reverse_fold

Synopsis

template<
      typename Sequence
    , typename State
    , typename BackwardOp
    , typename ForwardOp = _1
    >
struct reverse_fold
{
    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 reverse order. If ForwardOp is provided, then it is applied on forward traversal to form the result that is passed to the first BackwardOp call.

Header

#include <boost/mpl/reverse_fold.hpp>

Parameters

Parameter	Requirement	Description
`Sequence`	Forward Sequence	A sequence to iterate.
`State`	Any type	The initial state for the first `BackwardOp` / `ForwardOp` application.
`BackwardOp`	Binary Lambda Expression	The operation to be executed on backward traversal.
`ForwardOp`	Binary Lambda Expression	The operation to be executed on forward traversal.

Expression semantics

For any Forward Sequence s, binary Lambda Expression backward_op and forward_op, and arbitrary type state:

typedef reverse_fold< s,state,backward_op >::type t;

Return type:

Return type:	A type
Semantics:	Equivalent to typedef lambda<backward_op>::type op; typedef reverse_iter_fold< s , state , apply_wrap2< op, _1, deref<_2> > >::type t;

A type

Semantics:

Equivalent to

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

typedef reverse_fold< s,state,backward_op,forward_op >::type t;

Return type:

Return type:	A type.
Semantics:	Equivalent to typedef reverse_fold< Sequence , fold<s,state,forward_op>::type , backward_op >::type t;

A type.

Semantics:

Equivalent to

typedef reverse_fold<
      Sequence
    , fold<s,state,forward_op>::type
    , backward_op
    >::type t;

Complexity

Linear. Exactly size<s>::value applications of backward_op and forward_op.

Example

Remove non-negative elements from a sequence [2].

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

BOOST_MPL_ASSERT(( equal< negatives,result > ));

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