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## Class template flat_set

boost::container::flat_set

## Synopsis

    // In header: <boost/container/flat_set.hpp>

    template<typename Key, typename Compare = std::less<Key>,
             typename AllocatorOrContainer = new_allocator<Key> >
    class flat_set {
    public:
      // types
      typedef Key                                                    key_type;
      typedef Compare                                                key_compare;
      typedef Key                                                    value_type;
      typedef implementation_defined                                 sequence_type;
      typedef sequence_type::allocator_type                          allocator_type;
      typedef ::boost::container::allocator_traits< allocator_type > allocator_traits_type;
      typedef sequence_type::pointer                                 pointer;
      typedef sequence_type::const_pointer                           const_pointer;
      typedef sequence_type::reference                               reference;
      typedef sequence_type::const_reference                         const_reference;
      typedef sequence_type::size_type                               size_type;
      typedef sequence_type::difference_type                         difference_type;
      typedef implementation_defined                                 stored_allocator_type;
      typedef implementation_defined                                 value_compare;
      typedef sequence_type::iterator                                iterator;
      typedef sequence_type::const_iterator                          const_iterator;
      typedef sequence_type::reverse_iterator                        reverse_iterator;
      typedef sequence_type::const_reverse_iterator                  const_reverse_iterator;

      // construct/copy/destruct
      flat_set() noexcept(dtl::is_nothrow_default_constructible< AllocatorOrContainer >::value &&dtl::is_nothrow_default_constructible< Compare >::value));
      explicit flat_set(const Compare &);
      explicit flat_set(const allocator_type &);
      flat_set(const Compare &, const allocator_type &);
      template<typename InputIterator> flat_set(InputIterator, InputIterator);
      template<typename InputIterator>
        flat_set(InputIterator, InputIterator, const allocator_type &);
      template<typename InputIterator>
        flat_set(InputIterator, InputIterator, const Compare &);
      template<typename InputIterator>
        flat_set(InputIterator, InputIterator, const Compare &,
                 const allocator_type &);
      template<typename InputIterator>
        flat_set(ordered_unique_range_t, InputIterator, InputIterator);
      template<typename InputIterator>
        flat_set(ordered_unique_range_t, InputIterator, InputIterator,
                 const Compare &);
      template<typename InputIterator>
        flat_set(ordered_unique_range_t, InputIterator, InputIterator,
                 const Compare &, const allocator_type &);
      template<typename InputIterator>
        flat_set(ordered_unique_range_t, InputIterator, InputIterator,
                 const allocator_type &);
      flat_set(std::initializer_list< value_type >);
      flat_set(std::initializer_list< value_type >, const allocator_type &);
      flat_set(std::initializer_list< value_type >, const Compare &);
      flat_set(std::initializer_list< value_type >, const Compare &,
               const allocator_type &);
      flat_set(ordered_unique_range_t, std::initializer_list< value_type >);
      flat_set(ordered_unique_range_t, std::initializer_list< value_type >,
               const Compare &);
      flat_set(ordered_unique_range_t, std::initializer_list< value_type >,
               const Compare &, const allocator_type &);
      flat_set(const flat_set &);
      flat_set(flat_set &&) noexcept(boost::container::dtl::is_nothrow_move_constructible< Compare >::value));
      flat_set(const flat_set &, const allocator_type &);
      flat_set(flat_set &&, const allocator_type &);
      flat_set & operator=(const flat_set &);
      flat_set & operator=(flat_set &&) noexcept((allocator_traits_type::propagate_on_container_move_assignment::value||allocator_traits_type::is_always_equal::value) &&boost::container::dtl::is_nothrow_move_assignable< Compare >::value));
      flat_set & operator=(std::initializer_list< value_type >);

