has_key

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##### has_key

###### Description

Returns a type convertible to `bool` that evaluates to `true` if the sequence contains an element associated with a Key, else, evaluates to `false`.

###### Synopsis

    template <typename Key, typename Sequence>
    typename result_of::has_key<Sequence, Key>::type
    has_key(Sequence const& seq);

###### Parameters

Parameter | Requirement | Description
--- | --- | ---
`seq` | Model of Associative Sequence | The sequence we wish to investigate.
`Key` | Any type | The queried key.

###### Expression Semantics

    has_key<Key>(seq);

Return type: Convertible to `bool`.

Semantics: Evaluates to `true` if the sequence contains an element associated with Key, else, evaluates to `false`.

###### Header

    #include <boost/fusion/sequence/intrinsic/has_key.hpp>
    #include <boost/fusion/include/has_key.hpp>

###### Example

    set<int, char, bool> s(1, 'x', true);
    assert(has_key<char>(s) == true);