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< title > The MPL Reference Manual: at< / title >
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< body class = "docframe refmanual" >
< 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 >
< / tr > < / table > < div class = "header-separator" > < / div >
< 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 -->
< / div >
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< div class = "footer-separator" > < / div >
< 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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