      // public member functions
      allocator_type get_allocator() const noexcept;
      stored_allocator_type & get_stored_allocator() noexcept;
      const stored_allocator_type & get_stored_allocator() const noexcept;
      iterator begin() noexcept;
      const_iterator begin() const noexcept;
      iterator end() noexcept;
      const_iterator end() const noexcept;
      reverse_iterator rbegin() noexcept;
      const_reverse_iterator rbegin() const noexcept;
      reverse_iterator rend() noexcept;
      const_reverse_iterator rend() const noexcept;
      const_iterator cbegin() const noexcept;
      const_iterator cend() const noexcept;
      const_reverse_iterator crbegin() const noexcept;
      const_reverse_iterator crend() const noexcept;
      bool empty() const noexcept;
      size_type size() const noexcept;
      size_type max_size() const noexcept;
      size_type capacity() const noexcept;
      void reserve(size_type);
      void shrink_to_fit();
      template<class... Args> std::pair< iterator, bool > emplace(Args &&...);
      template<class... Args> iterator emplace_hint(const_iterator, Args &&...);
      std::pair< iterator, bool > insert(const value_type &);
      std::pair< iterator, bool > insert(value_type &&);
      iterator insert(const_iterator, const value_type &);
      iterator insert(const_iterator, value_type &&);
      template<typename InputIterator> void insert(InputIterator, InputIterator);
      template<typename InputIterator>
        void insert(ordered_unique_range_t, InputIterator, InputIterator);
      void insert(std::initializer_list< value_type >);
      void insert(ordered_unique_range_t, std::initializer_list< value_type >);
      template<typename C2>
        void merge(flat_set< Key, C2, AllocatorOrContainer > &);
      template<typename C2>
        void merge(flat_set< Key, C2, AllocatorOrContainer > &&);
      template<typename C2>
        void merge(flat_multiset< Key, C2, AllocatorOrContainer > &);
      template<typename C2>
        void merge(flat_multiset< Key, C2, AllocatorOrContainer > &&);
      iterator erase(const_iterator);
      size_type erase(const key_type &);
      iterator erase(const_iterator, const_iterator);
      void swap(flat_set &) noexcept(allocator_traits_type::is_always_equal::value &&boost::container::dtl::is_nothrow_swappable< Compare >::value));
      void clear() noexcept;
      key_compare key_comp() const;
      value_compare value_comp() const;
      iterator find(const key_type &);
      const_iterator find(const key_type &) const;
      template<typename K> iterator find(const K &);
      template<typename K> const_iterator find(const K &) const;
      iterator nth(size_type) noexcept;
      const_iterator nth(size_type) const noexcept;
      size_type index_of(iterator) noexcept;
      size_type index_of(const_iterator) const noexcept;
      size_type count(const key_type &) const;
      template<typename K> size_type count(const K &) const;
      bool contains(const key_type &) const;
      template<typename K> bool contains(const K &) const;
      iterator lower_bound(const key_type &);
      const_iterator lower_bound(const key_type &) const;
      template<typename K> iterator lower_bound(const K &);
      template<typename K> const_iterator lower_bound(const K &) const;
      iterator upper_bound(const key_type &);
      const_iterator upper_bound(const key_type &) const;
      template<typename K> iterator upper_bound(const K &);
      template<typename K> const_iterator upper_bound(const K &) const;
      std::pair< const_iterator, const_iterator >
      equal_range(const key_type &) const;
      std::pair< iterator, iterator > equal_range(const key_type &);
      template<typename K> std::pair< iterator, iterator > equal_range(const K &);
      template<typename K>
        std::pair< const_iterator, const_iterator > equal_range(const K &) const;
      sequence_type extract_sequence();
      void adopt_sequence(sequence_type &&);
      void adopt_sequence(ordered_unique_range_t, sequence_type &&);

      // friend functions
      friend bool operator==(const flat_set &, const flat_set &);
      friend bool operator!=(const flat_set &, const flat_set &);
      friend bool operator<(const flat_set &, const flat_set &);
      friend bool operator>(const flat_set &, const flat_set &);
      friend bool operator<=(const flat_set &, const flat_set &);
      friend bool operator>=(const flat_set &, const flat_set &);
      friend void swap(flat_set &, flat_set &);
    };

## Description

flat_set is a Sorted Associative Container that stores objects of type Key. It is also a Unique Associative Container, meaning that no two elements are the same.

flat_set is similar to std::set but it's implemented by as an ordered sequence container. The underlying sequence container is by default vector but it can also work user-provided vector-like SequenceContainers (like static_vector or small_vector).

Using vector-like sequence containers means that inserting a new element into a flat_set might invalidate previous iterators and references (unless that sequence container is stable_vector or a similar container that offers stable pointers and references). Similarly, erasing an element might invalidate iterators and references pointing to elements that come after (their keys are bigger) the erased element.

This container provides random-access iterators.

### Template Parameters

1.     typename Key

   is the type to be inserted in the set, which is also the key_type

2.     typename Compare = std::less<Key>

   is the comparison functor used to order keys

3.     typename AllocatorOrContainer = new_allocator<Key>

   is either:

   - The allocator to allocate `value_type`s (e.g. allocator< std::pair<Key, T> > ). (in this case sequence_type will be vector<value_type, AllocatorOrContainer>)

   - The SequenceContainer to be used as the underlying sequence_type. It must be a vector-like sequence container with random-access iterators.

