.. Sequences/Intrinsic Metafunctions//at

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|

.. copyright:: Copyright 2001-2009 Aleksey Gurtovoy and David Abrahams 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)