentence-3)

Features that would conflict with
that goal have been omitted[.](#1.sentence-4)

— *end note*]

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L6972)

A forward_list meets all of the requirements
of a container ([[container.reqmts]](container.reqmts "23.2.2.2 Container requirements")),
except that the size() member function is not provided andoperator== has linear complexity[.](#2.sentence-1)

A forward_list also meets all of the requirements
for an allocator-aware container ([[container.alloc.reqmts]](container.alloc.reqmts "23.2.2.5 Allocator-aware containers"))[.](#2.sentence-2)

In addition, a forward_list provides the assign member functions and
several of the optional sequence container requirements ([[sequence.reqmts]](sequence.reqmts "23.2.4 Sequence containers"))[.](#2.sentence-3)

Descriptions are provided here only for operations onforward_list that are not described in that table or for operations where there
is additional semantic information[.](#2.sentence-4)

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L6986)

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

Modifying any list requires access to the element preceding the first element
of interest, but in a forward_list there is no constant-time way to access a
preceding element[.](#3.sentence-1)

For this reason, erase_after and splice_after take fully-open ranges, not semi-open ranges[.](#3.sentence-2)

— *end note*]

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L6995)

The types iterator and const_iterator meet
the constexpr iterator requirements ([[iterator.requirements.general]](iterator.requirements.general "24.3.1 General"))[.](#4.sentence-1)