### `flat_set` public construct/copy/destruct

1.     flat_set() noexcept(dtl::is_nothrow_default_constructible< AllocatorOrContainer >::value &&dtl::is_nothrow_default_constructible< Compare >::value));

   Effects: Default constructs an empty container.

   Complexity: Constant.

2.     explicit flat_set(const Compare & comp);

   Effects: Constructs an empty container using the specified comparison object.

   Complexity: Constant.

3.     explicit flat_set(const allocator_type & a);

   Effects: Constructs an empty container using the specified allocator.

   Complexity: Constant.

4.     flat_set(const Compare & comp, const allocator_type & a);

   Effects: Constructs an empty container using the specified comparison object and allocator.

   Complexity: Constant.

5.     template<typename InputIterator>
         flat_set(InputIterator first, InputIterator last);

   Effects: Constructs an empty container and inserts elements from the range [first ,last ).

   Complexity: Linear in N if the range [first ,last ) is already sorted using comp and otherwise N logN, where N is last - first.

6.     template<typename InputIterator>
         flat_set(InputIterator first, InputIterator last, const allocator_type & a);

   Effects: Constructs an empty container using the specified allocator, and inserts elements from the range [first ,last ).

   Complexity: Linear in N if the range [first ,last ) is already sorted using comp and otherwise N logN, where N is last - first.

7.     template<typename InputIterator>
         flat_set(InputIterator first, InputIterator last, const Compare & comp);

   Effects: Constructs an empty container using the specified comparison object and inserts elements from the range [first ,last ).

   Complexity: Linear in N if the range [first ,last ) is already sorted using comp and otherwise N logN, where N is last - first.

8.     template<typename InputIterator>
         flat_set(InputIterator first, InputIterator last, const Compare & comp,
                  const allocator_type & a);

   Effects: Constructs an empty container using the specified comparison object and allocator, and inserts elements from the range [first ,last ).

   Complexity: Linear in N if the range [first ,last ) is already sorted using comp and otherwise N logN, where N is last - first.

9.     template<typename InputIterator>
         flat_set(ordered_unique_range_t, InputIterator first, InputIterator last);

   Effects: Constructs an empty container and inserts elements from the ordered unique range [first ,last). This function is more efficient than the normal range creation for ordered ranges.

   Requires: [first ,last) must be ordered according to the predicate and must be unique values.

   Complexity: Linear in N.

   Note: Non-standard extension.

10.     template<typename InputIterator>
          flat_set(ordered_unique_range_t, InputIterator first, InputIterator last,
                   const Compare & comp);

    Effects: Constructs an empty container using the specified comparison object and inserts elements from the ordered unique range [first ,last). This function is more efficient than the normal range creation for ordered ranges.

    Requires: [first ,last) must be ordered according to the predicate and must be unique values.

    Complexity: Linear in N.

    Note: Non-standard extension.

11.     template<typename InputIterator>
          flat_set(ordered_unique_range_t, InputIterator first, InputIterator last,
                   const Compare & comp, const allocator_type & a);

    Effects: Constructs an empty container using the specified comparison object and allocator, and inserts elements from the ordered unique range [first ,last). This function is more efficient than the normal range creation for ordered ranges.

    Requires: [first ,last) must be ordered according to the predicate and must be unique values.

    Complexity: Linear in N.

    Note: Non-standard extension.

12.     template<typename InputIterator>
          flat_set(ordered_unique_range_t, InputIterator first, InputIterator last,
                   const allocator_type & a);

    Effects: Constructs an empty container using the specified allocator and inserts elements from the ordered unique range [first ,last). This function is more efficient than the normal range creation for ordered ranges.

    Requires: [first ,last) must be ordered according to the predicate and must be unique values.

    Complexity: Linear in N.

    Note: Non-standard extension.

13.     flat_set(std::initializer_list< value_type > il);

    Effects: Constructs an empty container and inserts elements from the range [il.begin(), il.end()).

    Complexity: Linear in N if the range [il.begin(), il.end()) is already sorted using comp and otherwise N logN, where N is il.begin() - il.end().

14.     flat_set(std::initializer_list< value_type > il, const allocator_type & a);

    Effects: Constructs an empty container using the specified allocator, and inserts elements from the range [il.begin(), il.end()).

    Complexity: Linear in N if the range [il.begin(), il.end()) is already sorted using comp and otherwise N logN, where N is il.begin() - il.end().

15.     flat_set(std::initializer_list< value_type > il, const Compare & comp);

    Effects: Constructs an empty container using the specified comparison object and inserts elements from the range [il.begin(), il.end()).

    Complexity: Linear in N if the range [il.begin(), il.end()) is already sorted using comp and otherwise N logN, where N is il.begin() - il.end().

