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< html >< head >< title > boost::mpl::at</ title >
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< link rel = "stylesheet" href = "../mpl_wiki.css" >
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< h1 >< a href = "../Table_of_Contents.html" >< img src = "../mpl_logo.jpg" alt = "[Home]" border = 0 align = "right" ></ a > at</ h1 >< h3 > Synopsis</ h3 >
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< p >
< pre >
template<
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typename Sequence
, typename N
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>
struct at
{
typedef < em > unspecified</ em > type;
};
</ pre >
< p >
< h3 > Description</ h3 >
< p >
Returns a type identical to the < code > N</ code > -th element from the beginning of the sequence.
< p >
< h3 > Definition</ h3 >
< p >
< pre >
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#include "< a href = "../../../../../boost/mpl/at.hpp" > boost/mpl/at.hpp</ a > "
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</ pre >
< p >
< h3 > Parameters</ h3 >
< table border = "1" >
< tr >< th > Parameter </ th >< th > Requirement </ th >< th > Description </ th ></ tr >
< tr >< td >< code > Sequence</ code ></ td >< td > A model of < a href = "../Forward_Sequence.html" > Forward Sequence</ a ></ td >< td > A sequence to be examined. </ td ></ tr >
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< tr >< td >< code > N</ code ></ td >< td > A model of < a href = "../Integral_Constant.html" > Integral Constant</ a ></ td >< td > The offset from the beginning of the sequence that specifies the element to be retrieved. </ td ></ tr >
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</ table >
< p >
< h3 > Expression semantics</ h3 >
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< table border = "1" >
< tr >< th > Expression </ th >< th > Expression type </ th >< th > Precondition </ th >< th > Semantics </ th >< th > Postcondition </ th ></ tr >
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< tr >< td >< code > typedef at< Sequence,N> ::type t;</ code ></ td >< td > A type</ td >< td >< code > 0 < = N::value < size< Sequence> ::type::value</ code ></ td >< td > Equivalent to < code > typedef advance< begin< Sequence> ::type,N > ::type::type t;</ code ></ td >< td ></ td ></ tr >
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</ table >
< p >
< h3 > Complexity</ h3 >
< p >
Depends on the implementation of the particular sequence it is applied to. Linear in the worst case, or amortized constant time.
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< h3 > Example</ h3 >
< p >
< pre >
typedef < a href = "../Reference/range_c.html" > range_c</ a > < long,10,50> range;
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BOOST_STATIC_ASSERT(at< range, int_< 0> > ::type::value == 10);
BOOST_STATIC_ASSERT(at< range, int_< 10> > ::type::value == 20);
BOOST_STATIC_ASSERT(at< range, int_< 40> > ::type::value == 50);
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</ pre >
< p >
< h3 > See also</ h3 >
< p >
< a href = "../Forward_Sequence.html" > Forward Sequence</ a > , < code >< a href = "../Reference/at_c.html" > at_c</ a ></ code > , < code >< a href = "../Reference/front.html" > front</ a ></ code > , < code >< a href = "../Reference/back.html" > back</ a ></ code >
< p >< hr >
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< a href = "../Table_of_Contents.html" > Table of Contents</ a >< br > Last edited March 12, 2003 6:26 am</ body ></ html >
