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< title > The MPL Reference Manual: at</ title >
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< table class = "header" >< tr class = "header" >< td class = "header-group navigation-bar" >< span class = "navigation-group" >< a href = "./intrinsic-metafunctions.html" class = "navigation-link" > Prev</ a > < a href = "./at-c.html" class = "navigation-link" > Next</ a ></ span >< span class = "navigation-group-separator" > | </ span >< span class = "navigation-group" > Back < a href = "./at-c.html" class = "navigation-link" > Along</ a ></ span >< span class = "navigation-group-separator" > | </ span >< span class = "navigation-group" >< a href = "./intrinsic-metafunctions.html" class = "navigation-link" > Up</ a > < a href = "../refmanual.html" class = "navigation-link" > Home</ a ></ span >< span class = "navigation-group-separator" > | </ span >< span class = "navigation-group" >< a href = "./refmanual_toc.html" class = "navigation-link" > Full TOC</ a ></ span ></ td >
< td class = "header-group page-location" >< a href = "../refmanual.html" class = "navigation-link" > Front Page</ a > / < a href = "./sequences.html" class = "navigation-link" > Sequences</ a > / < a href = "./intrinsic-metafunctions.html" class = "navigation-link" > Intrinsic Metafunctions</ a > / < a href = "./at.html" class = "navigation-link" > at</ a ></ td >
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< div class = "section" id = "at" >
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< h1 >< a class = "toc-backref" href = "./intrinsic-metafunctions.html#id1425" > at</ a ></ h1 >
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< div class = "section" id = "id177" >
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< h3 >< a class = "subsection-title" href = "#synopsis" name = "synopsis" > Synopsis</ a ></ h3 >
< pre class = "literal-block" >
template<
typename Sequence
, typename N
>
struct < a href = "./at.html" class = "identifier" > at</ a >
{
typedef < em > unspecified</ em > type;
};
template<
typename AssocSeq
, typename Key
, typename Default = < em > unspecified</ em >
>
struct < a href = "./at.html" class = "identifier" > at</ a >
{
typedef < em > unspecified</ em > type;
};
</ pre >
</ div >
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< div class = "section" id = "id178" >
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< h3 >< a class = "subsection-title" href = "#description" name = "description" > Description</ a ></ h3 >
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< p >< tt class = "literal" >< span class = "pre" >< a href = "./at.html" class = "identifier" > at</ a ></ span ></ tt > is an < a class = "reference internal" href = "./terminology.html#overloaded-name" > overloaded name</ a > :</ p >
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< ul class = "simple" >
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< li >< tt class = "literal" >< span class = "pre" >< a href = "./at.html" class = "identifier" > at</ a > < Sequence,N> </ span ></ tt > returns the < tt class = "literal" >< span class = "pre" > N</ span ></ tt > -th element from the beginning of the
< a class = "reference internal" href = "./forward-sequence.html" > Forward Sequence</ a > < tt class = "literal" >< span class = "pre" > Sequence</ span ></ tt > .</ li >
< li >< tt class = "literal" >< span class = "pre" >< a href = "./at.html" class = "identifier" > at</ a > < AssocSeq,Key,Default> </ span ></ tt > returns the first element associated with < tt class = "literal" >< span class = "pre" > Key</ span ></ tt >
in the < a class = "reference internal" href = "./associative-sequence.html" > Associative Sequence</ a > < tt class = "literal" >< span class = "pre" > AssocSeq</ span ></ tt > , or < tt class = "literal" >< span class = "pre" > Default</ span ></ tt > if no such element
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exists.</ li >
</ ul >
</ div >
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< div class = "section" id = "id179" >
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< h3 >< a class = "subsection-title" href = "#header" name = "header" > Header</ a ></ h3 >
< pre class = "literal-block" >
#include < < a href = "../../../../boost/mpl/at.hpp" class = "header" > boost/mpl/at.hpp</ a > >
</ pre >
</ div >
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< div class = "section" id = "id180" >
