fold

Synopsis

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

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:

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

A type.

Semantics:

Equivalent to

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

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 );

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