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Front Page / Algorithms / Iteration Algorithms / fold

fold

Synopsis

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<Sequence>::type, end<Sequence>::type) in order.

Header

#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:

typedef fold<s,state,op>::type t; 
Return type:A type.
Semantics:

Equivalent to

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

Complexity

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

Example

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