include/forward_list

// -*- C++ -*-
//===----------------------- forward_list ---------------------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is dual licensed under the MIT and the University of Illinois Open
// Source Licenses. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//

#ifndef _LIBCPP_FORWARD_LIST
#define _LIBCPP_FORWARD_LIST

/*
    forward_list synopsis

namespace std
{

template <class T, class Allocator = allocator<T>>
class forward_list
{
public:
    typedef T         value_type;
    typedef Allocator allocator_type;

    typedef value_type&                                                reference;
    typedef const value_type&                                          const_reference;
    typedef typename allocator_traits<allocator_type>::pointer         pointer;
    typedef typename allocator_traits<allocator_type>::const_pointer   const_pointer;
    typedef typename allocator_traits<allocator_type>::size_type       size_type;
    typedef typename allocator_traits<allocator_type>::difference_type difference_type;

    typedef <details> iterator;
    typedef <details> const_iterator;

    forward_list()
        noexcept(is_nothrow_default_constructible<allocator_type>::value);
    explicit forward_list(const allocator_type& a);
    explicit forward_list(size_type n);
    explicit forward_list(size_type n, const allocator_type& a); // C++14
    forward_list(size_type n, const value_type& v);
    forward_list(size_type n, const value_type& v, const allocator_type& a);
    template <class InputIterator>
        forward_list(InputIterator first, InputIterator last);
    template <class InputIterator>
        forward_list(InputIterator first, InputIterator last, const allocator_type& a);
    forward_list(const forward_list& x);
    forward_list(const forward_list& x, const allocator_type& a);
    forward_list(forward_list&& x)
        noexcept(is_nothrow_move_constructible<allocator_type>::value);
    forward_list(forward_list&& x, const allocator_type& a);
    forward_list(initializer_list<value_type> il);
    forward_list(initializer_list<value_type> il, const allocator_type& a);

    ~forward_list();

    forward_list& operator=(const forward_list& x);
    forward_list& operator=(forward_list&& x)
        noexcept(
             allocator_type::propagate_on_container_move_assignment::value &&
             is_nothrow_move_assignable<allocator_type>::value);
    forward_list& operator=(initializer_list<value_type> il);

    template <class InputIterator>
        void assign(InputIterator first, InputIterator last);
    void assign(size_type n, const value_type& v);
    void assign(initializer_list<value_type> il);

    allocator_type get_allocator() 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 cend() const noexcept;

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

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

    reference       front();
    const_reference front() const;

    template <class... Args> void emplace_front(Args&&... args);
    void push_front(const value_type& v);
    void push_front(value_type&& v);

    void pop_front();

    template <class... Args>
        iterator emplace_after(const_iterator p, Args&&... args);
    iterator insert_after(const_iterator p, const value_type& v);
    iterator insert_after(const_iterator p, value_type&& v);
    iterator insert_after(const_iterator p, size_type n, const value_type& v);
    template <class InputIterator>
        iterator insert_after(const_iterator p,
                              InputIterator first, InputIterator last);
    iterator insert_after(const_iterator p, initializer_list<value_type> il);

    iterator erase_after(const_iterator p);
    iterator erase_after(const_iterator first, const_iterator last);

    void swap(forward_list& x)
        noexcept(allocator_traits<allocator_type>::is_always_equal::value);  // C++17

    void resize(size_type n);
    void resize(size_type n, const value_type& v);
    void clear() noexcept;

    void splice_after(const_iterator p, forward_list& x);
    void splice_after(const_iterator p, forward_list&& x);
    void splice_after(const_iterator p, forward_list& x, const_iterator i);
    void splice_after(const_iterator p, forward_list&& x, const_iterator i);
    void splice_after(const_iterator p, forward_list& x,
                      const_iterator first, const_iterator last);
    void splice_after(const_iterator p, forward_list&& x,
                      const_iterator first, const_iterator last);
    void remove(const value_type& v);
    template <class Predicate> void remove_if(Predicate pred);
    void unique();
    template <class BinaryPredicate> void 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 T, class Allocator>
    bool operator==(const forward_list<T, Allocator>& x,
                    const forward_list<T, Allocator>& y);

template <class T, class Allocator>
    bool operator< (const forward_list<T, Allocator>& x,
                    const forward_list<T, Allocator>& y);

template <class T, class Allocator>
    bool operator!=(const forward_list<T, Allocator>& x,
                    const forward_list<T, Allocator>& y);

template <class T, class Allocator>
    bool operator> (const forward_list<T, Allocator>& x,
                    const forward_list<T, Allocator>& y);

template <class T, class Allocator>
    bool operator>=(const forward_list<T, Allocator>& x,
                    const forward_list<T, Allocator>& y);

template <class T, class Allocator>
    bool 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)));

}  // std

*/

#include <__config>

#include <initializer_list>
#include <memory>
#include <limits>
#include <iterator>
#include <algorithm>

#include <__undef_min_max>

#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
#pragma GCC system_header
#endif

_LIBCPP_BEGIN_NAMESPACE_STD

template <class _Tp, class _VoidPtr> struct __forward_list_node;

template <class _NodePtr>
struct __forward_begin_node
{
    typedef _NodePtr pointer;

    pointer __next_;

     _LIBCPP_INLINE_VISIBILITY __forward_begin_node() : __next_(nullptr) {}
};

template <class _Tp, class _VoidPtr>
struct _LIBCPP_HIDDEN __begin_node_of
{
    typedef __forward_begin_node
        <
             typename pointer_traits<_VoidPtr>::template
#ifndef _LIBCPP_HAS_NO_TEMPLATE_ALIASES
                 rebind<__forward_list_node<_Tp, _VoidPtr> >
#else
                 rebind<__forward_list_node<_Tp, _VoidPtr> >::other
#endif
         > type;
};

template <class _Tp, class _VoidPtr>
struct __forward_list_node
    : public __begin_node_of<_Tp, _VoidPtr>::type
{
    typedef _Tp value_type;

    value_type __value_;
};

template <class _Tp, class _Alloc = allocator<_Tp> > class _LIBCPP_TYPE_VIS_ONLY forward_list;
template<class _NodeConstPtr> class _LIBCPP_TYPE_VIS_ONLY __forward_list_const_iterator;

template <class _NodePtr>
class _LIBCPP_TYPE_VIS_ONLY __forward_list_iterator
{
    typedef _NodePtr __node_pointer;

    __node_pointer __ptr_;

    _LIBCPP_INLINE_VISIBILITY
    explicit __forward_list_iterator(__node_pointer __p) _NOEXCEPT : __ptr_(__p) {}

    template<class, class> friend class _LIBCPP_TYPE_VIS_ONLY forward_list;
    template<class> friend class _LIBCPP_TYPE_VIS_ONLY __forward_list_const_iterator;

public:
    typedef forward_iterator_tag                              iterator_category;
    typedef typename pointer_traits<__node_pointer>::element_type::value_type
                                                              value_type;
    typedef value_type&                                       reference;
    typedef typename pointer_traits<__node_pointer>::difference_type
                                                              difference_type;
    typedef typename pointer_traits<__node_pointer>::template
#ifndef _LIBCPP_HAS_NO_TEMPLATE_ALIASES
            rebind<value_type>
#else
            rebind<value_type>::other
#endif
                                                              pointer;

    _LIBCPP_INLINE_VISIBILITY
    __forward_list_iterator() _NOEXCEPT : __ptr_(nullptr) {}

