.. Sequences/Intrinsic Metafunctions//at.. Copyright Aleksey Gurtovoy, David Abrahams 2007... Distributed under the Boost.. Software License, Version 1.0. (See accompanying.. file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)at==Synopsis--------.. parsed-literal::        template<          typename Sequence        , typename N        >    struct at    {        typedef |unspecified| type;    };    template<          typename AssocSeq        , typename Key        , typename Default = |unspecified|        >    struct at    {        typedef |unspecified| type;    };Description-----------``at`` is an |overloaded name|:* ``at<Sequence,N>`` returns the ``N``-th element from the beginning of the   |Forward Sequence| ``Sequence``.* ``at<AssocSeq,Key,Default>`` returns the first element associated with ``Key``   in the |Associative Sequence| ``AssocSeq``, or ``Default`` if no such element  exists.Header------.. parsed-literal::        #include <boost/mpl/at.hpp>Model of--------|Tag Dispatched Metafunction|Parameters----------+---------------+---------------------------+-----------------------------------------------+| Parameter     | Requirement               | Description                                   |+===============+===========================+===============================================+| ``Sequence``  | |Forward Sequence|        | A sequence to be examined.                    |+---------------+---------------------------+-----------------------------------------------+| ``AssocSeq``  | |Associative Sequence|    | A sequence to be examined.                    |+---------------+---------------------------+-----------------------------------------------+| ``N``         | |Integral Constant|       | An offset from the beginning of the sequence  ||               |                           | specifying the element to be retrieved.       |+---------------+---------------------------+-----------------------------------------------+| ``Key``       | Any type                  | A key for the element to be retrieved.        |+---------------+---------------------------+-----------------------------------------------+| ``Default``   | Any type                  | A default value to return if the element is   ||               |                           | not found.                                    |+---------------+---------------------------+-----------------------------------------------+Expression semantics--------------------.. compound::    :class: expression-semantics    For any |Forward Sequence| ``s``, and |Integral Constant| ``n``:    .. parsed-literal::        typedef at<s,n>::type t;     :Return type:        A type.    :Precondition:        ``0 <= n::value < size<s>::value``.    :Semantics:        Equivalent to                 .. parsed-literal::                      typedef deref< advance< begin<s>::type,n >::type >::type t;.. compound::    :class: expression-semantics    For any |Associative Sequence| ``s``, and arbitrary types ``key`` and ``x``:    .. parsed-literal::        typedef at<s,key,x>::type t;     :Return type:        A type.    :Semantics:        If ``has_key<s,key>::value == true``, ``t`` is the value type associated with ``key``;        otherwise ``t`` is identical to ``x``.    .. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~    .. parsed-literal::        typedef at<s,key>::type t;         :Return type:        A type.    :Semantics:        Equivalent to        .. parsed-literal::                      typedef at<s,key,void\_>::type t;Complexity----------+-------------------------------+-----------------------------------+| Sequence archetype            | Complexity                        |+===============================+===================================+| |Forward Sequence|            | Linear.                           |+-------------------------------+-----------------------------------+| |Random Access Sequence|      | Amortized constant time.          |+-------------------------------+-----------------------------------+| |Associative Sequence|        | Amortized constant time.          |+-------------------------------+-----------------------------------+Example-------.. parsed-literal::        typedef range_c<long,10,50> range;    BOOST_MPL_ASSERT_RELATION( (at< range, int_<0> >::value), ==, 10 );    BOOST_MPL_ASSERT_RELATION( (at< range, int_<10> >::value), ==, 20 );    BOOST_MPL_ASSERT_RELATION( (at< range, int_<40> >::value), ==, 50 );.. parsed-literal::    typedef set< int const,long*,double > s;    BOOST_MPL_ASSERT(( is_same< at<s,char>::type, void\_ > ));    BOOST_MPL_ASSERT(( is_same< at<s,int>::type, int > ));See also--------|Forward Sequence|, |Random Access Sequence|, |Associative Sequence|, |at_c|, |front|, |back|