.. 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)