    _LIBCPP_INLINE_VISIBILITY
    reference operator*() const {return __ptr_->__value_;}
    _LIBCPP_INLINE_VISIBILITY
    pointer operator->() const {return pointer_traits<pointer>::pointer_to(__ptr_->__value_);}

    _LIBCPP_INLINE_VISIBILITY
    __forward_list_iterator& operator++()
    {
        __ptr_ = __ptr_->__next_;
        return *this;
    }
    _LIBCPP_INLINE_VISIBILITY
    __forward_list_iterator operator++(int)
    {
        __forward_list_iterator __t(*this);
        ++(*this);
        return __t;
    }

    friend _LIBCPP_INLINE_VISIBILITY
    bool operator==(const __forward_list_iterator& __x,
                    const __forward_list_iterator& __y)
        {return __x.__ptr_ == __y.__ptr_;}
    friend _LIBCPP_INLINE_VISIBILITY
    bool operator!=(const __forward_list_iterator& __x,
                    const __forward_list_iterator& __y)
        {return !(__x == __y);}
};

template <class _NodeConstPtr>
class _LIBCPP_TYPE_VIS_ONLY __forward_list_const_iterator
{
    typedef _NodeConstPtr __node_const_pointer;

    __node_const_pointer __ptr_;

    _LIBCPP_INLINE_VISIBILITY
    explicit __forward_list_const_iterator(__node_const_pointer __p) _NOEXCEPT
        : __ptr_(__p) {}

    typedef typename remove_const
        <
            typename pointer_traits<__node_const_pointer>::element_type
        >::type                                               __node;
    typedef typename pointer_traits<__node_const_pointer>::template
#ifndef _LIBCPP_HAS_NO_TEMPLATE_ALIASES
            rebind<__node>
#else
            rebind<__node>::other
#endif
                                                              __node_pointer;

    template<class, class> friend class forward_list;

public:
    typedef forward_iterator_tag                              iterator_category;
    typedef typename __node::value_type                       value_type;
    typedef const value_type&                                 reference;
    typedef typename pointer_traits<__node_const_pointer>::difference_type
                                                              difference_type;
    typedef typename pointer_traits<__node_const_pointer>::template
#ifndef _LIBCPP_HAS_NO_TEMPLATE_ALIASES
            rebind<const value_type>
#else
            rebind<const value_type>::other
#endif
                                                              pointer;

    _LIBCPP_INLINE_VISIBILITY
    __forward_list_const_iterator() _NOEXCEPT : __ptr_(nullptr) {}
    _LIBCPP_INLINE_VISIBILITY
    __forward_list_const_iterator(__forward_list_iterator<__node_pointer> __p) _NOEXCEPT
        : __ptr_(__p.__ptr_) {}

    _LIBCPP_INLINE_VISIBILITY
    reference operator*() const {return __ptr_->__value_;}
    _LIBCPP_INLINE_VISIBILITY
    pointer operator->() const {return pointer_traits<pointer>::pointer_to(__ptr_->__value_);}

    _LIBCPP_INLINE_VISIBILITY
    __forward_list_const_iterator& operator++()
    {
        __ptr_ = __ptr_->__next_;
        return *this;
    }
    _LIBCPP_INLINE_VISIBILITY
    __forward_list_const_iterator operator++(int)
    {
        __forward_list_const_iterator __t(*this);
        ++(*this);
        return __t;
    }

    friend _LIBCPP_INLINE_VISIBILITY
    bool operator==(const __forward_list_const_iterator& __x,
                    const __forward_list_const_iterator& __y)
        {return __x.__ptr_ == __y.__ptr_;}
    friend _LIBCPP_INLINE_VISIBILITY
    bool operator!=(const __forward_list_const_iterator& __x,
                           const __forward_list_const_iterator& __y)
        {return !(__x == __y);}
};

template <class _Tp, class _Alloc>
class __forward_list_base
{
protected:
    typedef _Tp    value_type;
    typedef _Alloc allocator_type;

    typedef typename allocator_traits<allocator_type>::void_pointer  void_pointer;
    typedef __forward_list_node<value_type, void_pointer>            __node;
    typedef typename __begin_node_of<value_type, void_pointer>::type __begin_node;
    typedef typename __rebind_alloc_helper<allocator_traits<allocator_type>, __node>::type __node_allocator;
    typedef allocator_traits<__node_allocator>        __node_traits;
    typedef typename __node_traits::pointer           __node_pointer;
    typedef typename __node_traits::pointer           __node_const_pointer;

    typedef typename __rebind_alloc_helper<allocator_traits<allocator_type>, __begin_node>::type __begin_node_allocator;
    typedef typename allocator_traits<__begin_node_allocator>::pointer __begin_node_pointer;

    __compressed_pair<__begin_node, __node_allocator> __before_begin_;

    _LIBCPP_INLINE_VISIBILITY
    __node_pointer        __before_begin() _NOEXCEPT
        {return static_cast<__node_pointer>(pointer_traits<__begin_node_pointer>::
                                        pointer_to(__before_begin_.first()));}
    _LIBCPP_INLINE_VISIBILITY
    __node_const_pointer  __before_begin() const _NOEXCEPT
        {return static_cast<__node_const_pointer>(pointer_traits<__begin_node_pointer>::
                                        pointer_to(const_cast<__begin_node&>(__before_begin_.first())));}

    _LIBCPP_INLINE_VISIBILITY
          __node_allocator& __alloc() _NOEXCEPT
            {return __before_begin_.second();}
    _LIBCPP_INLINE_VISIBILITY
    const __node_allocator& __alloc() const _NOEXCEPT
        {return __before_begin_.second();}

    typedef __forward_list_iterator<__node_pointer>             iterator;
    typedef __forward_list_const_iterator<__node_pointer>       const_iterator;

    _LIBCPP_INLINE_VISIBILITY
    __forward_list_base()
        _NOEXCEPT_(is_nothrow_default_constructible<__node_allocator>::value)
        : __before_begin_(__begin_node()) {}
    _LIBCPP_INLINE_VISIBILITY
    __forward_list_base(const allocator_type& __a)
        : __before_begin_(__begin_node(), __node_allocator(__a)) {}

#ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES
public:
    __forward_list_base(__forward_list_base&& __x)
        _NOEXCEPT_(is_nothrow_move_constructible<__node_allocator>::value);
    __forward_list_base(__forward_list_base&& __x, const allocator_type& __a);
#endif  // _LIBCPP_HAS_NO_RVALUE_REFERENCES

private:
    __forward_list_base(const __forward_list_base&);
    __forward_list_base& operator=(const __forward_list_base&);

public:
    ~__forward_list_base();

protected:
    _LIBCPP_INLINE_VISIBILITY
    void __copy_assign_alloc(const __forward_list_base& __x)
        {__copy_assign_alloc(__x, integral_constant<bool,
              __node_traits::propagate_on_container_copy_assignment::value>());}

    _LIBCPP_INLINE_VISIBILITY
    void __move_assign_alloc(__forward_list_base& __x)
        _NOEXCEPT_(!__node_traits::propagate_on_container_move_assignment::value ||
                   is_nothrow_move_assignable<__node_allocator>::value)
        {__move_assign_alloc(__x, integral_constant<bool,
              __node_traits::propagate_on_container_move_assignment::value>());}

public:
    void swap(__forward_list_base& __x)
#if _LIBCPP_STD_VER >= 14
        _NOEXCEPT;
#else
        _NOEXCEPT_(!__node_traits::propagate_on_container_move_assignment::value || 
                    __is_nothrow_swappable<__node_allocator>::value);
#endif
protected:
    void clear() _NOEXCEPT;

private:
    _LIBCPP_INLINE_VISIBILITY
    void __copy_assign_alloc(const __forward_list_base&, false_type) {}
    _LIBCPP_INLINE_VISIBILITY
    void __copy_assign_alloc(const __forward_list_base& __x, true_type)
    {
        if (__alloc() != __x.__alloc())
            clear();
        __alloc() = __x.__alloc();
    }

