24 Containers library [containers]

24.3 Sequence containers [sequences]

24.3.9 Class template forward_list [forward.list]

24.3.9.1 Overview [forward.list.overview]

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.

Fast random access to list elements is not supported.

[Note 1:

It is intended that forward_list have zero space or time overhead relative to a hand-written C-style singly linked list.

Features that would conflict with that goal have been omitted.

— end note]

A forward_list meets all of the requirements of a container ([container.reqmts]), except that the size() member function is not provided and operator== has linear complexity, A forward_list also meets all of the requirements for an allocator-aware container ([container.alloc.reqmts]).

In addition, a forward_list provides the assign member functions and several of the optional sequence container requirements ([sequence.reqmts]).

Descriptions are provided here only for operations on forward_list that are not described in that table or for operations where there is additional semantic information.

[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.

For this reason, erase_after and splice_after take fully-open ranges, not semi-open ranges.

— end note]

namespace std { template<class T, class Allocator = allocator<T>> class forward_list { public: // types using 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] using difference_type = implementation-defined; // see [container.requirements] using iterator = implementation-defined; // see [container.requirements] using const_iterator = implementation-defined; // see [container.requirements] // [forward.list.cons], construct/copy/destroy forward_list() : forward_list(Allocator()) { } explicit forward_list(const Allocator&); explicit forward_list(size_type n, const Allocator& = Allocator()); forward_list(size_type n, const T& value, const Allocator& = Allocator()); template<class InputIterator> forward_list(InputIterator first, InputIterator last, const Allocator& = Allocator()); template<container-compatible-range<T> R> forward_list(from_range_t, R&& rg, const Allocator& = Allocator()); forward_list(const forward_list& x); forward_list(forward_list&& x); forward_list(const forward_list& x, const type_identity_t<Allocator>&); forward_list(forward_list&& x, const type_identity_t<Allocator>&); forward_list(initializer_list<T>, const Allocator& = Allocator()); ~forward_list(); forward_list& operator=(const forward_list& x); forward_list& operator=(forward_list&& x) noexcept(allocator_traits<Allocator>::is_always_equal::value); forward_list& operator=(initializer_list<T>); template<class InputIterator> void assign(InputIterator first, InputIterator last); template<container-compatible-range<T> R> void assign_range(R&& rg); void assign(size_type n, const T& t); void assign(initializer_list<T>); allocator_type get_allocator() const noexcept; // [forward.list.iter], iterators iterator before_begin() noexcept; const_iterator before_begin() const noexcept; iterator begin() noexcept; const_iterator begin() const noexcept; iterator end() noexcept; const_iterator end() const noexcept; const_iterator cbegin() const noexcept; const_iterator cbefore_begin() const noexcept; const_iterator cend() const noexcept; // capacity [[nodiscard]] bool empty() const noexcept; size_type max_size() const noexcept; // [forward.list.access], element access reference front(); const_reference front() const; // [forward.list.modifiers], modifiers template<class... Args> reference emplace_front(Args&&... args); void push_front(const T& x); void push_front(T&& x); template<container-compatible-range<T> R> void prepend_range(R&& rg); void pop_front(); template<class... Args> iterator emplace_after(const_iterator position, Args&&... args); iterator insert_after(const_iterator position, const T& x); iterator insert_after(const_iterator position, T&& x); iterator insert_after(const_iterator position, size_type n, const T& x); template<class InputIterator> iterator insert_after(const_iterator position, InputIterator first, InputIterator last); iterator insert_after(const_iterator position, initializer_list<T> il); template<container-compatible-range<T> R> iterator insert_range_after(const_iterator position, R&& rg); iterator erase_after(const_iterator position); iterator erase_after(const_iterator position, const_iterator last); void swap(forward_list&) noexcept(allocator_traits<Allocator>::is_always_equal::value); void resize(size_type sz); void resize(size_type sz, const value_type& c); void clear() noexcept; // [forward.list.ops], forward_list operations void splice_after(const_iterator position, forward_list& x); void splice_after(const_iterator position, forward_list&& x); void splice_after(const_iterator position, forward_list& x, const_iterator i); void splice_after(const_iterator position, forward_list&& x, const_iterator i); void splice_after(const_iterator position, forward_list& x, const_iterator first, const_iterator last); void splice_after(const_iterator position, forward_list&& x, const_iterator first, const_iterator last); size_type remove(const T& value); template<class Predicate> size_type remove_if(Predicate pred); size_type unique(); template<class BinaryPredicate> size_type unique(BinaryPredicate binary_pred); void merge(forward_list& x); void merge(forward_list&& x); template<class Compare> void merge(forward_list& x, Compare comp); template<class Compare> void merge(forward_list&& x, Compare comp); void sort(); template<class Compare> void sort(Compare comp); 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 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>; }

