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# 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 > ));

### See also

Forward Sequence, Random Access Sequence, Associative Sequence, `at_c`, `front`, `back`