    _LIBCPP_INLINE_VISIBILITY
    void __move_assign_alloc(__forward_list_base& __x, false_type) _NOEXCEPT
        {}
    _LIBCPP_INLINE_VISIBILITY
    void __move_assign_alloc(__forward_list_base& __x, true_type)
        _NOEXCEPT_(is_nothrow_move_assignable<__node_allocator>::value)
        {__alloc() = _VSTD::move(__x.__alloc());}
};

#ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES

template <class _Tp, class _Alloc>
inline _LIBCPP_INLINE_VISIBILITY
__forward_list_base<_Tp, _Alloc>::__forward_list_base(__forward_list_base&& __x)
        _NOEXCEPT_(is_nothrow_move_constructible<__node_allocator>::value)
    : __before_begin_(_VSTD::move(__x.__before_begin_))
{
    __x.__before_begin()->__next_ = nullptr;
}

template <class _Tp, class _Alloc>
inline _LIBCPP_INLINE_VISIBILITY
__forward_list_base<_Tp, _Alloc>::__forward_list_base(__forward_list_base&& __x,
                                                      const allocator_type& __a)
    : __before_begin_(__begin_node(), __node_allocator(__a))
{
    if (__alloc() == __x.__alloc())
    {
        __before_begin()->__next_ = __x.__before_begin()->__next_;
        __x.__before_begin()->__next_ = nullptr;
    }
}

#endif  // _LIBCPP_HAS_NO_RVALUE_REFERENCES

template <class _Tp, class _Alloc>
__forward_list_base<_Tp, _Alloc>::~__forward_list_base()
{
    clear();
}

template <class _Tp, class _Alloc>
inline _LIBCPP_INLINE_VISIBILITY
void
__forward_list_base<_Tp, _Alloc>::swap(__forward_list_base& __x)
#if _LIBCPP_STD_VER >= 14
        _NOEXCEPT
#else
        _NOEXCEPT_(!__node_traits::propagate_on_container_move_assignment::value || 
                    __is_nothrow_swappable<__node_allocator>::value)
#endif
{
    __swap_allocator(__alloc(), __x.__alloc(), 
            integral_constant<bool, __node_traits::propagate_on_container_swap::value>());
    using _VSTD::swap;
    swap(__before_begin()->__next_, __x.__before_begin()->__next_);
}

template <class _Tp, class _Alloc>
void
__forward_list_base<_Tp, _Alloc>::clear() _NOEXCEPT
{
    __node_allocator& __a = __alloc();
    for (__node_pointer __p = __before_begin()->__next_; __p != nullptr;)
    {
        __node_pointer __next = __p->__next_;
        __node_traits::destroy(__a, _VSTD::addressof(__p->__value_));
        __node_traits::deallocate(__a, __p, 1);
        __p = __next;
    }
    __before_begin()->__next_ = nullptr;
}

template <class _Tp, class _Alloc /*= allocator<_Tp>*/>
class _LIBCPP_TYPE_VIS_ONLY forward_list
    : private __forward_list_base<_Tp, _Alloc>
{
    typedef __forward_list_base<_Tp, _Alloc> base;
    typedef typename base::__node_allocator  __node_allocator;
    typedef typename base::__node            __node;
    typedef typename base::__node_traits     __node_traits;
    typedef typename base::__node_pointer    __node_pointer;

public:
    typedef _Tp    value_type;
    typedef _Alloc allocator_type;

    typedef value_type&                                                reference;
    typedef const value_type&                                          const_reference;
    typedef typename allocator_traits<allocator_type>::pointer         pointer;
    typedef typename allocator_traits<allocator_type>::const_pointer   const_pointer;
    typedef typename allocator_traits<allocator_type>::size_type       size_type;
    typedef typename allocator_traits<allocator_type>::difference_type difference_type;

    typedef typename base::iterator       iterator;
    typedef typename base::const_iterator const_iterator;

    _LIBCPP_INLINE_VISIBILITY
    forward_list()
        _NOEXCEPT_(is_nothrow_default_constructible<__node_allocator>::value)
        {} // = default;
    explicit forward_list(const allocator_type& __a);
    explicit forward_list(size_type __n);
#if _LIBCPP_STD_VER > 11
    explicit forward_list(size_type __n, const allocator_type& __a);
#endif
    forward_list(size_type __n, const value_type& __v);
    forward_list(size_type __n, const value_type& __v, const allocator_type& __a);
    template <class _InputIterator>
        forward_list(_InputIterator __f, _InputIterator __l,
                     typename enable_if<
                       __is_input_iterator<_InputIterator>::value
                     >::type* = nullptr);
    template <class _InputIterator>
        forward_list(_InputIterator __f, _InputIterator __l,
                     const allocator_type& __a,
                     typename enable_if<
                       __is_input_iterator<_InputIterator>::value
                     >::type* = nullptr);
    forward_list(const forward_list& __x);
    forward_list(const forward_list& __x, const allocator_type& __a);
#ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES
    _LIBCPP_INLINE_VISIBILITY
    forward_list(forward_list&& __x)
        _NOEXCEPT_(is_nothrow_move_constructible<base>::value)
        : base(_VSTD::move(__x)) {}
    forward_list(forward_list&& __x, const allocator_type& __a);
#endif  // _LIBCPP_HAS_NO_RVALUE_REFERENCES
#ifndef _LIBCPP_HAS_NO_GENERALIZED_INITIALIZERS
    forward_list(initializer_list<value_type> __il);
    forward_list(initializer_list<value_type> __il, const allocator_type& __a);
#endif  // _LIBCPP_HAS_NO_GENERALIZED_INITIALIZERS

    // ~forward_list() = default;

    forward_list& operator=(const forward_list& __x);
#ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES
    forward_list& operator=(forward_list&& __x)
        _NOEXCEPT_(
             __node_traits::propagate_on_container_move_assignment::value &&
             is_nothrow_move_assignable<allocator_type>::value);
#endif
#ifndef  _LIBCPP_HAS_NO_GENERALIZED_INITIALIZERS
    forward_list& operator=(initializer_list<value_type> __il);
#endif  // _LIBCPP_HAS_NO_GENERALIZED_INITIALIZERS

    template <class _InputIterator>
        typename enable_if
        <
            __is_input_iterator<_InputIterator>::value,
            void
        >::type
        assign(_InputIterator __f, _InputIterator __l);
    void assign(size_type __n, const value_type& __v);
#ifndef _LIBCPP_HAS_NO_GENERALIZED_INITIALIZERS
    void assign(initializer_list<value_type> __il);
#endif  // _LIBCPP_HAS_NO_GENERALIZED_INITIALIZERS

    _LIBCPP_INLINE_VISIBILITY
    allocator_type get_allocator() const _NOEXCEPT
        {return allocator_type(base::__alloc());}