16.     flat_set(std::initializer_list< value_type > il, const Compare & comp,
                 const allocator_type & a);

    Effects: Constructs an empty container using the specified comparison object and allocator, and inserts elements from the range [il.begin(), il.end()).

    Complexity: Linear in N if the range [il.begin(), il.end()) is already sorted using comp and otherwise N logN, where N is il.begin() - il.end().

17.     flat_set(ordered_unique_range_t, std::initializer_list< value_type > il);

    Effects: Constructs an empty container using the specified comparison object and inserts elements from the ordered unique range [il.begin(), il.end()). This function is more efficient than the normal range creation for ordered ranges.

    Requires: [il.begin(), il.end()) must be ordered according to the predicate and must be unique values.

    Complexity: Linear in N.

    Note: Non-standard extension.

18.     flat_set(ordered_unique_range_t, std::initializer_list< value_type > il,
                 const Compare & comp);

    Effects: Constructs an empty container using the specified comparison object and inserts elements from the ordered unique range [il.begin(), il.end()). This function is more efficient than the normal range creation for ordered ranges.

    Requires: [il.begin(), il.end()) must be ordered according to the predicate and must be unique values.

    Complexity: Linear in N.

    Note: Non-standard extension.

19.     flat_set(ordered_unique_range_t, std::initializer_list< value_type > il,
                 const Compare & comp, const allocator_type & a);

    Effects: Constructs an empty container using the specified comparison object and allocator, and inserts elements from the ordered unique range [il.begin(), il.end()). This function is more efficient than the normal range creation for ordered ranges.

    Requires: [il.begin(), il.end()) must be ordered according to the predicate and must be unique values.

    Complexity: Linear in N.

    Note: Non-standard extension.

20.     flat_set(const flat_set & x);

    Effects: Copy constructs the container.

    Complexity: Linear in x.size().

21.     flat_set(flat_set && x) noexcept(boost::container::dtl::is_nothrow_move_constructible< Compare >::value));

    Effects: Move constructs thecontainer. Constructs *this using x's resources.

    Complexity: Constant.

    Postcondition: x is emptied.

22.     flat_set(const flat_set & x, const allocator_type & a);

    Effects: Copy constructs a container using the specified allocator.

    Complexity: Linear in x.size().

23.     flat_set(flat_set && x, const allocator_type & a);

    Effects: Move constructs a container using the specified allocator. Constructs *this using x's resources.

    Complexity: Constant if a == x.get_allocator(), linear otherwise

24.     flat_set & operator=(const flat_set & x);

    Effects: Makes *this a copy of x.

    Complexity: Linear in x.size().

25.     flat_set & operator=(flat_set && x) noexcept((allocator_traits_type::propagate_on_container_move_assignment::value||allocator_traits_type::is_always_equal::value) &&boost::container::dtl::is_nothrow_move_assignable< Compare >::value));

    Throws: If allocator_traits_type::propagate_on_container_move_assignment is false and (allocation throws or value_type's move constructor throws)

    Complexity: Constant if allocator_traits_type:: propagate_on_container_move_assignment is true or this->get>allocator() == x.get_allocator(). Linear otherwise.

26.     flat_set & operator=(std::initializer_list< value_type > il);

    Effects: Copy all elements from il to *this.

    Complexity: Linear in il.size().

### `flat_set` public member functions

1.     allocator_type get_allocator() const noexcept;

   Effects: Returns a copy of the allocator that was passed to the object's constructor.

   Complexity: Constant.

2.     stored_allocator_type & get_stored_allocator() noexcept;

   Effects: Returns a reference to the internal allocator.

   Throws: Nothing

   Complexity: Constant.

   Note: Non-standard extension.

3.     const stored_allocator_type & get_stored_allocator() const noexcept;

   Effects: Returns a reference to the internal allocator.

   Throws: Nothing

   Complexity: Constant.

   Note: Non-standard extension.

4.     iterator begin() noexcept;

   Effects: Returns an iterator to the first element contained in the container.

   Throws: Nothing.

   Complexity: Constant.

5.     const_iterator begin() const noexcept;

   Effects: Returns a const_iterator to the first element contained in the container.

   Throws: Nothing.

   Complexity: Constant.

6.     iterator end() noexcept;

   Effects: Returns an iterator to the end of the container.

   Throws: Nothing.

   Complexity: Constant.

7.     const_iterator end() const noexcept;

   Effects: Returns a const_iterator to the end of the container.

   Throws: Nothing.

   Complexity: Constant.

