.. Sequences/Intrinsic Metafunctions//begin

begin

Synopsis

.. parsed-literal::

template<
      typename X
    >
struct begin
{
    typedef |unspecified| type;
};

Description

Returns an iterator that points to the first element of the sequence. If the argument is not a |Forward Sequence|, returns |void_|.

Header

.. parsed-literal::

#include <boost/mpl/begin_end.hpp>

Model of

|Tag Dispatched Metafunction|

Parameters

+---------------+-------------------+---------------------------------------------------+ | Parameter | Requirement | Description | +===============+===================+===================================================+ | X | Any type | A type whose begin iterator, if any, will be | | | | returned. | +---------------+-------------------+---------------------------------------------------+

Expression semantics

For any arbitrary type x:

.. parsed-literal::

typedef begin<x>::type first;

:Return type: |Forward Iterator| or |void_|.

:Semantics: If x is a |Forward Sequence|, first is an iterator pointing to the first element of s; otherwise first is |void_|.

:Postcondition: If first is an iterator, it is either dereferenceable or past-the-end; it is past-the-end if and only if size<x>::value == 0.

Complexity

Amortized constant time.

Example

.. parsed-literal::

typedef vector< unsigned char,unsigned short,
    unsigned int,unsigned long > unsigned_types;

typedef begin<unsigned_types>::type iter;
BOOST_MPL_ASSERT(( is_same< deref<iter>::type, unsigned char > ));

BOOST_MPL_ASSERT(( is_same< begin<int>::type, void\_ > ));

See also

|Iterators|, |Forward Sequence|, |end|, |size|, |empty|

.. 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)