ceph/src/boost/libs/vmd/doc/vmd_sequence_access.qbk

   1 [/
   2   (C) Copyright Edward Diener 2011-2015
   3   Distributed under the Boost Software License, Version 1.0.
   4   (See accompanying file LICENSE_1_0.txt or copy at
   5   http://www.boost.org/LICENSE_1_0.txt).
   6 ]
   7
   8 [section:vmd_sequence_access Accessing a sequence element]
   9
  10 It is possible to access an individual element of a sequence.
  11 The macro to do this is called BOOST_VMD_ELEM. The macro takes two
  12 required parameters. The required parameters are the element number
  13 to access and the sequence, in that order. The element number is a
  14 0-based number and its maximum value should be one less than the size
  15 of the sequence.
  16
  17 The BOOST_VMD_ELEM macro returns the actual sequence element. If the
  18 first required parameter is greater or equal to the size of the
  19 sequence the macro returns emptiness. Because of this using
  20 BOOST_VMD_ELEM on an empty sequence, whose size is 0, always returns
  21 emptiness.
  22
  23  #include <boost/vmd/elem.hpp>
  24
  25  #define BOOST_VMD_REGISTER_ANAME (ANAME)
  26  #define A_SEQUENCE (1,2,3) 46 (list_data1,(list_data2,BOOST_PP_NIL)) BOOST_VMD_TYPE_SEQ ANAME
  27  #define AN_EMPTY_SEQUENCE
  28
  29  BOOST_VMD_ELEM(0,A_SEQUENCE) will return (1,2,3)
  30  BOOST_VMD_ELEM(1,A_SEQUENCE) will return 46
  31  BOOST_VMD_ELEM(2,A_SEQUENCE) will return (list_data1,(list_data2,BOOST_PP_NIL))
  32  BOOST_VMD_ELEM(3,A_SEQUENCE) will return BOOST_VMD_TYPE_SEQ
  33  BOOST_VMD_ELEM(4,A_SEQUENCE) will return ANAME
  34
  35  BOOST_VMD_ELEM(5,A_SEQUENCE) will return emptiness
  36  BOOST_VMD_ELEM(0,AN_EMPTY_SEQUENCE) will return emptiness
  37
  38 Accessing an element of a sequence directly is slower than accessing an element
  39 of a Boost PP data type or even variadic data, since each access has to directly
  40 cycle through each element of the sequence to get to the one being accessed.
  41 The process of sequentially parsing each element again each time is slower than
  42 accessing a Boost PP data type element.
  43
  44 [heading Usage]
  45
  46 You can use the general header file:
  47
  48  #include <boost/vmd/vmd.hpp>
  49
  50 or you can use the individual header file:
  51
  52  #include <boost/vmd/elem.hpp>
  53
  54 [endsect]