8.     reverse_iterator rbegin() noexcept;

   Effects: Returns a reverse_iterator pointing to the beginning of the reversed container.

   Throws: Nothing.

   Complexity: Constant.

9.     const_reverse_iterator rbegin() const noexcept;

   Effects: Returns a const_reverse_iterator pointing to the beginning of the reversed container.

   Throws: Nothing.

   Complexity: Constant.

10.     reverse_iterator rend() noexcept;

    Effects: Returns a reverse_iterator pointing to the end of the reversed container.

    Throws: Nothing.

    Complexity: Constant.

11.     const_reverse_iterator rend() const noexcept;

    Effects: Returns a const_reverse_iterator pointing to the end of the reversed container.

    Throws: Nothing.

    Complexity: Constant.

12.     const_iterator cbegin() const noexcept;

    Effects: Returns a const_iterator to the first element contained in the container.

    Throws: Nothing.

    Complexity: Constant.

13.     const_iterator cend() const noexcept;

    Effects: Returns a const_iterator to the end of the container.

    Throws: Nothing.

    Complexity: Constant.

14.     const_reverse_iterator crbegin() const noexcept;

    Effects: Returns a const_reverse_iterator pointing to the beginning of the reversed container.

    Throws: Nothing.

    Complexity: Constant.

15.     const_reverse_iterator crend() const noexcept;

    Effects: Returns a const_reverse_iterator pointing to the end of the reversed container.

    Throws: Nothing.

    Complexity: Constant.

16.     bool empty() const noexcept;

    Effects: Returns true if the container contains no elements.

    Throws: Nothing.

    Complexity: Constant.

17.     size_type size() const noexcept;

    Effects: Returns the number of the elements contained in the container.

    Throws: Nothing.

    Complexity: Constant.

18.     size_type max_size() const noexcept;

    Effects: Returns the largest possible size of the container.

    Throws: Nothing.

    Complexity: Constant.

19.     size_type capacity() const noexcept;

    Effects: Number of elements for which memory has been allocated. capacity() is always greater than or equal to size().

    Throws: Nothing.

    Complexity: Constant.

20.     void reserve(size_type cnt);

    Effects: If n is less than or equal to capacity(), or the underlying container has no `reserve` member, this call has no effect. Otherwise, it is a request for allocation of additional memory. If the request is successful, then capacity() is greater than or equal to n; otherwise, capacity() is unchanged. In either case, size() is unchanged.

    Throws: If memory allocation allocation throws or T's copy constructor throws.

    Note: If capacity() is less than "cnt", iterators and references to to values might be invalidated.

21.     void shrink_to_fit();

    Effects: Tries to deallocate the excess of memory created

    Throws: If memory allocation throws, or Key's copy constructor throws.

    Complexity: Linear to size().

22.     template<class... Args> std::pair< iterator, bool > emplace(Args &&... args);

    Effects: Inserts an object x of type Key constructed with std::forward<Args>(args)... if and only if there is no element in the container with key equivalent to the key of x.

    Returns: The bool component of the returned pair is true if and only if the insertion takes place, and the iterator component of the pair points to the element with key equivalent to the key of x.

    Complexity: Logarithmic search time plus linear insertion to the elements with bigger keys than x.

    Note: If an element is inserted it might invalidate elements.

23.     template<class... Args>
          iterator emplace_hint(const_iterator p, Args &&... args);

    Effects: Inserts an object of type Key constructed with std::forward<Args>(args)... in the container if and only if there is no element in the container with key equivalent to the key of x. p is a hint pointing to where the insert should start to search.

    Returns: An iterator pointing to the element with key equivalent to the key of x.

    Complexity: Logarithmic search time (constant if x is inserted right before p) plus insertion linear to the elements with bigger keys than x.

    Note: If an element is inserted it might invalidate elements.

24.     std::pair< iterator, bool > insert(const value_type & x);

    Effects: Inserts x if and only if there is no element in the container with key equivalent to the key of x.

    Returns: The bool component of the returned pair is true if and only if the insertion takes place, and the iterator component of the pair points to the element with key equivalent to the key of x.

    Complexity: Logarithmic search time plus linear insertion to the elements with bigger keys than x.

    Note: If an element is inserted it might invalidate elements.

25.     std::pair< iterator, bool > insert(value_type && x);

    Effects: Inserts a new value_type move constructed from the pair if and only if there is no element in the container with key equivalent to the key of x.

    Returns: The bool component of the returned pair is true if and only if the insertion takes place, and the iterator component of the pair points to the element with key equivalent to the key of x.

    Complexity: Logarithmic search time plus linear insertion to the elements with bigger keys than x.

