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// Map implementation -*- C++ -*-

// Copyright (C) 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010

// Free Software Foundation, Inc.

//

// This file is part of the GNU ISO C++ Library.  This library is free

// software; you can redistribute it and/or modify it under the

// terms of the GNU General Public License as published by the

// Free Software Foundation; either version 3, or (at your option)

// any later version.

// This library is distributed in the hope that it will be useful,

// but WITHOUT ANY WARRANTY; without even the implied warranty of

// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the

// GNU General Public License for more details.

// Under Section 7 of GPL version 3, you are granted additional

// permissions described in the GCC Runtime Library Exception, version

// 3.1, as published by the Free Software Foundation.

// You should have received a copy of the GNU General Public License and

// a copy of the GCC Runtime Library Exception along with this program;

// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see

// <http://www.gnu.org/licenses/>.

/*

 *

 * Copyright (c) 1994

 * Hewlett-Packard Company

 *

 * Permission to use, copy, modify, distribute and sell this software

 * and its documentation for any purpose is hereby granted without fee,

 * provided that the above copyright notice appear in all copies and

 * that both that copyright notice and this permission notice appear

 * in supporting documentation.  Hewlett-Packard Company makes no

 * representations about the suitability of this software for any

 * purpose.  It is provided "as is" without express or implied warranty.

 *

 *

 * Copyright (c) 1996,1997

 * Silicon Graphics Computer Systems, Inc.

 *

 * Permission to use, copy, modify, distribute and sell this software

 * and its documentation for any purpose is hereby granted without fee,

 * provided that the above copyright notice appear in all copies and

 * that both that copyright notice and this permission notice appear

 * in supporting documentation.  Silicon Graphics makes no

 * representations about the suitability of this software for any

 * purpose.  It is provided "as is" without express or implied warranty.

 */

/** @file stl_map.h

 *  This is an internal header file, included by other library headers.

 *  You should not attempt to use it directly.

 */

#ifndef _STL_MAP_H

#define _STL_MAP_H 1

#include <bits/functexcept.h>

#include <bits/concept_check.h>

#include <initializer_list>

_GLIBCXX_BEGIN_NESTED_NAMESPACE(std, _GLIBCXX_STD_D)

  /**

   *  @brief A standard container made up of (key,value) pairs, which can be

   *  retrieved based on a key, in logarithmic time.

   *

   *  @ingroup associative_containers

   *

   *  Meets the requirements of a <a href="tables.html#65">container</a>, a

   *  <a href="tables.html#66">reversible container</a>, and an

   *  <a href="tables.html#69">associative container</a> (using unique keys).

   *  For a @c map<Key,T> the key_type is Key, the mapped_type is T, and the

   *  value_type is std::pair<const Key,T>.

   *

   *  Maps support bidirectional iterators.

   *

   *  The private tree data is declared exactly the same way for map and

   *  multimap; the distinction is made entirely in how the tree functions are

   *  called (*_unique versus *_equal, same as the standard).

  */

  template <typename _Key, typename _Tp, typename _Compare = std::less<_Key>,

            typename _Alloc = std::allocator<std::pair<const _Key, _Tp> > >

    class map

    {

    public:

      typedef _Key                                          key_type;

      typedef _Tp                                           mapped_type;

      typedef std::pair<const _Key, _Tp>                    value_type;

      typedef _Compare                                      key_compare;

      typedef _Alloc                                        allocator_type;

    private:

      // concept requirements

      typedef typename _Alloc::value_type                   _Alloc_value_type;

      __glibcxx_class_requires(_Tp, _SGIAssignableConcept)

      __glibcxx_class_requires4(_Compare, bool, _Key, _Key,

                                _BinaryFunctionConcept)

      __glibcxx_class_requires2(value_type, _Alloc_value_type, _SameTypeConcept)

    public:

      class value_compare

      : public std::binary_function<value_type, value_type, bool>

      {

        friend class map<_Key, _Tp, _Compare, _Alloc>;

      protected:

        _Compare comp;

        value_compare(_Compare __c)

        : comp(__c) { }

      public:

        bool operator()(const value_type& __x, const value_type& __y) const

        { return comp(__x.first, __y.first); }

      };

    private:

      /// This turns a red-black tree into a [multi]map. 

      typedef typename _Alloc::template rebind<value_type>::other

        _Pair_alloc_type;

      typedef _Rb_tree<key_type, value_type, _Select1st<value_type>,

                       key_compare, _Pair_alloc_type> _Rep_type;

      /// The actual tree structure.