namespace std {template<class T, class Allocator = allocator<T>>class forward_list {public:// typesusing value_type = T; using allocator_type = Allocator; using pointer = typename allocator_traits<Allocator>::pointer; using const_pointer = typename allocator_traits<Allocator>::const_pointer; using reference = value_type&; using const_reference = const value_type&; using size_type = *implementation-defined*; // see [[container.requirements]](container.requirements "23.2 Requirements")using difference_type = *implementation-defined*; // see [[container.requirements]](container.requirements "23.2 Requirements")using iterator = *implementation-defined*; // see [[container.requirements]](container.requirements "23.2 Requirements")using const_iterator = *implementation-defined*; // see [[container.requirements]](container.requirements "23.2 Requirements")// [[forward.list.cons]](forward.list.cons "23.3.7.2 Constructors, copy, and assignment"), construct/copy/destroyconstexpr forward_list() : forward_list(Allocator()) { }constexpr explicit forward_list(const Allocator&); constexpr explicit forward_list(size_type n, const Allocator& = Allocator()); constexpr forward_list(size_type n, const T& value, const Allocator& = Allocator()); template<class InputIterator>constexpr forward_list(InputIterator first, InputIterator last, const Allocator& = Allocator()); template<[*container-compatible-range*](container.intro.reqmts#concept:container-compatible-range "23.2.2.1 Introduction [container.intro.reqmts]")<T> R>constexpr forward_list(from_range_t, R&& rg, const Allocator& = Allocator()); constexpr forward_list(const forward_list& x); constexpr forward_list(forward_list&& x); constexpr forward_list(const forward_list& x, const type_identity_t<Allocator>&); constexpr forward_list(forward_list&& x, const type_identity_t<Allocator>&); constexpr forward_list(initializer_list<T>, const Allocator& = Allocator()); constexpr ~forward_list(); constexpr forward_list& operator=(const forward_list& x); constexpr forward_list& operator=(forward_list&& x)noexcept(allocator_traits<Allocator>::is_always_equal::value); constexpr forward_list& operator=(initializer_list<T>); template<class InputIterator>constexpr void assign(InputIterator first, InputIterator last); template<[*container-compatible-range*](container.intro.reqmts#concept:container-compatible-range "23.2.2.1 Introduction [container.intro.reqmts]")<T> R>constexpr void assign_range(R&& rg); constexpr void assign(size_type n, const T& t); constexpr void assign(initializer_list<T>); constexpr allocator_type get_allocator() const noexcept; // [[forward.list.iter]](forward.list.iter "23.3.7.3 Iterators"), iteratorsconstexpr iterator before_begin() noexcept; constexpr const_iterator before_begin() const noexcept; constexpr iterator begin() noexcept; constexpr const_iterator begin() const noexcept; constexpr iterator end() noexcept; constexpr const_iterator end() const noexcept; constexpr const_iterator cbegin() const noexcept; constexpr const_iterator cbefore_begin() const noexcept; constexpr const_iterator cend() const noexcept; // capacityconstexpr bool empty() const noexcept; constexpr size_type max_size() const noexcept; // [[forward.list.access]](forward.list.access "23.3.7.4 Element access"), element accessconstexpr reference front(); constexpr const_reference front() const; // [[forward.list.modifiers]](forward.list.modifiers "23.3.7.5 Modifiers"), modifierstemplate<class... Args> constexpr reference emplace_front(Args&&... args); constexpr void push_front(const T& x); constexpr void push_front(T&& x); template<[*container-compatible-range*](container.intro.reqmts#concept:container-compatible-range "23.2.2.1 Introduction [container.intro.reqmts]")<T> R>constexpr void prepend_range(R&& rg); constexpr void pop_front(); template<class... Args>constexpr iterator emplace_after(const_iterator position, Args&&... args); constexpr iterator insert_after(const_iterator position, const T& x); constexpr iterator insert_after(const_iterator position, T&& x); constexpr iterator insert_after(const_iterator position, size_type n, const T& x); template<class InputIterator>constexpr iterator insert_after(const_iterator position,
 InputIterator first, InputIterator last); constexpr iterator insert_after(const_iterator position, initializer_list<T> il); template<[*container-compatible-range*](container.intro.reqmts#concept:container-compatible-range "23.2.2.1 Introduction [container.intro.reqmts]")<T> R>constexpr iterator insert_range_after(const_iterator position, R&& rg); constexpr iterator erase_after(const_iterator position); constexpr iterator erase_after(const_iterator position, const_iterator last); constexpr void swap(forward_list&)noexcept(allocator_traits<Allocator>::is_always_equal::value); constexpr void resize(size_type sz); constexpr void resize(size_type sz, const value_type& c); constexpr void clear() noexcept; // [[forward.list.ops]](forward.list.ops "23.3.7.6 Operations"), forward_list operationsconstexpr void splice_after(const_iterator position, forward_list& x); constexpr void splice_after(const_iterator position, forward_list&& x); constexpr void splice_after(const_iterator position, forward_list& x, const_iterator i); constexpr void splice_after(const_iterator position, forward_list&& x, const_iterator i); constexpr void splice_after(const_iterator position, forward_list& x,
 const_iterator first, const_iterator last); constexpr void splice_after(const_iterator position, forward_list&& x,
 const_iterator first, const_iterator last); constexpr size_type remove(const T& value); template<class Predicate> constexpr size_type remove_if(Predicate pred);

 size_type unique(); template<class BinaryPredicate> constexpr size_type unique(BinaryPredicate binary_pred); constexpr void merge(forward_list& x); constexpr void merge(forward_list&& x); template<class Compare> constexpr void merge(forward_list& x, Compare comp); template<class Compare> constexpr void merge(forward_list&& x, Compare comp); constexpr void sort(); template<class Compare> constexpr void sort(Compare comp); constexpr void reverse() noexcept; }; template<class InputIterator, class Allocator = allocator<*iter-value-type*<InputIterator>>> forward_list(InputIterator, InputIterator, Allocator = Allocator())-> forward_list<*iter-value-type*<InputIterator>, Allocator>; template<ranges::[input_range](range.refinements#concept:input_range "25.4.6 Other range refinements [range.refinements]") R, class Allocator = allocator<ranges::range_value_t<R>>> forward_list(from_range_t, R&&, Allocator = Allocator())-> forward_list<ranges::range_value_t<R>, Allocator>;}

[5](#5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L7131)

An incomplete type T may be used when instantiating forward_list if the allocator meets the[allocator completeness requirements](allocator.requirements.completeness "16.4.4.6.2 Allocator completeness requirements [allocator.requirements.completeness]")[.](#5.sentence-1)

T shall be complete before any member of the resulting specialization
of forward_list is referenced[.](#5.sentence-2)