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< h3 >< a class = "subsection-title" href = "#model-of" name = "model-of" > Model of</ a ></ h3 >
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< p >< a class = "reference internal" href = "./tag-dispatched-metafunction.html" > Tag Dispatched Metafunction</ a ></ p >
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</ div >
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< div class = "section" id = "id181" >
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< h3 >< a class = "subsection-title" href = "#parameters" name = "parameters" > Parameters</ a ></ h3 >
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< table border = "1" class = "docutils table" >
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< colgroup >
< col width = "17%" />
< col width = "30%" />
< col width = "53%" />
</ colgroup >
< thead valign = "bottom" >
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< tr >< th class = "head" > Parameter</ th >
< th class = "head" > Requirement</ th >
< th class = "head" > Description</ th >
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</ tr >
</ thead >
< tbody valign = "top" >
< tr >< td >< tt class = "literal" >< span class = "pre" > Sequence</ span ></ tt ></ td >
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< td >< a class = "reference internal" href = "./forward-sequence.html" > Forward Sequence</ a ></ td >
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< td > A sequence to be examined.</ td >
</ tr >
< tr >< td >< tt class = "literal" >< span class = "pre" > AssocSeq</ span ></ tt ></ td >
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< td >< a class = "reference internal" href = "./associative-sequence.html" > Associative Sequence</ a ></ td >
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< td > A sequence to be examined.</ td >
</ tr >
< tr >< td >< tt class = "literal" >< span class = "pre" > N</ span ></ tt ></ td >
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< td >< a class = "reference internal" href = "./integral-constant.html" > Integral Constant</ a ></ td >
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< td > An offset from the beginning of the sequence
specifying the element to be retrieved.</ td >
</ tr >
< tr >< td >< tt class = "literal" >< span class = "pre" > Key</ span ></ tt ></ td >
< td > Any type</ td >
< td > A key for the element to be retrieved.</ td >
</ tr >
< tr >< td >< tt class = "literal" >< span class = "pre" > Default</ span ></ tt ></ td >
< td > Any type</ td >
< td > A default value to return if the element is
not found.</ td >
</ tr >
</ tbody >
</ table >
</ div >
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< div class = "section" id = "id182" >
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< h3 >< a class = "subsection-title" href = "#expression-semantics" name = "expression-semantics" > Expression semantics</ a ></ h3 >
< div class = "expression-semantics compound" >
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< p class = "compound-first" > For any < a class = "reference internal" href = "./forward-sequence.html" > Forward Sequence</ a > < tt class = "literal" >< span class = "pre" > s</ span ></ tt > , and < a class = "reference internal" href = "./integral-constant.html" > Integral Constant</ a > < tt class = "literal" >< span class = "pre" > n</ span ></ tt > :</ p >
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< pre class = "compound-middle literal-block" >
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typedef < a href = "./at.html" class = "identifier" > at</ a > < s,n> ::type t;
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</ pre >
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< table class = "compound-last docutils field-list" frame = "void" rules = "none" >
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< col class = "field-name" />
< col class = "field-body" />
< tbody valign = "top" >
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< tr class = "field" >< th class = "field-name" > Return type:</ th >< td class = "field-body" >< p class = "first" > A type.</ p >
</ td >
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</ tr >
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< tr class = "field" >< th class = "field-name" > Precondition:</ th >< td class = "field-body" >< p class = "first" >< tt class = "literal" >< span class = "pre" > 0</ span > < span class = "pre" > < =</ span > < span class = "pre" > n::value</ span > < span class = "pre" > < </ span > < span class = "pre" >< a href = "./size.html" class = "identifier" > size</ a > < s> ::value</ span ></ tt > .</ p >