    _LIBCPP_INLINE_VISIBILITY
    iterator       begin() _NOEXCEPT
        {return       iterator(base::__before_begin()->__next_);}
    _LIBCPP_INLINE_VISIBILITY
    const_iterator begin() const _NOEXCEPT
        {return const_iterator(base::__before_begin()->__next_);}
    _LIBCPP_INLINE_VISIBILITY
    iterator       end() _NOEXCEPT
        {return       iterator(nullptr);}
    _LIBCPP_INLINE_VISIBILITY
    const_iterator end() const _NOEXCEPT
        {return const_iterator(nullptr);}

    _LIBCPP_INLINE_VISIBILITY
    const_iterator cbegin() const _NOEXCEPT
        {return const_iterator(base::__before_begin()->__next_);}
    _LIBCPP_INLINE_VISIBILITY
    const_iterator cend() const _NOEXCEPT
        {return const_iterator(nullptr);}

    _LIBCPP_INLINE_VISIBILITY
    iterator       before_begin() _NOEXCEPT
        {return       iterator(base::__before_begin());}
    _LIBCPP_INLINE_VISIBILITY
    const_iterator before_begin() const _NOEXCEPT
        {return const_iterator(base::__before_begin());}
    _LIBCPP_INLINE_VISIBILITY
    const_iterator cbefore_begin() const _NOEXCEPT
        {return const_iterator(base::__before_begin());}

    _LIBCPP_INLINE_VISIBILITY
    bool empty() const _NOEXCEPT
        {return base::__before_begin()->__next_ == nullptr;}
    _LIBCPP_INLINE_VISIBILITY
    size_type max_size() const _NOEXCEPT
        {return numeric_limits<size_type>::max();}

    _LIBCPP_INLINE_VISIBILITY
    reference       front()       {return base::__before_begin()->__next_->__value_;}
    _LIBCPP_INLINE_VISIBILITY
    const_reference front() const {return base::__before_begin()->__next_->__value_;}

#ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES
#ifndef _LIBCPP_HAS_NO_VARIADICS
    template <class... _Args> void emplace_front(_Args&&... __args);
#endif
    void push_front(value_type&& __v);
#endif  // _LIBCPP_HAS_NO_RVALUE_REFERENCES
    void push_front(const value_type& __v);

    void pop_front();

#ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES
#ifndef _LIBCPP_HAS_NO_VARIADICS
    template <class... _Args>
        iterator emplace_after(const_iterator __p, _Args&&... __args);
#endif  // _LIBCPP_HAS_NO_VARIADICS
    iterator insert_after(const_iterator __p, value_type&& __v);
#endif  // _LIBCPP_HAS_NO_RVALUE_REFERENCES
    iterator insert_after(const_iterator __p, const value_type& __v);
    iterator insert_after(const_iterator __p, size_type __n, const value_type& __v);
    template <class _InputIterator>
        _LIBCPP_INLINE_VISIBILITY
        typename enable_if
        <
            __is_input_iterator<_InputIterator>::value,
            iterator
        >::type
        insert_after(const_iterator __p, _InputIterator __f, _InputIterator __l);
#ifndef _LIBCPP_HAS_NO_GENERALIZED_INITIALIZERS
    iterator insert_after(const_iterator __p, initializer_list<value_type> __il)
        {return insert_after(__p, __il.begin(), __il.end());}
#endif  // _LIBCPP_HAS_NO_GENERALIZED_INITIALIZERS

    iterator erase_after(const_iterator __p);
    iterator erase_after(const_iterator __f, const_iterator __l);

    _LIBCPP_INLINE_VISIBILITY
    void swap(forward_list& __x)
#if _LIBCPP_STD_VER >= 14
        _NOEXCEPT
#else
        _NOEXCEPT_(!__node_traits::propagate_on_container_swap::value ||
                   __is_nothrow_swappable<__node_allocator>::value)
#endif
        {base::swap(__x);}

    void resize(size_type __n);
    void resize(size_type __n, const value_type& __v);
    _LIBCPP_INLINE_VISIBILITY
    void clear() _NOEXCEPT {base::clear();}

#ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES
    _LIBCPP_INLINE_VISIBILITY
    void splice_after(const_iterator __p, forward_list&& __x);
    _LIBCPP_INLINE_VISIBILITY
    void splice_after(const_iterator __p, forward_list&& __x, const_iterator __i);
    _LIBCPP_INLINE_VISIBILITY
    void splice_after(const_iterator __p, forward_list&& __x,
                      const_iterator __f, const_iterator __l);
#endif  // _LIBCPP_HAS_NO_RVALUE_REFERENCES
    void splice_after(const_iterator __p, forward_list& __x);
    void splice_after(const_iterator __p, forward_list& __x, const_iterator __i);
    void splice_after(const_iterator __p, forward_list& __x,
                      const_iterator __f, const_iterator __l);
    void remove(const value_type& __v);
    template <class _Predicate> void remove_if(_Predicate __pred);
    _LIBCPP_INLINE_VISIBILITY
    void unique() {unique(__equal_to<value_type>());}
    template <class _BinaryPredicate> void unique(_BinaryPredicate __binary_pred);
#ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES
    _LIBCPP_INLINE_VISIBILITY
    void merge(forward_list&& __x) {merge(__x, __less<value_type>());}
    template <class _Compare>
        _LIBCPP_INLINE_VISIBILITY
        void merge(forward_list&& __x, _Compare __comp)
        {merge(__x, _VSTD::move(__comp));}
#endif  // _LIBCPP_HAS_NO_RVALUE_REFERENCES
    _LIBCPP_INLINE_VISIBILITY
    void merge(forward_list& __x) {merge(__x, __less<value_type>());}
    template <class _Compare> void merge(forward_list& __x, _Compare __comp);
    _LIBCPP_INLINE_VISIBILITY
    void sort() {sort(__less<value_type>());}
    template <class _Compare> void sort(_Compare __comp);
    void reverse() _NOEXCEPT;

private:

#ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES
    void __move_assign(forward_list& __x, true_type)
        _NOEXCEPT_(is_nothrow_move_assignable<allocator_type>::value);
    void __move_assign(forward_list& __x, false_type);
#endif  // _LIBCPP_HAS_NO_RVALUE_REFERENCES

    template <class _Compare>
        static
        __node_pointer
        __merge(__node_pointer __f1, __node_pointer __f2, _Compare& __comp);

    template <class _Compare>
        static
        __node_pointer
        __sort(__node_pointer __f, difference_type __sz, _Compare& __comp);
};

template <class _Tp, class _Alloc>
inline _LIBCPP_INLINE_VISIBILITY
forward_list<_Tp, _Alloc>::forward_list(const allocator_type& __a)
    : base(__a)
{
}

template <class _Tp, class _Alloc>
forward_list<_Tp, _Alloc>::forward_list(size_type __n)
{
    if (__n > 0)
    {
        __node_allocator& __a = base::__alloc();
        typedef __allocator_destructor<__node_allocator> _Dp;
        unique_ptr<__node, _Dp> __h(nullptr, _Dp(__a, 1));
        for (__node_pointer __p = base::__before_begin(); __n > 0; --__n,
                                                             __p = __p->__next_)
        {
            __h.reset(__node_traits::allocate(__a, 1));
            __node_traits::construct(__a, _VSTD::addressof(__h->__value_));
            __h->__next_ = nullptr;
            __p->__next_ = __h.release();
        }
    }
}

