.. Algorithms/Iteration Algorithms//fold

fold

Synopsis

.. parsed-literal::

template<
      typename Sequence
    , typename State
    , typename ForwardOp
    >
struct fold
{
    typedef |unspecified| type;
};

Description

Returns the result of the successive application of binary ForwardOp to the result of the previous ForwardOp invocation (State if it's the first call) and every element of the sequence in the range |begin/end<Sequence>| in order.

Header

.. parsed-literal::

#include <boost/mpl/fold.hpp>

Parameters

+---------------+-------------------------------+---------------------------------------------------+ | Parameter | Requirement | Description | +===============+===============================+===================================================+ | Sequence | |Forward Sequence| | A sequence to iterate. | +---------------+-------------------------------+---------------------------------------------------+ | State | Any type | The initial state for the first ForwardOp | | | | application. | +---------------+-------------------------------+---------------------------------------------------+ | ForwardOp | Binary |Lambda Expression| | The operation to be executed on forward | | | | traversal. | +---------------+-------------------------------+---------------------------------------------------+

Expression semantics

For any |Forward Sequence| s, binary |Lambda Expression| op, and arbitrary type state:

.. parsed-literal::

typedef fold<s,state,op>::type t; 

:Return type: A type.

:Semantics: Equivalent to

.. parsed-literal::

    typedef iter_fold<
          s
        , state
        , apply_wrap2< lambda<op>::type, _1, deref<_2> >
        >::type t; 

Complexity

Linear. Exactly size<s>::value applications of op.

Example

.. parsed-literal::

typedef vector<long,float,short,double,float,long,long double> types;
typedef fold<
      types
    , int_<0>
    , if_< is_float<_2>,next<_1>,_1 >
    >::type number_of_floats;

BOOST_MPL_ASSERT_RELATION( number_of_floats::value, ==, 4 );

See also

|Algorithms|, |accumulate|, |reverse_fold|, |iter_fold|, |reverse_iter_fold|, |copy|, |copy_if|

.. 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)