</ td >
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</ tr >
< tr class = "field" >< th class = "field-name" > Semantics:</ th >< td class = "field-body" >< p class = "first" > Equivalent to</ p >
< pre class = "last literal-block" >
typedef < a href = "./deref.html" class = "identifier" > deref</ a > < < a href = "./advance.html" class = "identifier" > advance</ a > < < a href = "./begin.html" class = "identifier" > begin</ a > < s> ::type,n > ::type > ::type t;
</ pre >
</ td >
</ tr >
</ tbody >
</ table >
</ div >
< div class = "expression-semantics compound" >
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< p class = "compound-first" > For any < a class = "reference internal" href = "./associative-sequence.html" > Associative Sequence</ a > < tt class = "literal" >< span class = "pre" > s</ span ></ tt > , and arbitrary types < tt class = "literal" >< span class = "pre" > key</ span ></ tt > and < tt class = "literal" >< span class = "pre" > x</ span ></ tt > :</ p >
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< pre class = "compound-middle literal-block" >
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typedef < a href = "./at.html" class = "identifier" > at</ a > < s,key,x> ::type t;
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</ pre >
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< table class = "compound-middle docutils field-list" frame = "void" rules = "none" >
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< col class = "field-name" />
< col class = "field-body" />
< tbody valign = "top" >
< tr class = "field" >< th class = "field-name" > Return type:</ th >< td class = "field-body" > A type.</ td >
</ tr >
< tr class = "field" >< th class = "field-name" > Semantics:</ th >< td class = "field-body" > If < tt class = "literal" >< span class = "pre" >< a href = "./has-key.html" class = "identifier" > has_key</ a > < s,key> ::value</ span > < span class = "pre" > ==</ span > < span class = "pre" > true</ span ></ tt > , < tt class = "literal" >< span class = "pre" > t</ span ></ tt > is the value type associated with < tt class = "literal" >< span class = "pre" > key</ span ></ tt > ;
otherwise < tt class = "literal" >< span class = "pre" > t</ span ></ tt > is identical to < tt class = "literal" >< span class = "pre" > x</ span ></ tt > .</ td >
</ tr >
</ tbody >
</ table >
<!-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -->
< pre class = "compound-middle literal-block" >
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typedef < a href = "./at.html" class = "identifier" > at</ a > < s,key> ::type t;
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</ pre >
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< table class = "compound-last docutils field-list" frame = "void" rules = "none" >
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< col class = "field-name" />
< col class = "field-body" />
< tbody valign = "top" >
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< tr class = "field" >< th class = "field-name" > Return type:</ th >< td class = "field-body" >< p class = "first" > A type.</ p >
</ td >
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</ tr >
< tr class = "field" >< th class = "field-name" > Semantics:</ th >< td class = "field-body" >< p class = "first" > Equivalent to</ p >
< pre class = "last literal-block" >
typedef < a href = "./at.html" class = "identifier" > at</ a > < s,key,< a href = "./void.html" class = "identifier" > void_</ a > > ::type t;
</ pre >
</ td >
</ tr >
</ tbody >
</ table >
</ div >
</ div >
< div class = "section" id = "complexity" >
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< h3 > Complexity</ h3 >
< table border = "1" class = "docutils table" >
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< colgroup >
< col width = "47%" />
< col width = "53%" />
</ colgroup >
< thead valign = "bottom" >
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< tr >< th class = "head" > Sequence archetype</ th >
< th class = "head" > Complexity</ th >
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</ tr >
</ thead >
< tbody valign = "top" >
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< tr >< td >< a class = "reference internal" href = "./forward-sequence.html" > Forward Sequence</ a ></ td >
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< td > Linear.</ td >
</ tr >