    Note: If an element is inserted it might invalidate elements.

26.     iterator insert(const_iterator p, const value_type & x);

    Effects: Inserts a copy of x in the container if and only if there is no element in the container with key equivalent to the key of x. p is a hint pointing to where the insert should start to search.

    Returns: An iterator pointing to the element with key equivalent to the key of x.

    Complexity: Logarithmic search time (constant if x is inserted right before p) plus insertion linear to the elements with bigger keys than x.

    Note: If an element is inserted it might invalidate elements.

27.     iterator insert(const_iterator p, value_type && x);

    Effects: Inserts an element move constructed from x in the container. p is a hint pointing to where the insert should start to search.

    Returns: An iterator pointing to the element with key equivalent to the key of x.

    Complexity: Logarithmic search time (constant if x is inserted right before p) plus insertion linear to the elements with bigger keys than x.

    Note: If an element is inserted it might invalidate elements.

28.     template<typename InputIterator>
          void insert(InputIterator first, InputIterator last);

    Requires: first, last are not iterators into *this.

    Effects: inserts each element from the range [first,last) if and only if there is no element with key equivalent to the key of that element.

    Complexity: N log(N).

    Note: If an element is inserted it might invalidate elements.

29.     template<typename InputIterator>
          void insert(ordered_unique_range_t, InputIterator first, InputIterator last);

    Requires: first, last are not iterators into *this and must be ordered according to the predicate and must be unique values.

    Effects: inserts each element from the range [first,last) .This function is more efficient than the normal range creation for ordered ranges.

    Complexity: Linear.

    Note: Non-standard extension. If an element is inserted it might invalidate elements.

30.     void insert(std::initializer_list< value_type > il);

    Effects: inserts each element from the range [il.begin(), il.end()) if and only if there is no element with key equivalent to the key of that element.

    Complexity: N log(N).

    Note: If an element is inserted it might invalidate elements.

31.     void insert(ordered_unique_range_t, std::initializer_list< value_type > il);

    Requires: Range [il.begin(), il.end()) must be ordered according to the predicate and must be unique values.

    Effects: inserts each element from the range [il.begin(), il.end()) .This function is more efficient than the normal range creation for ordered ranges.

    Complexity: Linear.

    Note: Non-standard extension. If an element is inserted it might invalidate elements.

32.     template<typename C2>
          void merge(flat_set< Key, C2, AllocatorOrContainer > & source);

    Requires: this->get_allocator() == source.get_allocator().

    Effects: Attempts to extract each element in source and insert it into a using the comparison object of *this. If there is an element in a with key equivalent to the key of an element from source, then that element is not extracted from source.

    Postcondition: Pointers and references to the transferred elements of source refer to those same elements but as members of *this. Iterators referring to the transferred elements will continue to refer to their elements, but they now behave as iterators into *this, not into source.

    Throws: Nothing unless the comparison object throws.

    Complexity: N log(a.size() + N) (N has the value source.size())

33.     template<typename C2>
          void merge(flat_set< Key, C2, AllocatorOrContainer > && source);

    Requires: this->get_allocator() == source.get_allocator().

    Effects: Attempts to extract each element in source and insert it into a using the comparison object of *this. If there is an element in a with key equivalent to the key of an element from source, then that element is not extracted from source.

    Postcondition: Pointers and references to the transferred elements of source refer to those same elements but as members of *this. Iterators referring to the transferred elements will continue to refer to their elements, but they now behave as iterators into *this, not into source.

    Throws: Nothing unless the comparison object throws.

    Complexity: N log(a.size() + N) (N has the value source.size())

34.     template<typename C2>
          void merge(flat_multiset< Key, C2, AllocatorOrContainer > & source);

    Requires: this->get_allocator() == source.get_allocator().

    Effects: Attempts to extract each element in source and insert it into a using the comparison object of *this. If there is an element in a with key equivalent to the key of an element from source, then that element is not extracted from source.

    Postcondition: Pointers and references to the transferred elements of source refer to those same elements but as members of *this. Iterators referring to the transferred elements will continue to refer to their elements, but they now behave as iterators into *this, not into source.

    Throws: Nothing unless the comparison object throws.

    Complexity: N log(a.size() + N) (N has the value source.size())

35.     template<typename C2>
          void merge(flat_multiset< Key, C2, AllocatorOrContainer > && source);

    Requires: this->get_allocator() == source.get_allocator().

    Effects: Attempts to extract each element in source and insert it into a using the comparison object of *this. If there is an element in a with key equivalent to the key of an element from source, then that element is not extracted from source.