#if _LIBCPP_STD_VER > 11
template <class _Tp, class _Alloc>
forward_list<_Tp, _Alloc>::forward_list(size_type __n, const allocator_type& __a)
    : base ( __a )
{
    if (__n > 0)
    {
        __node_allocator& __a = base::__alloc();
        typedef __allocator_destructor<__node_allocator> _Dp;
        unique_ptr<__node, _Dp> __h(nullptr, _Dp(__a, 1));
        for (__node_pointer __p = base::__before_begin(); __n > 0; --__n,
                                                             __p = __p->__next_)
        {
            __h.reset(__node_traits::allocate(__a, 1));
            __node_traits::construct(__a, _VSTD::addressof(__h->__value_));
            __h->__next_ = nullptr;
            __p->__next_ = __h.release();
        }
    }
}
#endif

template <class _Tp, class _Alloc>
forward_list<_Tp, _Alloc>::forward_list(size_type __n, const value_type& __v)
{
    insert_after(cbefore_begin(), __n, __v);
}

template <class _Tp, class _Alloc>
forward_list<_Tp, _Alloc>::forward_list(size_type __n, const value_type& __v,
                                        const allocator_type& __a)
    : base(__a)
{
    insert_after(cbefore_begin(), __n, __v);
}

template <class _Tp, class _Alloc>
template <class _InputIterator>
forward_list<_Tp, _Alloc>::forward_list(_InputIterator __f, _InputIterator __l,
                                        typename enable_if<
                                          __is_input_iterator<_InputIterator>::value
                                        >::type*)
{
    insert_after(cbefore_begin(), __f, __l);
}

template <class _Tp, class _Alloc>
template <class _InputIterator>
forward_list<_Tp, _Alloc>::forward_list(_InputIterator __f, _InputIterator __l,
                                        const allocator_type& __a,
                                        typename enable_if<
                                          __is_input_iterator<_InputIterator>::value
                                        >::type*)
    : base(__a)
{
    insert_after(cbefore_begin(), __f, __l);
}

template <class _Tp, class _Alloc>
forward_list<_Tp, _Alloc>::forward_list(const forward_list& __x)
    : base(allocator_type(
             __node_traits::select_on_container_copy_construction(__x.__alloc())
                         )
          )
{
    insert_after(cbefore_begin(), __x.begin(), __x.end());
}

template <class _Tp, class _Alloc>
forward_list<_Tp, _Alloc>::forward_list(const forward_list& __x,
                                        const allocator_type& __a)
    : base(__a)
{
    insert_after(cbefore_begin(), __x.begin(), __x.end());
}

#ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES

template <class _Tp, class _Alloc>
forward_list<_Tp, _Alloc>::forward_list(forward_list&& __x,
                                        const allocator_type& __a)
    : base(_VSTD::move(__x), __a)
{
    if (base::__alloc() != __x.__alloc())
    {
        typedef move_iterator<iterator> _Ip;
        insert_after(cbefore_begin(), _Ip(__x.begin()), _Ip(__x.end()));
    }
}

#endif  // _LIBCPP_HAS_NO_RVALUE_REFERENCES

#ifndef _LIBCPP_HAS_NO_GENERALIZED_INITIALIZERS

template <class _Tp, class _Alloc>
forward_list<_Tp, _Alloc>::forward_list(initializer_list<value_type> __il)
{
    insert_after(cbefore_begin(), __il.begin(), __il.end());
}

template <class _Tp, class _Alloc>
forward_list<_Tp, _Alloc>::forward_list(initializer_list<value_type> __il,
                                        const allocator_type& __a)
    : base(__a)
{
    insert_after(cbefore_begin(), __il.begin(), __il.end());
}

#endif  // _LIBCPP_HAS_NO_GENERALIZED_INITIALIZERS

template <class _Tp, class _Alloc>
forward_list<_Tp, _Alloc>&
forward_list<_Tp, _Alloc>::operator=(const forward_list& __x)
{
    if (this != &__x)
    {
        base::__copy_assign_alloc(__x);
        assign(__x.begin(), __x.end());
    }
    return *this;
}

#ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES

template <class _Tp, class _Alloc>
void
forward_list<_Tp, _Alloc>::__move_assign(forward_list& __x, true_type)
    _NOEXCEPT_(is_nothrow_move_assignable<allocator_type>::value)
{
    clear();
    base::__move_assign_alloc(__x);
    base::__before_begin()->__next_ = __x.__before_begin()->__next_;
    __x.__before_begin()->__next_ = nullptr;
}

template <class _Tp, class _Alloc>
void
forward_list<_Tp, _Alloc>::__move_assign(forward_list& __x, false_type)
{
    if (base::__alloc() == __x.__alloc())
        __move_assign(__x, true_type());
    else
    {
        typedef move_iterator<iterator> _Ip;
        assign(_Ip(__x.begin()), _Ip(__x.end()));
    }
}

template <class _Tp, class _Alloc>
inline _LIBCPP_INLINE_VISIBILITY
forward_list<_Tp, _Alloc>&
forward_list<_Tp, _Alloc>::operator=(forward_list&& __x)
    _NOEXCEPT_(
             __node_traits::propagate_on_container_move_assignment::value &&
             is_nothrow_move_assignable<allocator_type>::value)
{
    __move_assign(__x, integral_constant<bool,
          __node_traits::propagate_on_container_move_assignment::value>());
    return *this;
}

#endif  // _LIBCPP_HAS_NO_RVALUE_REFERENCES

#ifndef _LIBCPP_HAS_NO_GENERALIZED_INITIALIZERS

template <class _Tp, class _Alloc>
inline _LIBCPP_INLINE_VISIBILITY
forward_list<_Tp, _Alloc>&
forward_list<_Tp, _Alloc>::operator=(initializer_list<value_type> __il)
{
    assign(__il.begin(), __il.end());
    return *this;
}

#endif  // _LIBCPP_HAS_NO_GENERALIZED_INITIALIZERS

template <class _Tp, class _Alloc>
template <class _InputIterator>
typename enable_if
<
    __is_input_iterator<_InputIterator>::value,
    void
>::type
forward_list<_Tp, _Alloc>::assign(_InputIterator __f, _InputIterator __l)
{
    iterator __i = before_begin();
    iterator __j = _VSTD::next(__i);
    iterator __e = end();
    for (; __j != __e && __f != __l; ++__i, (void) ++__j, ++__f)
        *__j = *__f;
    if (__j == __e)
        insert_after(__i, __f, __l);
    else
        erase_after(__i, __e);
}

template <class _Tp, class _Alloc>
void
forward_list<_Tp, _Alloc>::assign(size_type __n, const value_type& __v)
{
    iterator __i = before_begin();
    iterator __j = _VSTD::next(__i);
    iterator __e = end();
    for (; __j != __e && __n > 0; --__n, ++__i, ++__j)
        *__j = __v;
    if (__j == __e)
        insert_after(__i, __n, __v);
    else
        erase_after(__i, __e);
}

#ifndef _LIBCPP_HAS_NO_GENERALIZED_INITIALIZERS

template <class _Tp, class _Alloc>
inline _LIBCPP_INLINE_VISIBILITY
void
forward_list<_Tp, _Alloc>::assign(initializer_list<value_type> __il)
{
    assign(__il.begin(), __il.end());
}