      _Rep_type _M_t;

    public:

      // many of these are specified differently in ISO, but the following are

      // "functionally equivalent"

      typedef typename _Pair_alloc_type::pointer         pointer;

      typedef typename _Pair_alloc_type::const_pointer   const_pointer;

      typedef typename _Pair_alloc_type::reference       reference;

      typedef typename _Pair_alloc_type::const_reference const_reference;

      typedef typename _Rep_type::iterator               iterator;

      typedef typename _Rep_type::const_iterator         const_iterator;

      typedef typename _Rep_type::size_type              size_type;

      typedef typename _Rep_type::difference_type        difference_type;

      typedef typename _Rep_type::reverse_iterator       reverse_iterator;

      typedef typename _Rep_type::const_reverse_iterator const_reverse_iterator;

      // [23.3.1.1] construct/copy/destroy

      // (get_allocator() is normally listed in this section, but seems to have

      // been accidentally omitted in the printed standard)

      /**

       *  @brief  Default constructor creates no elements.

       */

      map()

      : _M_t() { }

      /**

       *  @brief  Creates a %map with no elements.

       *  @param  comp  A comparison object.

       *  @param  a  An allocator object.

       */

      explicit

      map(const _Compare& __comp,

          const allocator_type& __a = allocator_type())

      : _M_t(__comp, __a) { }

      /**

       *  @brief  %Map copy constructor.

       *  @param  x  A %map of identical element and allocator types.

       *

       *  The newly-created %map uses a copy of the allocation object

       *  used by @a x.

       */

      map(const map& __x)

      : _M_t(__x._M_t) { }

#ifdef __GXX_EXPERIMENTAL_CXX0X__

      /**

       *  @brief  %Map move constructor.

       *  @param  x  A %map of identical element and allocator types.

       *

       *  The newly-created %map contains the exact contents of @a x.

       *  The contents of @a x are a valid, but unspecified %map.

       */

      map(map&& __x)

      : _M_t(std::forward<_Rep_type>(__x._M_t)) { }

      /**

       *  @brief  Builds a %map from an initializer_list.

       *  @param  l  An initializer_list.

       *  @param  comp  A comparison object.

       *  @param  a  An allocator object.

       *

       *  Create a %map consisting of copies of the elements in the

       *  initializer_list @a l.

       *  This is linear in N if the range is already sorted, and NlogN

       *  otherwise (where N is @a l.size()).

       */

      map(initializer_list<value_type> __l,

          const _Compare& __c = _Compare(),

          const allocator_type& __a = allocator_type())

      : _M_t(__c, __a)

      { _M_t._M_insert_unique(__l.begin(), __l.end()); }

#endif

      /**

       *  @brief  Builds a %map from a range.

       *  @param  first  An input iterator.

       *  @param  last  An input iterator.

       *

       *  Create a %map consisting of copies of the elements from [first,last).

       *  This is linear in N if the range is already sorted, and NlogN

       *  otherwise (where N is distance(first,last)).

       */

      template<typename _InputIterator>

        map(_InputIterator __first, _InputIterator __last)

        : _M_t()

        { _M_t._M_insert_unique(__first, __last); }

      /**

       *  @brief  Builds a %map from a range.

       *  @param  first  An input iterator.

       *  @param  last  An input iterator.

       *  @param  comp  A comparison functor.

       *  @param  a  An allocator object.

       *

       *  Create a %map consisting of copies of the elements from [first,last).

       *  This is linear in N if the range is already sorted, and NlogN

       *  otherwise (where N is distance(first,last)).

       */

      template<typename _InputIterator>

        map(_InputIterator __first, _InputIterator __last,

            const _Compare& __comp,

            const allocator_type& __a = allocator_type())

        : _M_t(__comp, __a)

        { _M_t._M_insert_unique(__first, __last); }

      // FIXME There is no dtor declared, but we should have something

      // generated by Doxygen.  I don't know what tags to add to this

      // paragraph to make that happen:

      /**

       *  The dtor only erases the elements, and note that if the elements

       *  themselves are pointers, the pointed-to memory is not touched in any

       *  way.  Managing the pointer is the user's responsibility.

       */

      /**

       *  @brief  %Map assignment operator.

