...one of the most highly
regarded and expertly designed C++ library projects in the
world.

— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards

template< typename Sequence , typename State , typename BackwardOp , typename ForwardOp = _1 > struct fold_backward { typedefunspecifiedtype; };

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 |
---|---|---|---|

`Sequence` | A model of Sequence | A sequence to iterate. | |

`State` | A type | The initial state for the first `BackwardOp` /`ForwardOp` application. | |

`BackwardOp` | A model of [Lambda Function] | The operation to be executed on backward traversal. | |

`ForwardOp` | A model of [Lambda Function] | The operation to be executed on forward traversal. | `arg<1>` |

Expression | Expression type | Precondition | Semantics | Postcondition |
---|---|---|---|---|

`typedef fold_backward< Sequence,T,BackwardOp >::type t;` | A type | Equivalent 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 type | Equivalent 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< numbers , 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.

Algorithms, `fold`

, `iter_fold_backward`

, `iter_fold`

, `copy`

, `copy_backward`

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Last edited March 12, 2003 6:30 am