#endif  // _LIBCPP_HAS_NO_GENERALIZED_INITIALIZERS

#ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES
#ifndef _LIBCPP_HAS_NO_VARIADICS

template <class _Tp, class _Alloc>
template <class... _Args>
void
forward_list<_Tp, _Alloc>::emplace_front(_Args&&... __args)
{
    __node_allocator& __a = base::__alloc();
    typedef __allocator_destructor<__node_allocator> _Dp;
    unique_ptr<__node, _Dp> __h(__node_traits::allocate(__a, 1), _Dp(__a, 1));
    __node_traits::construct(__a, _VSTD::addressof(__h->__value_),
                                  _VSTD::forward<_Args>(__args)...);
    __h->__next_ = base::__before_begin()->__next_;
    base::__before_begin()->__next_ = __h.release();
}

#endif  // _LIBCPP_HAS_NO_VARIADICS

template <class _Tp, class _Alloc>
void
forward_list<_Tp, _Alloc>::push_front(value_type&& __v)
{
    __node_allocator& __a = base::__alloc();
    typedef __allocator_destructor<__node_allocator> _Dp;
    unique_ptr<__node, _Dp> __h(__node_traits::allocate(__a, 1), _Dp(__a, 1));
    __node_traits::construct(__a, _VSTD::addressof(__h->__value_), _VSTD::move(__v));
    __h->__next_ = base::__before_begin()->__next_;
    base::__before_begin()->__next_ = __h.release();
}

#endif  // _LIBCPP_HAS_NO_RVALUE_REFERENCES

template <class _Tp, class _Alloc>
void
forward_list<_Tp, _Alloc>::push_front(const value_type& __v)
{
    __node_allocator& __a = base::__alloc();
    typedef __allocator_destructor<__node_allocator> _Dp;
    unique_ptr<__node, _Dp> __h(__node_traits::allocate(__a, 1), _Dp(__a, 1));
    __node_traits::construct(__a, _VSTD::addressof(__h->__value_), __v);
    __h->__next_ = base::__before_begin()->__next_;
    base::__before_begin()->__next_ = __h.release();
}

template <class _Tp, class _Alloc>
void
forward_list<_Tp, _Alloc>::pop_front()
{
    __node_allocator& __a = base::__alloc();
    __node_pointer __p = base::__before_begin()->__next_;
    base::__before_begin()->__next_ = __p->__next_;
    __node_traits::destroy(__a, _VSTD::addressof(__p->__value_));
    __node_traits::deallocate(__a, __p, 1);
}

#ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES
#ifndef _LIBCPP_HAS_NO_VARIADICS

template <class _Tp, class _Alloc>
template <class... _Args>
typename forward_list<_Tp, _Alloc>::iterator
forward_list<_Tp, _Alloc>::emplace_after(const_iterator __p, _Args&&... __args)
{
    __node_pointer const __r = __p.__ptr_;
    __node_allocator& __a = base::__alloc();
    typedef __allocator_destructor<__node_allocator> _Dp;
    unique_ptr<__node, _Dp> __h(__node_traits::allocate(__a, 1), _Dp(__a, 1));
    __node_traits::construct(__a, _VSTD::addressof(__h->__value_),
                                  _VSTD::forward<_Args>(__args)...);
    __h->__next_ = __r->__next_;
    __r->__next_ = __h.release();
    return iterator(__r->__next_);
}

#endif  // _LIBCPP_HAS_NO_VARIADICS

template <class _Tp, class _Alloc>
typename forward_list<_Tp, _Alloc>::iterator
forward_list<_Tp, _Alloc>::insert_after(const_iterator __p, value_type&& __v)
{
    __node_pointer const __r = __p.__ptr_;
    __node_allocator& __a = base::__alloc();
    typedef __allocator_destructor<__node_allocator> _Dp;
    unique_ptr<__node, _Dp> __h(__node_traits::allocate(__a, 1), _Dp(__a, 1));
    __node_traits::construct(__a, _VSTD::addressof(__h->__value_), _VSTD::move(__v));
    __h->__next_ = __r->__next_;
    __r->__next_ = __h.release();
    return iterator(__r->__next_);
}

#endif  // _LIBCPP_HAS_NO_RVALUE_REFERENCES

template <class _Tp, class _Alloc>
typename forward_list<_Tp, _Alloc>::iterator
forward_list<_Tp, _Alloc>::insert_after(const_iterator __p, const value_type& __v)
{
    __node_pointer const __r = __p.__ptr_;
    __node_allocator& __a = base::__alloc();
    typedef __allocator_destructor<__node_allocator> _Dp;
    unique_ptr<__node, _Dp> __h(__node_traits::allocate(__a, 1), _Dp(__a, 1));
    __node_traits::construct(__a, _VSTD::addressof(__h->__value_), __v);
    __h->__next_ = __r->__next_;
    __r->__next_ = __h.release();
    return iterator(__r->__next_);
}

template <class _Tp, class _Alloc>
typename forward_list<_Tp, _Alloc>::iterator
forward_list<_Tp, _Alloc>::insert_after(const_iterator __p, size_type __n,
                                        const value_type& __v)
{
    __node_pointer __r = __p.__ptr_;
    if (__n > 0)
    {
        __node_allocator& __a = base::__alloc();
        typedef __allocator_destructor<__node_allocator> _Dp;
        unique_ptr<__node, _Dp> __h(__node_traits::allocate(__a, 1), _Dp(__a, 1));
        __node_traits::construct(__a, _VSTD::addressof(__h->__value_), __v);
        __node_pointer __first = __h.release();
        __node_pointer __last = __first;
#ifndef _LIBCPP_NO_EXCEPTIONS
        try
        {
#endif  // _LIBCPP_NO_EXCEPTIONS
            for (--__n; __n != 0; --__n, __last = __last->__next_)
            {
                __h.reset(__node_traits::allocate(__a, 1));
                __node_traits::construct(__a, _VSTD::addressof(__h->__value_), __v);
                __last->__next_ = __h.release();
            }
#ifndef _LIBCPP_NO_EXCEPTIONS
        }
        catch (...)
        {
            while (__first != nullptr)
            {
                __node_pointer __next = __first->__next_;
                __node_traits::destroy(__a, _VSTD::addressof(__first->__value_));
                __node_traits::deallocate(__a, __first, 1);
                __first = __next;
            }
            throw;
        }
#endif  // _LIBCPP_NO_EXCEPTIONS
        __last->__next_ = __r->__next_;
        __r->__next_ = __first;
        __r = __last;
    }
    return iterator(__r);
}

template <class _Tp, class _Alloc>
template <class _InputIterator>
typename enable_if
<
    __is_input_iterator<_InputIterator>::value,
    typename forward_list<_Tp, _Alloc>::iterator
>::type
forward_list<_Tp, _Alloc>::insert_after(const_iterator __p,
                                        _InputIterator __f, _InputIterator __l)
{
    __node_pointer __r = __p.__ptr_;
    if (__f != __l)
    {
        __node_allocator& __a = base::__alloc();
        typedef __allocator_destructor<__node_allocator> _Dp;
        unique_ptr<__node, _Dp> __h(__node_traits::allocate(__a, 1), _Dp(__a, 1));
        __node_traits::construct(__a, _VSTD::addressof(__h->__value_), *__f);
        __node_pointer __first = __h.release();
        __node_pointer __last = __first;
#ifndef _LIBCPP_NO_EXCEPTIONS
        try
        {
#endif  // _LIBCPP_NO_EXCEPTIONS
            for (++__f; __f != __l; ++__f, ((void)(__last = __last->__next_)))
            {
                __h.reset(__node_traits::allocate(__a, 1));
                __node_traits::construct(__a, _VSTD::addressof(__h->__value_), *__f);
                __last->__next_ = __h.release();
            }
#ifndef _LIBCPP_NO_EXCEPTIONS
        }
        catch (...)
        {
            while (__first != nullptr)
            {
                __node_pointer __next = __first->__next_;
                __node_traits::destroy(__a, _VSTD::addressof(__first->__value_));
                __node_traits::deallocate(__a, __first, 1);
                __first = __next;
            }
            throw;
        }
#endif  // _LIBCPP_NO_EXCEPTIONS
        __last->__next_ = __r->__next_;
        __r->__next_ = __first;
        __r = __last;
    }
    return iterator(__r);
}

