| .. 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) |