    Postcondition: Pointers and references to the transferred elements of source refer to those same elements but as members of *this. Iterators referring to the transferred elements will continue to refer to their elements, but they now behave as iterators into *this, not into source.

    Throws: Nothing unless the comparison object throws.

    Complexity: N log(a.size() + N) (N has the value source.size())

36.     iterator erase(const_iterator p);

    Effects: Erases the element pointed to by p.

    Returns: Returns an iterator pointing to the element immediately following q prior to the element being erased. If no such element exists, returns end().

    Complexity: Linear to the elements with keys bigger than p

    Note: Invalidates elements with keys not less than the erased element.

37.     size_type erase(const key_type & x);

    Effects: Erases all elements in the container with key equivalent to x.

    Returns: Returns the number of erased elements.

    Complexity: Logarithmic search time plus erasure time linear to the elements with bigger keys.

38.     iterator erase(const_iterator first, const_iterator last);

    Effects: Erases all the elements in the range [first, last).

    Returns: Returns last.

    Complexity: size()*N where N is the distance from first to last.

    Complexity: Logarithmic search time plus erasure time linear to the elements with bigger keys.

39.     void swap(flat_set & x) noexcept(allocator_traits_type::is_always_equal::value &&boost::container::dtl::is_nothrow_swappable< Compare >::value));

    Effects: Swaps the contents of *this and x.

    Throws: Nothing.

    Complexity: Constant.

40.     void clear() noexcept;

    Effects: erase(a.begin(),a.end()).

    Postcondition: size() == 0.

    Complexity: linear in size().

41.     key_compare key_comp() const;

    Effects: Returns the comparison object out of which a was constructed.

    Complexity: Constant.

42.     value_compare value_comp() const;

    Effects: Returns an object of value_compare constructed out of the comparison object.

    Complexity: Constant.

43.     iterator find(const key_type & x);

    Returns: An iterator pointing to an element with the key equivalent to x, or end() if such an element is not found.

    Complexity: Logarithmic.

44.     const_iterator find(const key_type & x) const;

    Returns: A const_iterator pointing to an element with the key equivalent to x, or end() if such an element is not found.

    Complexity: Logarithmic.

45.     template<typename K> iterator find(const K & x);

    Requires: This overload is available only if key_compare::is_transparent exists.

    Returns: An iterator pointing to an element with the key equivalent to x, or end() if such an element is not found.

    Complexity: Logarithmic.

46.     template<typename K> const_iterator find(const K & x) const;

    Requires: This overload is available only if key_compare::is_transparent exists.

    Returns: A const_iterator pointing to an element with the key equivalent to x, or end() if such an element is not found.

    Complexity: Logarithmic.

47.     iterator nth(size_type n) noexcept;

    Requires: size() >= n.

    Effects: Returns an iterator to the nth element from the beginning of the container. Returns end() if n == size().

    Throws: Nothing.

    Complexity: Constant.

    Note: Non-standard extension

48.     const_iterator nth(size_type n) const noexcept;

    Requires: size() >= n.

    Effects: Returns a const_iterator to the nth element from the beginning of the container. Returns end() if n == size().

    Throws: Nothing.

    Complexity: Constant.

    Note: Non-standard extension

49.     size_type index_of(iterator p) noexcept;

    Requires: begin() <= p <= end().

    Effects: Returns the index of the element pointed by p and size() if p == end().

    Throws: Nothing.

    Complexity: Constant.

    Note: Non-standard extension

50.     size_type index_of(const_iterator p) const noexcept;

    Requires: begin() <= p <= end().

    Effects: Returns the index of the element pointed by p and size() if p == end().

    Throws: Nothing.

    Complexity: Constant.

    Note: Non-standard extension

51.     size_type count(const key_type & x) const;

    Returns: The number of elements with key equivalent to x.

    Complexity: log(size())+count(k)

52.     template<typename K> size_type count(const K & x) const;

    Requires: This overload is available only if key_compare::is_transparent exists.

    Returns: The number of elements with key equivalent to x.

    Complexity: log(size())+count(k)

53.     bool contains(const key_type & x) const;

    Returns: Returns true if there is an element with key equivalent to key in the container, otherwise false.

    Complexity: log(size()).

54.     template<typename K> bool contains(const K & x) const;

    Requires: This overload is available only if key_compare::is_transparent exists.

    Returns: Returns true if there is an element with key equivalent to key in the container, otherwise false.

    Complexity: log(size()).

55.     iterator lower_bound(const key_type & x);

    Returns: An iterator pointing to the first element with key not less than k, or a.end() if such an element is not found.