template <class _Tp, class _Alloc>
typename forward_list<_Tp, _Alloc>::iterator
forward_list<_Tp, _Alloc>::erase_after(const_iterator __f)
{
    __node_pointer __p = __f.__ptr_;
    __node_pointer __n = __p->__next_;
    __p->__next_ = __n->__next_;
    __node_allocator& __a = base::__alloc();
    __node_traits::destroy(__a, _VSTD::addressof(__n->__value_));
    __node_traits::deallocate(__a, __n, 1);
    return iterator(__p->__next_);
}

template <class _Tp, class _Alloc>
typename forward_list<_Tp, _Alloc>::iterator
forward_list<_Tp, _Alloc>::erase_after(const_iterator __f, const_iterator __l)
{
    __node_pointer __e = __l.__ptr_;
    if (__f != __l)
    {
        __node_pointer __p = __f.__ptr_;
        __node_pointer __n = __p->__next_;
        if (__n != __e)
        {
            __p->__next_ = __e;
            __node_allocator& __a = base::__alloc();
            do
            {
                __p = __n->__next_;
                __node_traits::destroy(__a, _VSTD::addressof(__n->__value_));
                __node_traits::deallocate(__a, __n, 1);
                __n = __p;
            } while (__n != __e);
        }
    }
    return iterator(__e);
}

template <class _Tp, class _Alloc>
void
forward_list<_Tp, _Alloc>::resize(size_type __n)
{
    size_type __sz = 0;
    iterator __p = before_begin();
    iterator __i = begin();
    iterator __e = end();
    for (; __i != __e && __sz < __n; ++__p, ++__i, ++__sz)
        ;
    if (__i != __e)
        erase_after(__p, __e);
    else
    {
        __n -= __sz;
        if (__n > 0)
        {
            __node_allocator& __a = base::__alloc();
            typedef __allocator_destructor<__node_allocator> _Dp;
            unique_ptr<__node, _Dp> __h(nullptr, _Dp(__a, 1));
            for (__node_pointer __ptr = __p.__ptr_; __n > 0; --__n,
                                                         __ptr = __ptr->__next_)
            {
                __h.reset(__node_traits::allocate(__a, 1));
                __node_traits::construct(__a, _VSTD::addressof(__h->__value_));
                __h->__next_ = nullptr;
                __ptr->__next_ = __h.release();
            }
        }
    }
}

template <class _Tp, class _Alloc>
void
forward_list<_Tp, _Alloc>::resize(size_type __n, const value_type& __v)
{
    size_type __sz = 0;
    iterator __p = before_begin();
    iterator __i = begin();
    iterator __e = end();
    for (; __i != __e && __sz < __n; ++__p, ++__i, ++__sz)
        ;
    if (__i != __e)
        erase_after(__p, __e);
    else
    {
        __n -= __sz;
        if (__n > 0)
        {
            __node_allocator& __a = base::__alloc();
            typedef __allocator_destructor<__node_allocator> _Dp;
            unique_ptr<__node, _Dp> __h(nullptr, _Dp(__a, 1));
            for (__node_pointer __ptr = __p.__ptr_; __n > 0; --__n,
                                                         __ptr = __ptr->__next_)
            {
                __h.reset(__node_traits::allocate(__a, 1));
                __node_traits::construct(__a, _VSTD::addressof(__h->__value_), __v);
                __h->__next_ = nullptr;
                __ptr->__next_ = __h.release();
            }
        }
    }
}

template <class _Tp, class _Alloc>
void
forward_list<_Tp, _Alloc>::splice_after(const_iterator __p,
                                        forward_list& __x)
{
    if (!__x.empty())
    {
        if (__p.__ptr_->__next_ != nullptr)
        {
            const_iterator __lm1 = __x.before_begin();
            while (__lm1.__ptr_->__next_ != nullptr)
                ++__lm1;
            __lm1.__ptr_->__next_ = __p.__ptr_->__next_;
        }
        __p.__ptr_->__next_ = __x.__before_begin()->__next_;
        __x.__before_begin()->__next_ = nullptr;
    }
}

template <class _Tp, class _Alloc>
void
forward_list<_Tp, _Alloc>::splice_after(const_iterator __p,
                                        forward_list& __x,
                                        const_iterator __i)
{
    const_iterator __lm1 = _VSTD::next(__i);
    if (__p != __i && __p != __lm1)
    {
        __i.__ptr_->__next_ = __lm1.__ptr_->__next_;
        __lm1.__ptr_->__next_ = __p.__ptr_->__next_;
        __p.__ptr_->__next_ = __lm1.__ptr_;
    }
}

template <class _Tp, class _Alloc>
void
forward_list<_Tp, _Alloc>::splice_after(const_iterator __p,
                                        forward_list& __x,
                                        const_iterator __f, const_iterator __l)
{
    if (__f != __l && __p != __f)
    {
        const_iterator __lm1 = __f;
        while (__lm1.__ptr_->__next_ != __l.__ptr_)
            ++__lm1;
        if (__f != __lm1)
        {
            __lm1.__ptr_->__next_ = __p.__ptr_->__next_;
            __p.__ptr_->__next_ = __f.__ptr_->__next_;
            __f.__ptr_->__next_ = __l.__ptr_;
        }
    }
}

#ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES

template <class _Tp, class _Alloc>
inline _LIBCPP_INLINE_VISIBILITY
void
forward_list<_Tp, _Alloc>::splice_after(const_iterator __p,
                                        forward_list&& __x)
{
    splice_after(__p, __x);
}

template <class _Tp, class _Alloc>
inline _LIBCPP_INLINE_VISIBILITY
void
forward_list<_Tp, _Alloc>::splice_after(const_iterator __p,
                                        forward_list&& __x,
                                        const_iterator __i)
{
    splice_after(__p, __x, __i);
}

template <class _Tp, class _Alloc>
inline _LIBCPP_INLINE_VISIBILITY
void
forward_list<_Tp, _Alloc>::splice_after(const_iterator __p,
                                        forward_list&& __x,
                                        const_iterator __f, const_iterator __l)
{
    splice_after(__p, __x, __f, __l);
}