       *  @param  x  A %map of identical element and allocator types.

       *

       *  All the elements of @a x are copied, but unlike the copy constructor,

       *  the allocator object is not copied.

       */

      map&

      operator=(const map& __x)

      {

        _M_t = __x._M_t;

        return *this;

      }

#ifdef __GXX_EXPERIMENTAL_CXX0X__

      /**

       *  @brief  %Map move assignment operator.

       *  @param  x  A %map of identical element and allocator types.

       *

       *  The contents of @a x are moved into this map (without copying).

       *  @a x is a valid, but unspecified %map.

       */

      map&

      operator=(map&& __x)

      {

        // NB: DR 1204.

        // NB: DR 675.

        this->clear();

        this->swap(__x);

        return *this;

      }

      /**

       *  @brief  %Map list assignment operator.

       *  @param  l  An initializer_list.

       *

       *  This function fills a %map with copies of the elements in the

       *  initializer list @a l.

       *

       *  Note that the assignment completely changes the %map and

       *  that the resulting %map's size is the same as the number

       *  of elements assigned.  Old data may be lost.

       */

      map&

      operator=(initializer_list<value_type> __l)

      {

        this->clear();

        this->insert(__l.begin(), __l.end());

        return *this;

      }

#endif

      /// Get a copy of the memory allocation object.

      allocator_type

      get_allocator() const

      { return _M_t.get_allocator(); }

      // iterators

      /**

       *  Returns a read/write iterator that points to the first pair in the

       *  %map.

       *  Iteration is done in ascending order according to the keys.

       */

      iterator

      begin()

      { return _M_t.begin(); }

      /**

       *  Returns a read-only (constant) iterator that points to the first pair

       *  in the %map.  Iteration is done in ascending order according to the

       *  keys.

       */

      const_iterator

      begin() const

      { return _M_t.begin(); }

      /**

       *  Returns a read/write iterator that points one past the last

       *  pair in the %map.  Iteration is done in ascending order

       *  according to the keys.

       */

      iterator

      end()

      { return _M_t.end(); }

      /**

       *  Returns a read-only (constant) iterator that points one past the last

       *  pair in the %map.  Iteration is done in ascending order according to

       *  the keys.

       */

      const_iterator

      end() const

      { return _M_t.end(); }

      /**

       *  Returns a read/write reverse iterator that points to the last pair in

       *  the %map.  Iteration is done in descending order according to the

       *  keys.

       */

      reverse_iterator

      rbegin()

      { return _M_t.rbegin(); }

      /**

       *  Returns a read-only (constant) reverse iterator that points to the

       *  last pair in the %map.  Iteration is done in descending order

       *  according to the keys.

       */

      const_reverse_iterator

      rbegin() const

      { return _M_t.rbegin(); }

      /**

       *  Returns a read/write reverse iterator that points to one before the

       *  first pair in the %map.  Iteration is done in descending order

       *  according to the keys.

       */

      reverse_iterator

      rend()

      { return _M_t.rend(); }

      /**

       *  Returns a read-only (constant) reverse iterator that points to one

       *  before the first pair in the %map.  Iteration is done in descending

       *  order according to the keys.

       */

      const_reverse_iterator

      rend() const

      { return _M_t.rend(); }

#ifdef __GXX_EXPERIMENTAL_CXX0X__

      /**

       *  Returns a read-only (constant) iterator that points to the first pair

       *  in the %map.  Iteration is done in ascending order according to the

       *  keys.

       */

      const_iterator

      cbegin() const

      { return _M_t.begin(); }

      /**

       *  Returns a read-only (constant) iterator that points one past the last

       *  pair in the %map.  Iteration is done in ascending order according to

       *  the keys.

       */

      const_iterator

      cend() const

      { return _M_t.end(); }

      /**

       *  Returns a read-only (constant) reverse iterator that points to the

       *  last pair in the %map.  Iteration is done in descending order

       *  according to the keys.

       */

      const_reverse_iterator

      crbegin() const

      { return _M_t.rbegin(); }

      /**

       *  Returns a read-only (constant) reverse iterator that points to one

       *  before the first pair in the %map.  Iteration is done in descending

       *  order according to the keys.

       */

      const_reverse_iterator

      crend() const

      { return _M_t.rend(); }

#endif

      // capacity

      /** Returns true if the %map is empty.  (Thus begin() would equal

       *  end().)

