...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
Front Page / Sequences / Intrinsic Metafunctions / at |
template< typename Sequence , typename N > struct at { typedef unspecified type; }; template< typename AssocSeq , typename Key , typename Default = unspecified > struct at { typedef unspecified type; };
at is an overloaded name:
#include <boost/mpl/at.hpp>
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. |
For any Forward Sequence s, and Integral Constant n:
typedef at<s,n>::type t;
Return type: | A type. |
---|---|
Precondition: | 0 <= n::value < size<s>::value. |
Semantics: | Equivalent to typedef deref< advance< begin<s>::type,n >::type >::type t; |
For any Associative Sequence s, and arbitrary types key and x:
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. |
typedef at<s,key>::type t;
Return type: | A type. |
---|---|
Semantics: | Equivalent to typedef at<s,key,void_>::type t; |
Sequence archetype | Complexity |
---|---|
Forward Sequence | Linear. |
Random Access Sequence | Amortized constant time. |
Associative Sequence | Amortized constant time. |
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 );
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 > ));