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< tr >< td >< a class = "reference internal" href = "./random-access-sequence.html" > Random Access Sequence</ a ></ td >
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< td > Amortized constant time.</ td >
</ tr >
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< tr >< td >< a class = "reference internal" href = "./associative-sequence.html" > Associative Sequence</ a ></ td >
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< td > Amortized constant time.</ td >
</ tr >
</ tbody >
</ table >
</ div >
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< div class = "section" id = "id183" >
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< h3 >< a class = "subsection-title" href = "#example" name = "example" > Example</ a ></ h3 >
< pre class = "literal-block" >
typedef < a href = "./range-c.html" class = "identifier" > range_c</ a > < long,10,50> range;
< a href = "./assert-relation.html" class = "identifier" > BOOST_MPL_ASSERT_RELATION</ a > ( (< a href = "./at.html" class = "identifier" > at</ a > < range, < a href = "./int.html" class = "identifier" > int_</ a > < 0> > ::value), ==, 10 );
< a href = "./assert-relation.html" class = "identifier" > BOOST_MPL_ASSERT_RELATION</ a > ( (< a href = "./at.html" class = "identifier" > at</ a > < range, < a href = "./int.html" class = "identifier" > int_</ a > < 10> > ::value), ==, 20 );
< a href = "./assert-relation.html" class = "identifier" > BOOST_MPL_ASSERT_RELATION</ a > ( (< a href = "./at.html" class = "identifier" > at</ a > < range, < a href = "./int.html" class = "identifier" > int_</ a > < 40> > ::value), ==, 50 );
</ pre >
< pre class = "literal-block" >
typedef < a href = "./set.html" class = "identifier" > set</ a > < int const,long*,double > s;
< a href = "./assert.html" class = "identifier" > BOOST_MPL_ASSERT</ a > (( is_same< < a href = "./at.html" class = "identifier" > at</ a > < s,char> ::type, < a href = "./void.html" class = "identifier" > void_</ a > > ));
< a href = "./assert.html" class = "identifier" > BOOST_MPL_ASSERT</ a > (( is_same< < a href = "./at.html" class = "identifier" > at</ a > < s,int> ::type, int > ));
</ pre >
</ div >
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< div class = "section" id = "id184" >
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< h3 >< a class = "subsection-title" href = "#see-also" name = "see-also" > See also</ a ></ h3 >
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< p >< a class = "reference internal" href = "./forward-sequence.html" > Forward Sequence</ a > , < a class = "reference internal" href = "./random-access-sequence.html" > Random Access Sequence</ a > , < a class = "reference internal" href = "./associative-sequence.html" > Associative Sequence</ a > , < a class = "reference internal" href = "./at-c.html" > at_c</ a > , < a class = "reference internal" href = "./front.html" > front</ a > , < a class = "reference internal" href = "./back.html" > back</ a ></ p >
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<!-- Sequences/Intrinsic Metafunctions//at_c -->
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< table class = "footer" >< tr class = "footer" >< td class = "header-group navigation-bar" >< span class = "navigation-group" >< a href = "./intrinsic-metafunctions.html" class = "navigation-link" > Prev</ a > < a href = "./at-c.html" class = "navigation-link" > Next</ a ></ span >< span class = "navigation-group-separator" > | </ span >< span class = "navigation-group" > Back < a href = "./at-c.html" class = "navigation-link" > Along</ a ></ span >< span class = "navigation-group-separator" > | </ span >< span class = "navigation-group" >< a href = "./intrinsic-metafunctions.html" class = "navigation-link" > Up</ a > < a href = "../refmanual.html" class = "navigation-link" > Home</ a ></ span >< span class = "navigation-group-separator" > | </ span >< span class = "navigation-group" >< a href = "./refmanual_toc.html" class = "navigation-link" > Full TOC</ a ></ span ></ td >
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< td >< div class = "copyright-footer" >< div class = "copyright" > Copyright © 2001-2009 Aleksey Gurtovoy and David Abrahams</ div >
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at < a class = "reference external" href = "http://www.boost.org/LICENSE_1_0.txt" target = "_top" > http://www.boost.org/LICENSE_1_0.txt</ a > )</ div ></ td ></ tr ></ table ></ body >
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