An incomplete type T may be used when instantiating forward_list if the allocator meets the allocator completeness requirements .

T shall be complete before any member of the resulting specialization of forward_list is referenced.

24.3.9.2 Constructors, copy, and assignment [forward.list.cons]

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explicit forward_list(const Allocator&);

Effects: Constructs an empty forward_list object using the specified allocator.

Complexity: Constant.

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explicit forward_list(size_type n, const Allocator& = Allocator());

Preconditions: T is Cpp17DefaultInsertable into *this.

Effects: Constructs a forward_list object with n default-inserted elements using the specified allocator.

Complexity: Linear in n.

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forward_list(size_type n, const T& value, const Allocator& = Allocator());

Preconditions: T is Cpp17CopyInsertable into *this.

Effects: Constructs a forward_list object with n copies of value using the specified allocator.

Complexity: Linear in n.

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template<class InputIterator>
  forward_list(InputIterator first, InputIterator last, const Allocator& = Allocator());

Effects: Constructs a forward_list object equal to the range [first, last).

Complexity: Linear in distance(first, last).

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template<container-compatible-range<T> R>
  forward_list(from_range_t, R&& rg, const Allocator& = Allocator());

Effects: Constructs a forward_list object with the elements of the range rg.

Complexity: Linear in ranges::distance(rg).

24.3.9.3 Iterators [forward.list.iter]

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iterator before_begin() noexcept;
const_iterator before_begin() const noexcept;
const_iterator cbefore_begin() const noexcept;

Effects: cbefore_begin() is equivalent to const_cast<forward_list const&>(*this).before_begin().

Returns: A non-dereferenceable iterator that, when incremented, is equal to the iterator returned by begin().

Remarks: before_begin() == end() shall equal false.

24.3.9.4 Element access [forward.list.access]

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reference front();
const_reference front() const;

Returns: *begin()

24.3.9.5 Modifiers [forward.list.modifiers]

None of the overloads of insert_after shall affect the validity of iterators and references, and erase_after shall invalidate only iterators and references to the erased elements.

If an exception is thrown during insert_after there shall be no effect.

Inserting n elements into a forward_list is linear in n, and the number of calls to the copy or move constructor of T is exactly equal to n.

Erasing n elements from a forward_list is linear in n and the number of calls to the destructor of type T is exactly equal to n.

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template<class... Args> reference emplace_front(Args&&... args);

Effects: Inserts an object of type value_type constructed with value_type(std::forward<Args>(args)...) at the beginning of the list.

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void push_front(const T& x);
void push_front(T&& x);

Effects: Inserts a copy of x at the beginning of the list.

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template<container-compatible-range<T> R>
  void prepend_range(R&& rg);

Effects: Inserts a copy of each element of rg at the beginning of the list.

[Note 1:

The order of elements is not reversed.

— end note]

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void pop_front();

Effects: As if by erase_after(before_begin()).

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iterator insert_after(const_iterator position, const T& x);

Preconditions: T is Cpp17CopyInsertable into forward_list.

position is before_begin() or is a dereferenceable iterator in the range [begin(), end()).

Effects: Inserts a copy of x after position.

Returns: An iterator pointing to the copy of x.

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iterator insert_after(const_iterator position, T&& x);

Preconditions: T is Cpp17MoveInsertable into forward_list.

position is before_begin() or is a dereferenceable iterator in the range [begin(), end()).

Effects: Inserts a copy of x after position.

Returns: An iterator pointing to the copy of x.

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iterator insert_after(const_iterator position, size_type n, const T& x);

Preconditions: T is Cpp17CopyInsertable into forward_list.

position is before_begin() or is a dereferenceable iterator in the range [begin(), end()).