      */

      bool

      empty() const

      { return _M_t.empty(); }

      /** Returns the size of the %map.  */

      size_type

      size() const

      { return _M_t.size(); }

      /** Returns the maximum size of the %map.  */

      size_type

      max_size() const

      { return _M_t.max_size(); }

      // [23.3.1.2] element access

      /**

       *  @brief  Subscript ( @c [] ) access to %map data.

       *  @param  k  The key for which data should be retrieved.

       *  @return  A reference to the data of the (key,data) %pair.

       *

       *  Allows for easy lookup with the subscript ( @c [] )

       *  operator.  Returns data associated with the key specified in

       *  subscript.  If the key does not exist, a pair with that key

       *  is created using default values, which is then returned.

       *

       *  Lookup requires logarithmic time.

       */

      mapped_type&

      operator[](const key_type& __k)

      {

        // concept requirements

        __glibcxx_function_requires(_DefaultConstructibleConcept<mapped_type>)

        iterator __i = lower_bound(__k);

        // __i->first is greater than or equivalent to __k.

        if (__i == end() || key_comp()(__k, (*__i).first))

          __i = insert(__i, value_type(__k, mapped_type()));

        return (*__i).second;

      }

      // _GLIBCXX_RESOLVE_LIB_DEFECTS

      // DR 464. Suggestion for new member functions in standard containers.

      /**

       *  @brief  Access to %map data.

       *  @param  k  The key for which data should be retrieved.

       *  @return  A reference to the data whose key is equivalent to @a k, if

       *           such a data is present in the %map.

       *  @throw  std::out_of_range  If no such data is present.

       */

      mapped_type&

      at(const key_type& __k)

      {

        iterator __i = lower_bound(__k);

        if (__i == end() || key_comp()(__k, (*__i).first))

          __throw_out_of_range(__N("map::at"));

        return (*__i).second;

      }

      const mapped_type&

      at(const key_type& __k) const

      {

        const_iterator __i = lower_bound(__k);

        if (__i == end() || key_comp()(__k, (*__i).first))

          __throw_out_of_range(__N("map::at"));

        return (*__i).second;

      }

      // modifiers

      /**

       *  @brief Attempts to insert a std::pair into the %map.

       *  @param  x  Pair to be inserted (see std::make_pair for easy creation

       *             of pairs).

       *  @return  A pair, of which the first element is an iterator that

       *           points to the possibly inserted pair, and the second is

       *           a bool that is true if the pair was actually inserted.

       *

       *  This function attempts to insert a (key, value) %pair into the %map.

       *  A %map relies on unique keys and thus a %pair is only inserted if its

       *  first element (the key) is not already present in the %map.

       *

       *  Insertion requires logarithmic time.

       */

      std::pair<iterator, bool>

      insert(const value_type& __x)

      { return _M_t._M_insert_unique(__x); }

#ifdef __GXX_EXPERIMENTAL_CXX0X__

      /**

       *  @brief Attempts to insert a list of std::pairs into the %map.

       *  @param  list  A std::initializer_list<value_type> of pairs to be

       *                inserted.

       *

       *  Complexity similar to that of the range constructor.

       */

      void

      insert(std::initializer_list<value_type> __list)

      { insert (__list.begin(), __list.end()); }

#endif

      /**

       *  @brief Attempts to insert a std::pair into the %map.

       *  @param  position  An iterator that serves as a hint as to where the

       *                    pair should be inserted.

       *  @param  x  Pair to be inserted (see std::make_pair for easy creation

       *             of pairs).

       *  @return  An iterator that points to the element with key of @a x (may

       *           or may not be the %pair passed in).

       *

       *  This function is not concerned about whether the insertion

       *  took place, and thus does not return a boolean like the

       *  single-argument insert() does.  Note that the first

       *  parameter is only a hint and can potentially improve the

       *  performance of the insertion process.  A bad hint would

       *  cause no gains in efficiency.

       *

       *  See

       *  http://gcc.gnu.org/onlinedocs/libstdc++/manual/bk01pt07ch17.html

       *  for more on @a hinting.

       *

       *  Insertion requires logarithmic time (if the hint is not taken).

       */

      iterator

      insert(iterator __position, const value_type& __x)

      { return _M_t._M_insert_unique_(__position, __x); }

      /**

       *  @brief Template function that attempts to insert a range of elements.

