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.. Sequences/Intrinsic Metafunctions//has_key
has_key
=======
Synopsis
--------
.. parsed-literal::
template<
typename Sequence
, typename Key
>
struct has_key
{
typedef |unspecified| type;
};
Description
-----------
Returns a true-valued |Integral Constant| if ``Sequence`` contains an element
with key ``Key``.
Header
------
.. parsed-literal::
#include <boost/mpl/has_key.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``:
.. parsed-literal::
typedef has_key<s,key>::type c;
:Return type:
Boolean |Integral Constant|.
:Semantics:
``c::value == true`` if ``key`` is in ``s``'s set of keys; otherwise
``c::value == false``.
Complexity
----------
Amortized constant time.
Example
-------
.. parsed-literal::
typedef map< pair<int,unsigned>, pair<char,long> > m;
BOOST_MPL_ASSERT_NOT(( has_key<m,long> ));
typedef insert< m, pair<long,unsigned long> > m1;
BOOST_MPL_ASSERT(( has_key<m1,long> ));
See also
--------
|Associative Sequence|, |count|, |insert|, |erase_key|
.. copyright:: Copyright © 2001-2009 Aleksey Gurtovoy and David Abrahams
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)