Effects: Inserts n copies of x after position.

Returns: An iterator pointing to the last inserted copy of x, or position if n == 0 is true.

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template<class InputIterator>
  iterator insert_after(const_iterator position, InputIterator first, InputIterator last);

Preconditions: T is Cpp17EmplaceConstructible into forward_list from *first.

position is before_begin() or is a dereferenceable iterator in the range [begin(), end()).

Neither first nor last are iterators in *this.

Effects: Inserts copies of elements in [first, last) after position.

Returns: An iterator pointing to the last inserted element, or position if first == last is true.

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template<container-compatible-range<T> R>
  iterator insert_range_after(const_iterator position, R&& rg);

Preconditions: T is Cpp17EmplaceConstructible into forward_list from *ranges::begin(rg).

position is before_begin() or is a dereferenceable iterator in the range [begin(), end()).

rg and *this do not overlap.

Effects: Inserts copies of elements in the range rg after position.

Returns: An iterator pointing to the last inserted element, or position if rg is empty.

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iterator insert_after(const_iterator position, initializer_list<T> il);

Effects: Equivalent to: return insert_after(position, il.begin(), il.end());

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template<class... Args>
  iterator emplace_after(const_iterator position, Args&&... args);

Preconditions: T is Cpp17EmplaceConstructible into forward_list from std::forward<Args>(
args)....

position is before_begin() or is a dereferenceable iterator in the range [begin(), end()).

Effects: Inserts an object of type value_type direct-non-list-initialized with std::forward<Args>(
args)... after position.

Returns: An iterator pointing to the new object.

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iterator erase_after(const_iterator position);

Preconditions: The iterator following position is dereferenceable.

Effects: Erases the element pointed to by the iterator following position.

Returns: An iterator pointing to the element following the one that was erased, or end() if no such element exists.

Throws: Nothing.

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iterator erase_after(const_iterator position, const_iterator last);

Preconditions: All iterators in the range (position, last) are dereferenceable.

Effects: Erases the elements in the range (position, last).

Returns: last.

Throws: Nothing.

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void resize(size_type sz);

Preconditions: T is Cpp17DefaultInsertable into *this.

Effects: If sz < distance(begin(), end()), erases the last distance(begin(), end()) - sz elements from the list.

Otherwise, inserts sz - distance(begin(), end()) default-inserted elements at the end of the list.

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void resize(size_type sz, const value_type& c);

Preconditions: T is Cpp17CopyInsertable into *this.

Effects: If sz < distance(begin(), end()), erases the last distance(begin(), end()) - sz elements from the list.

Otherwise, inserts sz - distance(begin(), end()) copies of c at the end of the list.

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void clear() noexcept;

Effects: Erases all elements in the range [begin(), end()).

Remarks: Does not invalidate past-the-end iterators.

24.3.9.6 Operations [forward.list.ops]

In this subclause, arguments for a template parameter named Predicate or BinaryPredicate shall meet the corresponding requirements in [algorithms.requirements].

The semantics of i + n, where i is an iterator into the list and n is an integer, are the same as those of next(i, n).

The expression i - n, where i is an iterator into the list and n is an integer, means an iterator j such that j + n == i is true.

For merge and sort, the definitions and requirements in [alg.sorting] apply.

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void splice_after(const_iterator position, forward_list& x);
void splice_after(const_iterator position, forward_list&& x);

Preconditions: position is before_begin() or is a dereferenceable iterator in the range [begin(), end()).

get_allocator() == x.get_allocator() is true.

addressof(x) != this is true.

Effects: Inserts the contents of x after position, and x becomes empty.

Pointers and references to the moved elements of x now refer to those same elements but as members of *this.

Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into *this, not into x.

Throws: Nothing.

Complexity:

O (distance(x.begin(), x.end()))

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void splice_after(const_iterator position, forward_list& x, const_iterator i);
void splice_after(const_iterator position, forward_list&& x, const_iterator i);

Preconditions: position is before_begin() or is a dereferenceable iterator in the range [begin(), end()).

The iterator following i is a dereferenceable iterator in x.

get_allocator() == x.get_allocator() is true.

Effects: Inserts the element following i into *this, following position, and removes it from x.