#endif  // _LIBCPP_HAS_NO_RVALUE_REFERENCES

template <class _Tp, class _Alloc>
void
forward_list<_Tp, _Alloc>::remove(const value_type& __v)
{
    forward_list<_Tp, _Alloc> __deleted_nodes; // collect the nodes we're removing
    iterator __e = end();
    for (iterator __i = before_begin(); __i.__ptr_->__next_ != nullptr;)
    {
        if (__i.__ptr_->__next_->__value_ == __v)
        {
            iterator __j = _VSTD::next(__i, 2);
            for (; __j != __e && *__j == __v; ++__j)
                ;
            __deleted_nodes.splice_after(__deleted_nodes.before_begin(), *this, __i, __j);
            if (__j == __e)
                break;
            __i = __j;
        }
        else
            ++__i;
    }
}

template <class _Tp, class _Alloc>
template <class _Predicate>
void
forward_list<_Tp, _Alloc>::remove_if(_Predicate __pred)
{
    iterator __e = end();
    for (iterator __i = before_begin(); __i.__ptr_->__next_ != nullptr;)
    {
        if (__pred(__i.__ptr_->__next_->__value_))
        {
            iterator __j = _VSTD::next(__i, 2);
            for (; __j != __e && __pred(*__j); ++__j)
                ;
            erase_after(__i, __j);
            if (__j == __e)
                break;
            __i = __j;
        }
        else
            ++__i;
    }
}

template <class _Tp, class _Alloc>
template <class _BinaryPredicate>
void
forward_list<_Tp, _Alloc>::unique(_BinaryPredicate __binary_pred)
{
    for (iterator __i = begin(), __e = end(); __i != __e;)
    {
        iterator __j = _VSTD::next(__i);
        for (; __j != __e && __binary_pred(*__i, *__j); ++__j)
            ;
        if (__i.__ptr_->__next_ != __j.__ptr_)
            erase_after(__i, __j);
        __i = __j;
    }
}

template <class _Tp, class _Alloc>
template <class _Compare>
void
forward_list<_Tp, _Alloc>::merge(forward_list& __x, _Compare __comp)
{
    if (this != &__x)
    {
        base::__before_begin()->__next_ = __merge(base::__before_begin()->__next_,
                                                    __x.__before_begin()->__next_,
                                                    __comp);
        __x.__before_begin()->__next_ = nullptr;
    }
}

template <class _Tp, class _Alloc>
template <class _Compare>
typename forward_list<_Tp, _Alloc>::__node_pointer
forward_list<_Tp, _Alloc>::__merge(__node_pointer __f1, __node_pointer __f2,
                                   _Compare& __comp)
{
    if (__f1 == nullptr)
        return __f2;
    if (__f2 == nullptr)
        return __f1;
    __node_pointer __r;
    if (__comp(__f2->__value_, __f1->__value_))
    {
        __node_pointer __t = __f2;
        while (__t->__next_ != nullptr &&
                             __comp(__t->__next_->__value_, __f1->__value_))
            __t = __t->__next_;
        __r = __f2;
        __f2 = __t->__next_;
        __t->__next_ = __f1;
    }
    else
        __r = __f1;
    __node_pointer __p = __f1;
    __f1 = __f1->__next_;
    while (__f1 != nullptr && __f2 != nullptr)
    {
        if (__comp(__f2->__value_, __f1->__value_))
        {
            __node_pointer __t = __f2;
            while (__t->__next_ != nullptr &&
                                 __comp(__t->__next_->__value_, __f1->__value_))
                __t = __t->__next_;
            __p->__next_ = __f2;
            __f2 = __t->__next_;
            __t->__next_ = __f1;
        }
        __p = __f1;
        __f1 = __f1->__next_;
    }
    if (__f2 != nullptr)
        __p->__next_ = __f2;
    return __r;
}

template <class _Tp, class _Alloc>
template <class _Compare>
inline _LIBCPP_INLINE_VISIBILITY
void
forward_list<_Tp, _Alloc>::sort(_Compare __comp)
{
    base::__before_begin()->__next_ = __sort(base::__before_begin()->__next_,
                                       _VSTD::distance(begin(), end()), __comp);
}

template <class _Tp, class _Alloc>
template <class _Compare>
typename forward_list<_Tp, _Alloc>::__node_pointer
forward_list<_Tp, _Alloc>::__sort(__node_pointer __f1, difference_type __sz,
                                  _Compare& __comp)
{
    switch (__sz)
    {
    case 0:
    case 1:
        return __f1;
    case 2:
        if (__comp(__f1->__next_->__value_, __f1->__value_))
        {
            __node_pointer __t = __f1->__next_;
            __t->__next_ = __f1;
            __f1->__next_ = nullptr;
            __f1 = __t;
        }
        return __f1;
    }
    difference_type __sz1 = __sz / 2;
    difference_type __sz2 = __sz - __sz1;
    __node_pointer __t = _VSTD::next(iterator(__f1), __sz1 - 1).__ptr_;
    __node_pointer __f2 = __t->__next_;
    __t->__next_ = nullptr;
    return __merge(__sort(__f1, __sz1, __comp),
                   __sort(__f2, __sz2, __comp), __comp);
}

template <class _Tp, class _Alloc>
void
forward_list<_Tp, _Alloc>::reverse() _NOEXCEPT
{
    __node_pointer __p = base::__before_begin()->__next_;
    if (__p != nullptr)
    {
        __node_pointer __f = __p->__next_;
        __p->__next_ = nullptr;
        while (__f != nullptr)
        {
            __node_pointer __t = __f->__next_;
            __f->__next_ = __p;
            __p = __f;
            __f = __t;
        }
        base::__before_begin()->__next_ = __p;
    }
}

template <class _Tp, class _Alloc>
bool operator==(const forward_list<_Tp, _Alloc>& __x,
                const forward_list<_Tp, _Alloc>& __y)
{
    typedef forward_list<_Tp, _Alloc> _Cp;
    typedef typename _Cp::const_iterator _Ip;
    _Ip __ix = __x.begin();
    _Ip __ex = __x.end();
    _Ip __iy = __y.begin();
    _Ip __ey = __y.end();
    for (; __ix != __ex && __iy != __ey; ++__ix, ++__iy)
        if (!(*__ix == *__iy))
            return false;
    return (__ix == __ex) == (__iy == __ey);
}

template <class _Tp, class _Alloc>
inline _LIBCPP_INLINE_VISIBILITY
bool operator!=(const forward_list<_Tp, _Alloc>& __x,
                const forward_list<_Tp, _Alloc>& __y)
{
    return !(__x == __y);
}

template <class _Tp, class _Alloc>
inline _LIBCPP_INLINE_VISIBILITY
bool operator< (const forward_list<_Tp, _Alloc>& __x,
                const forward_list<_Tp, _Alloc>& __y)
{
    return _VSTD::lexicographical_compare(__x.begin(), __x.end(),
                                         __y.begin(), __y.end());
}

template <class _Tp, class _Alloc>
inline _LIBCPP_INLINE_VISIBILITY
bool operator> (const forward_list<_Tp, _Alloc>& __x,
                const forward_list<_Tp, _Alloc>& __y)
{
    return __y < __x;
}

template <class _Tp, class _Alloc>
inline _LIBCPP_INLINE_VISIBILITY
bool operator>=(const forward_list<_Tp, _Alloc>& __x,
                const forward_list<_Tp, _Alloc>& __y)
{
    return !(__x < __y);
}

template <class _Tp, class _Alloc>
inline _LIBCPP_INLINE_VISIBILITY
bool operator<=(const forward_list<_Tp, _Alloc>& __x,
                const forward_list<_Tp, _Alloc>& __y)
{
    return !(__y < __x);
}

template <class _Tp, class _Alloc>
inline _LIBCPP_INLINE_VISIBILITY
void
swap(forward_list<_Tp, _Alloc>& __x, forward_list<_Tp, _Alloc>& __y)
    _NOEXCEPT_(_NOEXCEPT_(__x.swap(__y)))
{
    __x.swap(__y);
}

_LIBCPP_END_NAMESPACE_STD

#endif  // _LIBCPP_FORWARD_LIST