# Standard library header <forward_list> (C++11)

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< cpp‎ | header

This header is part of the containers library.

### Includes

<compare> (C++20) | Three-way comparison operator support
<initializer_list> (C++11) | std::initializer_list class template

### Classes

forward_list (C++11) | singly-linked list (class template)

### Functions

operator==operator!=operator<operator<=operator>operator>=operator<=> (C++11)(C++11)(removed in C++20)(C++11)(removed in C++20)(C++11)(removed in C++20)(C++11)(removed in C++20)(C++11)(removed in C++20)(C++20) | lexicographically compares the values of two `forward_list`s (function template)
std::swap(std::forward_list) (C++11) | specializes the std::swap algorithm (function template)
erase(std::forward_list)erase_if(std::forward_list) (C++20) | erases all elements satisfying specific criteria (function template)

##### Range access

begincbegin (C++11)(C++14) | returns an iterator to the beginning of a container or array (function template)
endcend (C++11)(C++14) | returns an iterator to the end of a container or array (function template)
rbegincrbegin (C++14) | returns a reverse iterator to the beginning of a container or array (function template)
rendcrend (C++14) | returns a reverse end iterator for a container or array (function template)
sizessize (C++17)(C++20) | returns the size of a container or array (function template)
empty (C++17) | checks whether the container is empty (function template)
data (C++17) | obtains the pointer to the underlying array (function template)

### Synopsis

    #include <compare>
    #include <initializer_list>

    namespace std {
      // class template forward_list
      template<class T, class Allocator = allocator<T>>
      class forward_list;

      template<class T, class Allocator>
      bool operator==(const forward_list<T, Allocator>& x,
                      const forward_list<T, Allocator>& y);
      template<class T, class Allocator>
      /*synth-three-way-result*/<T> operator<=>(const forward_list<T, Allocator>& x,
                                                const forward_list<T, Allocator>& y);

      template<class T, class Allocator>
      void swap(forward_list<T, Allocator>& x,
                forward_list<T, Allocator>& y) noexcept(noexcept(x.swap(y)));

      // erasure
      template<class T, class Allocator, class U = T>
      typename forward_list<T, Allocator>::size_type erase(forward_list<T, Allocator>& c,
                                                           const U& value);
      template<class T, class Allocator, class Predicate>
      typename forward_list<T, Allocator>::size_type erase_if(forward_list<T, Allocator>& c,
                                                              Predicate pred);

      namespace pmr {
        template<class T>
        using forward_list = std::forward_list<T, polymorphic_allocator<T>>;
      }
    }

#### Class template std::forward_list

    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 */;
        using difference_type = /* implementation-defined */;
        using iterator        = /* implementation-defined */;
        using const_iterator  = /* implementation-defined */;

        // 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 InputIter>
        forward_list(InputIter first, InputIter 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 InputIter>
        void assign(InputIter first, InputIter 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;

        // 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
        bool empty() const noexcept;
        size_type max_size() const noexcept;

        // element access
        reference front();
        const_reference front() const;

        // 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 InputIter>
        iterator insert_after(const_iterator position, InputIter first, InputIter 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 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 InputIter, class Allocator = allocator</*iter-value-type*/<InputIter>>>
      forward_list(InputIter, InputIter, Allocator = Allocator())
        -> forward_list</*iter-value-type*/<InputIter>, 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>;
    }

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