| .. Sequences/Intrinsic Metafunctions//key_type |
| |
| key_type |
| ======== |
| |
| Synopsis |
| -------- |
| |
| .. parsed-literal:: |
| |
| template< |
| typename Sequence |
| , typename X |
| > |
| struct key_type |
| { |
| typedef |unspecified| type; |
| }; |
| |
| |
| |
| Description |
| ----------- |
| |
| Returns the |key| that would be used to identify ``X`` in ``Sequence``. |
| |
| |
| Header |
| ------ |
| |
| .. parsed-literal:: |
| |
| #include <boost/mpl/key_type.hpp> |
| |
| |
| Model of |
| -------- |
| |
| |Tag Dispatched Metafunction| |
| |
| |
| Parameters |
| ---------- |
| |
| +---------------+---------------------------+-----------------------------------------------+ |
| | Parameter | Requirement | Description | |
| +===============+===========================+===============================================+ |
| | ``Sequence`` | |Associative Sequence| | A sequence to query. | |
| +---------------+---------------------------+-----------------------------------------------+ |
| | ``X`` | Any type | The type to get the |key| for. | |
| +---------------+---------------------------+-----------------------------------------------+ |
| |
| |
| Expression semantics |
| -------------------- |
| |
| For any |Associative Sequence| ``s``, iterators ``pos1`` and ``pos2`` in ``s``, and an |
| artibrary type ``x``: |
| |
| .. parsed-literal:: |
| |
| typedef key_type<s,x>::type k; |
| |
| :Return type: |
| A type. |
| |
| :Precondition: |
| ``x`` can be put in ``s``. |
| |
| :Semantics: |
| ``k`` is the |key| that would be used to identify ``x`` in ``s``. |
| |
| :Postcondition: |
| If ``key_type< s,deref<pos1>::type >::type`` is identical to |
| ``key_type< s,deref<pos2>::type >::type`` then ``pos1`` is identical to ``pos2``. |
| |
| |
| |
| Complexity |
| ---------- |
| |
| Amortized constant time. |
| |
| |
| Example |
| ------- |
| |
| .. parsed-literal:: |
| |
| typedef key_type< map<>,pair<int,unsigned> >::type k1; |
| typedef key_type< set<>,pair<int,unsigned> >::type k2; |
| |
| BOOST_MPL_ASSERT(( is_same< k1,int > )); |
| BOOST_MPL_ASSERT(( is_same< k2,pair<int,unsigned> > )); |
| |
| |
| See also |
| -------- |
| |
| |Associative Sequence|, |value_type|, |has_key|, |set|, |map| |
| |
| |
| .. 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) |