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# order

### Synopsis

    template<
          typename Sequence
        , typename Key
        >
    struct order
    {
        typedef unspecified type;
    };

### Description

Returns a unique unsigned Integral Constant associated with the key `Key` in `Sequence`.

### Header

    #include <boost/mpl/order.hpp>

### Model of

Tag Dispatched Metafunction

### Parameters

Parameter | Requirement | Description
--- | --- | ---
`Sequence` | Associative Sequence | A sequence to query.
`Key` | Any type | The queried key.

### Expression semantics

For any Associative Sequence `s`, and arbitrary type `key`:

    typedef order<s,key>::type n;

Return type: | Unsigned Integral Constant.
Semantics: | If `has_key<s,key>::value == true`, `n` is a unique unsigned Integral Constant associated with `key` in `s`; otherwise, `n` is identical to `void_`.

### Complexity

Amortized constant time.

### Example

    typedef map< pair<int,unsigned>, pair<char,long> > m;

    BOOST_MPL_ASSERT_NOT(( is_same< order<m,int>::type, void_ > ));
    BOOST_MPL_ASSERT(( is_same< order<m,long>::type,void_ > ));

### See also

Associative Sequence, `has_key`, `count`, `map`