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      typename Sequence
    , typename State
    , typename BackwardOp
    , typename ForwardOp = _1
struct fold_backward
    typedef unspecified type;


Returns the result of the successive application of binary BackwardOp to the result of the previous BackwardOp invocation (State if it's the first call) and every element in the range [begin<Sequence>::type,end<Sequence>::type) in the reverse order. If ForwardOp is provided, then it's applied on forward traversal to form the result which is passed to the first BackwardOp call.


#include "boost/mpl/fold_backward.hpp"


 Parameter  Requirement  Description  Default value  
SequenceA model of SequenceA sequence to iterate.
StateA typeThe initial state for the first BackwardOp/ForwardOp application.
BackwardOpA model of [Lambda Function]The operation to be executed on backward traversal.
ForwardOpA model of [Lambda Function]The operation to be executed on forward traversal.arg<1>

Expression semantics

 Expression  Expression type  Precondition  Semantics  Postcondition 
typedef fold_backward< Sequence,T,BackwardOp >::type t;A typeEquivalent to typedef lambda<BackwardOp>::type bk_op; typedef begin<Sequence>::type i1; typedef i1::next i2; ...; typedef in::next last; typedef apply<bk_op,T,in::type>::type tn; typedef apply<bk_op,tn,in-1::type>::type tn-1; ...; typedef apply<bk_op,t2,i1::type>::type t1; typedef t1 t, where n == size<Sequence>::type::value and last is identical to end<Sequence>::type; Equivalent to typedef T t; if the sequence is empty.
typedef fold_backward< Sequence,T,BackwardOp,ForwardOp >::type t;A typeEquivalent to typedef fold_backward<Sequence, fold<Sequence,State,ForwardOp>::type, BackwardOp>::type t;.


Linear. Exactly size<Sequence>::type::value applications of BackwardOp and ForwardOp.


Removes negative elements from a sequence [1].

typedef list_c<int,5,-1,0,-7,-2,0,-5,4> numbers;
typedef list_c<int,-1,-7,-2,-5> negatives;
typedef fold_backward<
    , list_c<int>
    , if_< less< _2,int_<0> >, push_front<_1,_2,>, _1 >
    >::type result;

BOOST_STATIC_ASSERT(equal< negatives,result >::type::value);


[1] See remove_if for a more compact way to do this.

See also

Algorithms, fold, iter_fold_backward, iter_fold, copy, copy_backward

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Last edited March 12, 2003 6:30 am