| .. 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) |
