| .. Sequences/Intrinsic Metafunctions//front |
| |
| front |
| ===== |
| |
| Synopsis |
| -------- |
| |
| .. parsed-literal:: |
| |
| template< |
| typename Sequence |
| > |
| struct front |
| { |
| typedef |unspecified| type; |
| }; |
| |
| |
| |
| Description |
| ----------- |
| |
| Returns the first element in the sequence. |
| |
| |
| Header |
| ------ |
| |
| .. parsed-literal:: |
| |
| #include <boost/mpl/front.hpp> |
| |
| |
| Model of |
| -------- |
| |
| |Tag Dispatched Metafunction| |
| |
| |
| Parameters |
| ---------- |
| |
| +---------------+-----------------------+-----------------------------------------------+ |
| | Parameter | Requirement | Description | |
| +===============+=======================+===============================================+ |
| | ``Sequence`` | |Forward Sequence| | A sequence to be examined. | |
| +---------------+-----------------------+-----------------------------------------------+ |
| |
| |
| Expression semantics |
| -------------------- |
| |
| For any |Forward Sequence| ``s``: |
| |
| |
| .. parsed-literal:: |
| |
| typedef front<s>::type t; |
| |
| :Return type: |
| A type. |
| |
| :Precondition: |
| ``empty<s>::value == false``. |
| |
| :Semantics: |
| Equivalent to |
| |
| .. parsed-literal:: |
| |
| typedef deref< begin<s>::type >::type t; |
| |
| |
| |
| Complexity |
| ---------- |
| |
| Amortized constant time. |
| |
| |
| Example |
| ------- |
| |
| .. parsed-literal:: |
| |
| typedef list<long>::type types1; |
| typedef list<int,long>::type types2; |
| typedef list<char,int,long>::type types3; |
| |
| BOOST_MPL_ASSERT(( is_same< front<types1>::type, long > )); |
| BOOST_MPL_ASSERT(( is_same< front<types2>::type, int> )); |
| BOOST_MPL_ASSERT(( is_same< front<types3>::type, char> )); |
| |
| |
| |
| See also |
| -------- |
| |
| |Forward Sequence|, |back|, |push_front|, |begin|, |deref|, |at| |
| |
| |
| .. 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) |
