ceph/src/boost/libs/mpl/doc/src/refmanual/fold.rst

   1 .. Algorithms/Iteration Algorithms//fold
   2
   3 fold
   4 ====
   5
   6 Synopsis
   7 --------
   8
   9 .. parsed-literal::
  10
  11     template<
  12           typename Sequence
  13         , typename State
  14         , typename ForwardOp
  15         >
  16     struct fold
  17     {
  18         typedef |unspecified| type;
  19     };
  20
  21
  22
  23 Description
  24 -----------
  25
  26 Returns the result of the successive application of binary ``ForwardOp`` to the
  27 result of the previous ``ForwardOp`` invocation (``State`` if it's the first call)
  28 and every element of the sequence in the range |begin/end<Sequence>| in order.
  29
  30
  31 Header
  32 ------
  33
  34 .. parsed-literal::
  35
  36     #include <boost/mpl/fold.hpp>
  37
  38
  39 Parameters
  40 ----------
  41
  42 +---------------+-------------------------------+---------------------------------------------------+
  43 | Parameter     | Requirement                   | Description                                       |
  44 +===============+===============================+===================================================+
  45 | ``Sequence``  | |Forward Sequence|            | A sequence to iterate.                            |
  46 +---------------+-------------------------------+---------------------------------------------------+
  47 | ``State``     | Any type                      | The initial state for the first ``ForwardOp``     |
  48 |               |                               | application.                                      |
  49 +---------------+-------------------------------+---------------------------------------------------+
  50 | ``ForwardOp`` | Binary |Lambda Expression|    | The operation to be executed on forward           |
  51 |               |                               | traversal.                                        |
  52 +---------------+-------------------------------+---------------------------------------------------+
  53
  54
  55 Expression semantics
  56 --------------------
  57
  58 For any |Forward Sequence| ``s``, binary |Lambda Expression| ``op``, and arbitrary type ``state``:
  59
  60
  61 .. parsed-literal::
  62
  63     typedef fold<s,state,op>::type t;
  64
  65 :Return type:
  66     A type.
  67
  68 :Semantics:
  69     Equivalent to
  70
  71     .. parsed-literal::
  72
  73         typedef iter_fold<
  74               s
  75             , state
  76             , apply_wrap2< lambda<op>::type, _1, deref<_2> >
  77             >::type t;
  78
  79
  80 Complexity
  81 ----------
  82
  83 Linear. Exactly ``size<s>::value`` applications of ``op``.
  84
  85
  86 Example
  87 -------
  88
  89 .. parsed-literal::
  90
  91     typedef vector<long,float,short,double,float,long,long double> types;
  92     typedef fold<
  93           types
  94         , int_<0>
  95         , if_< is_float<_2>,next<_1>,_1 >
  96         >::type number_of_floats;
  97
  98     BOOST_MPL_ASSERT_RELATION( number_of_floats::value, ==, 4 );
  99
 100
 101 See also
 102 --------
 103
 104 |Algorithms|, |accumulate|, |reverse_fold|, |iter_fold|, |reverse_iter_fold|, |copy|, |copy_if|
 105
 106
 107 .. copyright:: Copyright ©  2001-2009 Aleksey Gurtovoy and David Abrahams
 108    Distributed under the Boost Software License, Version 1.0. (See accompanying
 109    file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)