at

Synopsis

template<
      typename Sequence
    , typename N
    >
struct at
{
    typedef unspecified type;
};

template<
      typename AssocSeq
    , typename Key
    , typename Default = unspecified
    >
struct at
{
    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.

Header

#include <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<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;

Complexity

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

Example

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 > ));