       *  @param  first  Iterator pointing to the start of the range to be

       *                 inserted.

       *  @param  last  Iterator pointing to the end of the range.

       *

       *  Complexity similar to that of the range constructor.

       */

      template<typename _InputIterator>

        void

        insert(_InputIterator __first, _InputIterator __last)

        { _M_t._M_insert_unique(__first, __last); }

#ifdef __GXX_EXPERIMENTAL_CXX0X__

      // _GLIBCXX_RESOLVE_LIB_DEFECTS

      // DR 130. Associative erase should return an iterator.

      /**

       *  @brief Erases an element from a %map.

       *  @param  position  An iterator pointing to the element to be erased.

       *  @return An iterator pointing to the element immediately following

       *          @a position prior to the element being erased. If no such

       *          element exists, end() is returned.

       *

       *  This function erases an element, pointed to by the given

       *  iterator, from a %map.  Note that this function only erases

       *  the element, and that if the element is itself a pointer,

       *  the pointed-to memory is not touched in any way.  Managing

       *  the pointer is the user's responsibility.

       */

      iterator

      erase(iterator __position)

      { return _M_t.erase(__position); }

#else

      /**

       *  @brief Erases an element from a %map.

       *  @param  position  An iterator pointing to the element to be erased.

       *

       *  This function erases an element, pointed to by the given

       *  iterator, from a %map.  Note that this function only erases

       *  the element, and that if the element is itself a pointer,

       *  the pointed-to memory is not touched in any way.  Managing

       *  the pointer is the user's responsibility.

       */

      void

      erase(iterator __position)

      { _M_t.erase(__position); }

#endif

      /**

       *  @brief Erases elements according to the provided key.

       *  @param  x  Key of element to be erased.

       *  @return  The number of elements erased.

       *

       *  This function erases all the elements located by the given key from

       *  a %map.

       *  Note that this function only erases the element, and that if

       *  the element is itself a pointer, the pointed-to memory is not touched

       *  in any way.  Managing the pointer is the user's responsibility.

       */

      size_type

      erase(const key_type& __x)

      { return _M_t.erase(__x); }

#ifdef __GXX_EXPERIMENTAL_CXX0X__

      // _GLIBCXX_RESOLVE_LIB_DEFECTS

      // DR 130. Associative erase should return an iterator.

      /**

       *  @brief Erases a [first,last) range of elements from a %map.

       *  @param  first  Iterator pointing to the start of the range to be

       *                 erased.

       *  @param  last  Iterator pointing to the end of the range to be erased.

       *  @return The iterator @a last.

       *

       *  This function erases a sequence of elements from a %map.

       *  Note that this function only erases the element, and that if

       *  the element is itself a pointer, the pointed-to memory is not touched

       *  in any way.  Managing the pointer is the user's responsibility.

       */

      iterator

      erase(iterator __first, iterator __last)

      { return _M_t.erase(__first, __last); }

#else

      /**

       *  @brief Erases a [first,last) range of elements from a %map.

       *  @param  first  Iterator pointing to the start of the range to be

       *                 erased.

       *  @param  last  Iterator pointing to the end of the range to be erased.

       *

       *  This function erases a sequence of elements from a %map.

       *  Note that this function only erases the element, and that if

       *  the element is itself a pointer, the pointed-to memory is not touched

       *  in any way.  Managing the pointer is the user's responsibility.

       */

      void

      erase(iterator __first, iterator __last)

      { _M_t.erase(__first, __last); }

#endif

      /**

       *  @brief  Swaps data with another %map.

       *  @param  x  A %map of the same element and allocator types.

       *

       *  This exchanges the elements between two maps in constant

       *  time.  (It is only swapping a pointer, an integer, and an

       *  instance of the @c Compare type (which itself is often

       *  stateless and empty), so it should be quite fast.)  Note

       *  that the global std::swap() function is specialized such

       *  that std::swap(m1,m2) will feed to this function.

       */

      void

      swap(map& __x)

      { _M_t.swap(__x._M_t); }

      /**

       *  Erases all elements in a %map.  Note that this function only

       *  erases the elements, and that if the elements themselves are

       *  pointers, the pointed-to memory is not touched in any way.

       *  Managing the pointer is the user's responsibility.