The result is unchanged if position == i or position == ++i.

Pointers and references to *++i continue to refer to the same element but as a member of *this.

Iterators to *++i continue to refer to the same element, but now behave as iterators into *this, not into x.

Throws: Nothing.

Complexity:

O (1)

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void splice_after(const_iterator position, forward_list& x,
                  const_iterator first, const_iterator last);
void splice_after(const_iterator position, forward_list&& x,
                  const_iterator first, const_iterator last);

Preconditions: position is before_begin() or is a dereferenceable iterator in the range [begin(), end()).

(first, last) is a valid range in x, and all iterators in the range (first, last) are dereferenceable.

position is not an iterator in the range (first, last).

get_allocator() == x.get_allocator() is true.

Effects: Inserts elements in the range (first, last) after position and removes the elements from x.

Pointers and references to the moved elements of x now refer to those same elements but as members of *this.

Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into *this, not into x.

Complexity:

O (distance(first, last))

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size_type remove(const T& value);
template<class Predicate> size_type remove_if(Predicate pred);

Effects: Erases all the elements in the list referred to by a list iterator i for which the following conditions hold: *i == value (for remove()), pred(*i) is true (for remove_if()).

Invalidates only the iterators and references to the erased elements.

Returns: The number of elements erased.

Throws: Nothing unless an exception is thrown by the equality comparison or the predicate.

Complexity: Exactly distance(begin(), end()) applications of the corresponding predicate.

Remarks: Stable .

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size_type unique();
template<class BinaryPredicate> size_type unique(BinaryPredicate binary_pred);

Let binary_pred be equal_to<>{} for the first overload.

Preconditions: binary_pred is an equivalence relation.

Effects: Erases all but the first element from every consecutive group of equivalent elements.

That is, for a nonempty list, erases all elements referred to by the iterator i in the range [begin() + 1, end()) for which binary_pred(*i, *(i - 1)) is true.

Invalidates only the iterators and references to the erased elements.

Returns: The number of elements erased.

Throws: Nothing unless an exception is thrown by the predicate.

Complexity: If empty() is false, exactly distance(begin(), end()) - 1 applications of the corresponding predicate, otherwise no applications of the predicate.

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void merge(forward_list& x);
void merge(forward_list&& x);
template<class Compare> void merge(forward_list& x, Compare comp);
template<class Compare> void merge(forward_list&& x, Compare comp);

Let comp be less<> for the first two overloads.

Preconditions: *this and x are both sorted with respect to the comparator comp, and get_allocator() == x.get_allocator() is true.

Effects: If addressof(x) == this, there are no effects.

Otherwise, merges the two sorted ranges [begin(), end()) and [x.begin(), x.end()).

The result is a range that is sorted with respect to the comparator comp.

Pointers and references to the moved elements of x now refer to those same elements but as members of *this.

Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into *this, not into x.

Complexity: At most distance(begin(), end()) + distance(x.begin(), x.end()) - 1 comparisons if addressof(x) != this; otherwise, no comparisons are performed.

Remarks: Stable ([algorithm.stable]).

If addressof(x) != this, x is empty after the merge.

No elements are copied by this operation.

If an exception is thrown other than by a comparison, there are no effects.

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void sort();
template<class Compare> void sort(Compare comp);

Effects: Sorts the list according to the operator< or the comp function object.

If an exception is thrown, the order of the elements in *this is unspecified.

Does not affect the validity of iterators and references.

Complexity: Approximately

N log N

comparisons, where N is distance(begin(), end()).

Remarks: Stable .

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void reverse() noexcept;

Effects: Reverses the order of the elements in the list.

Does not affect the validity of iterators and references.

Complexity: Linear time.

24.3.9.7 Erasure [forward.list.erasure]

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template<class T, class Allocator, class U>
  typename forward_list<T, Allocator>::size_type
    erase(forward_list<T, Allocator>& c, const U& value);

Effects: Equivalent to: return erase_if(c, [&](auto& elem) { return elem == value; });

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template<class T, class Allocator, class Predicate>
  typename forward_list<T, Allocator>::size_type
    erase_if(forward_list<T, Allocator>& c, Predicate pred);

Effects: Equivalent to: return c.remove_if(pred);