at

Synopsis

template<
      typename Sequence
    , typename N
    >
struct[at](./at.html){
    typedef_unspecified_type;
};

template<
      typename AssocSeq
    , typename Key
    , typename Default =_unspecified_>
struct[at](./at.html){
    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.

#include <[boost/mpl/at.hpp](../../../../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

For any Forward Sequence s, and Integral Constant n:

typedef[at](./at.html)<s,n>::type t;

| Return type: |

A type.

| | Precondition: |

0 <= n::value < size<s>::value.

| | Semantics: |

Equivalent to

typedef[deref](./deref.html)<[advance](./advance.html)<[begin](./begin.html)<s>::type,n >::type >::type t;

For any Associative Sequence s, and arbitrary types key and x:

typedef[at](./at.html)<s,key,x>::type t;

typedef[at](./at.html)<s,key>::type t;

| Return type: |

A type.

| | Semantics: |

Equivalent to

typedef[at](./at.html)<s,key,[void\_](./void.html)>::type t;

Complexity

Sequence archetype	Complexity
Forward Sequence	Linear.
Random Access Sequence	Amortized constant time.
Associative Sequence	Amortized constant time.

Example

typedef[range\_c](./range-c.html)<long,10,50> range;[BOOST\_MPL\_ASSERT\_RELATION](./assert-relation.html)( ([at](./at.html)< range,[int\_](./int.html)<0> >::value), ==, 10 );[BOOST\_MPL\_ASSERT\_RELATION](./assert-relation.html)( ([at](./at.html)< range,[int\_](./int.html)<10> >::value), ==, 20 );[BOOST\_MPL\_ASSERT\_RELATION](./assert-relation.html)( ([at](./at.html)< range,[int\_](./int.html)<40> >::value), ==, 50 );

typedef[set](./set.html)< int const,long*,double > s;[BOOST\_MPL\_ASSERT](./assert.html)(( is_same<[at](./at.html)<s,char>::type,[void\_](./void.html)> ));[BOOST\_MPL\_ASSERT](./assert.html)(( is_same<[at](./at.html)<s,int>::type, int > ));

The MPL Reference Manual: at

at