    Complexity: Logarithmic

56.     const_iterator lower_bound(const key_type & x) const;

    Returns: A const iterator pointing to the first element with key not less than k, or a.end() if such an element is not found.

    Complexity: Logarithmic

57.     template<typename K> iterator lower_bound(const K & x);

    Requires: This overload is available only if key_compare::is_transparent exists.

    Returns: An iterator pointing to the first element with key not less than k, or a.end() if such an element is not found.

    Complexity: Logarithmic

58.     template<typename K> const_iterator lower_bound(const K & x) const;

    Requires: This overload is available only if key_compare::is_transparent exists.

    Returns: A const iterator pointing to the first element with key not less than k, or a.end() if such an element is not found.

    Complexity: Logarithmic

59.     iterator upper_bound(const key_type & x);

    Returns: An iterator pointing to the first element with key not less than x, or end() if such an element is not found.

    Complexity: Logarithmic

60.     const_iterator upper_bound(const key_type & x) const;

    Returns: A const iterator pointing to the first element with key not less than x, or end() if such an element is not found.

    Complexity: Logarithmic

61.     template<typename K> iterator upper_bound(const K & x);

    Requires: This overload is available only if key_compare::is_transparent exists.

    Returns: An iterator pointing to the first element with key not less than x, or end() if such an element is not found.

    Complexity: Logarithmic

62.     template<typename K> const_iterator upper_bound(const K & x) const;

    Requires: This overload is available only if key_compare::is_transparent exists.

    Returns: A const iterator pointing to the first element with key not less than x, or end() if such an element is not found.

    Complexity: Logarithmic

63.     std::pair< const_iterator, const_iterator >
        equal_range(const key_type & x) const;

    Effects: Equivalent to std::make_pair(this->lower_bound(k), this->upper_bound(k)).

    Complexity: Logarithmic

64.     std::pair< iterator, iterator > equal_range(const key_type & x);

    Effects: Equivalent to std::make_pair(this->lower_bound(k), this->upper_bound(k)).

    Complexity: Logarithmic

65.     template<typename K> std::pair< iterator, iterator > equal_range(const K & x);

    Requires: This overload is available only if key_compare::is_transparent exists.

    Effects: Equivalent to std::make_pair(this->lower_bound(k), this->upper_bound(k)).

    Complexity: Logarithmic

66.     template<typename K>
          std::pair< const_iterator, const_iterator > equal_range(const K & x) const;

    Requires: This overload is available only if key_compare::is_transparent exists.

    Effects: Equivalent to std::make_pair(this->lower_bound(k), this->upper_bound(k)).

    Complexity: Logarithmic

67.     sequence_type extract_sequence();

    Effects: Extracts the internal sequence container.

    Complexity: Same as the move constructor of sequence_type, usually constant.

    Postcondition: this->empty()

    Throws: If secuence_type's move constructor throws

68.     void adopt_sequence(sequence_type && seq);

    Effects: Discards the internally hold sequence container and adopts the one passed externally using the move assignment. Erases non-unique elements.

    Complexity: Assuming O(1) move assignment, O(NlogN) with N = seq.size()

    Throws: If the comparison or the move constructor throws

69.     void adopt_sequence(ordered_unique_range_t, sequence_type && seq);

    Requires: seq shall be ordered according to this->compare() and shall contain unique elements.

    Effects: Discards the internally hold sequence container and adopts the one passed externally using the move assignment.

    Complexity: Assuming O(1) move assignment, O(1)

    Throws: If the move assignment throws

### `flat_set` friend functions

1.     friend bool operator==(const flat_set & x, const flat_set & y);

   Effects: Returns true if x and y are equal

   Complexity: Linear to the number of elements in the container.

2.     friend bool operator!=(const flat_set & x, const flat_set & y);

   Effects: Returns true if x and y are unequal

   Complexity: Linear to the number of elements in the container.

3.     friend bool operator<(const flat_set & x, const flat_set & y);

   Effects: Returns true if x is less than y

   Complexity: Linear to the number of elements in the container.

4.     friend bool operator>(const flat_set & x, const flat_set & y);

   Effects: Returns true if x is greater than y

   Complexity: Linear to the number of elements in the container.

5.     friend bool operator<=(const flat_set & x, const flat_set & y);

   Effects: Returns true if x is equal or less than y

   Complexity: Linear to the number of elements in the container.

6.     friend bool operator>=(const flat_set & x, const flat_set & y);

   Effects: Returns true if x is equal or greater than y

   Complexity: Linear to the number of elements in the container.

7.     friend void swap(flat_set & x, flat_set & y);

   Effects: x.swap(y)

   Complexity: Constant.