       */

      void

      clear()

      { _M_t.clear(); }

      // observers

      /**

       *  Returns the key comparison object out of which the %map was

       *  constructed.

       */

      key_compare

      key_comp() const

      { return _M_t.key_comp(); }

      /**

       *  Returns a value comparison object, built from the key comparison

       *  object out of which the %map was constructed.

       */

      value_compare

      value_comp() const

      { return value_compare(_M_t.key_comp()); }

      // [23.3.1.3] map operations

      /**

       *  @brief Tries to locate an element in a %map.

       *  @param  x  Key of (key, value) %pair to be located.

       *  @return  Iterator pointing to sought-after element, or end() if not

       *           found.

       *

       *  This function takes a key and tries to locate the element with which

       *  the key matches.  If successful the function returns an iterator

       *  pointing to the sought after %pair.  If unsuccessful it returns the

       *  past-the-end ( @c end() ) iterator.

       */

      iterator

      find(const key_type& __x)

      { return _M_t.find(__x); }

      /**

       *  @brief Tries to locate an element in a %map.

       *  @param  x  Key of (key, value) %pair to be located.

       *  @return  Read-only (constant) iterator pointing to sought-after

       *           element, or end() if not found.

       *

       *  This function takes a key and tries to locate the element with which

       *  the key matches.  If successful the function returns a constant

       *  iterator pointing to the sought after %pair. If unsuccessful it

       *  returns the past-the-end ( @c end() ) iterator.

       */

      const_iterator

      find(const key_type& __x) const

      { return _M_t.find(__x); }

      /**

       *  @brief  Finds the number of elements with given key.

       *  @param  x  Key of (key, value) pairs to be located.

       *  @return  Number of elements with specified key.

       *

       *  This function only makes sense for multimaps; for map the result will

       *  either be 0 (not present) or 1 (present).

       */

      size_type

      count(const key_type& __x) const

      { return _M_t.find(__x) == _M_t.end() ? 0 : 1; }

      /**

       *  @brief Finds the beginning of a subsequence matching given key.

       *  @param  x  Key of (key, value) pair to be located.

       *  @return  Iterator pointing to first element equal to or greater

       *           than key, or end().

       *

       *  This function returns the first element of a subsequence of elements

       *  that matches the given key.  If unsuccessful it returns an iterator

       *  pointing to the first element that has a greater value than given key

       *  or end() if no such element exists.

       */

      iterator

      lower_bound(const key_type& __x)

      { return _M_t.lower_bound(__x); }

      /**

       *  @brief Finds the beginning of a subsequence matching given key.

       *  @param  x  Key of (key, value) pair to be located.

       *  @return  Read-only (constant) iterator pointing to first element

       *           equal to or greater than key, or end().

       *

       *  This function returns the first element of a subsequence of elements

       *  that matches the given key.  If unsuccessful it returns an iterator

       *  pointing to the first element that has a greater value than given key

       *  or end() if no such element exists.

       */

      const_iterator

      lower_bound(const key_type& __x) const

      { return _M_t.lower_bound(__x); }

      /**

       *  @brief Finds the end of a subsequence matching given key.

       *  @param  x  Key of (key, value) pair to be located.

       *  @return Iterator pointing to the first element

       *          greater than key, or end().

       */

      iterator

      upper_bound(const key_type& __x)

      { return _M_t.upper_bound(__x); }

      /**

       *  @brief Finds the end of a subsequence matching given key.

       *  @param  x  Key of (key, value) pair to be located.

       *  @return  Read-only (constant) iterator pointing to first iterator

       *           greater than key, or end().

       */

      const_iterator

      upper_bound(const key_type& __x) const

      { return _M_t.upper_bound(__x); }

      /**

       *  @brief Finds a subsequence matching given key.

       *  @param  x  Key of (key, value) pairs to be located.

       *  @return  Pair of iterators that possibly points to the subsequence

       *           matching given key.

       *

       *  This function is equivalent to

       *  @code

       *    std::make_pair(c.lower_bound(val),

       *                   c.upper_bound(val))

       *  @endcode

       *  (but is faster than making the calls separately).

       *

       *  This function probably only makes sense for multimaps.

       */

      std::pair<iterator, iterator>

      equal_range(const key_type& __x)

      { return _M_t.equal_range(__x); }

      /**

       *  @brief Finds a subsequence matching given key.

       *  @param  x  Key of (key, value) pairs to be located.